The shape, structure, function, and evolution of the pterosaurian uropatagium

Abstract

The wing membrane of pterosaurs is the earliest innovation in the evolution of vertebrate powered flight and led to pterosaurs dominating the Mesozoic skies. Most studies of pterosaur flight focus on the main wing surface, the brachiopatagium, whereas the small but aerodynamically potentially important tail membrane, the uropatagium, is comparatively understudied. This review presents the current knowledge of the shape, extent, structure and function of the uropatagium that lies between the legs in pterosaurs, based on the available fossil evidence. Both direct evidence from soft tissue preservation and indirect evidence from osteological and ichnological data, suggest variation in the uropatagium among taxa, although evolutionary trends can still be outlined. While early branching pterosaurs had a proportionally large, confluent uropatagium integrated to varying extent with the tail and attached to the elongate fifth toe, pterodactyloid pterosaurs had a greatly reduced and split uropatagium spanning a much smaller triangular area between the knee joint and the base of the tail. The uropatagium was an important component of flight in early pterosaurs and its modification and reduction in derived forms is part of a major transition with the origins of the pterodactyloids and their changing flight and walking apparatus.

Key words
Pterosauria; flight apparatus; evolution; cruropatagium

INTRODUCTION

Pterosaurs are an extinct group of Mesozoic flying reptiles and the first vertebrates to evolve powered flight. Pterosaurs possessed a tripartite, skin-derived wing membrane composed of 1) the propatagium that sat in the crux of the forearm and was supported by a neomorphic bone, the pteroid (Bennett 2007a), 2) the brachiopatagium, the main wing that spanned the area between the tip of the elongate fourth finger (referred to as wing finger) and the ankle (Elgin et al. 2011) and 3) the uropatagium (occasionally called the cruropatagium) that sat between the legs as a large sheet or split pair of membranes (Unwin 1999) (Figure 1a).

Figure 1
a) Dorsal view of two hypothetical pterosaurs highlighting the main differences between the two major pterosaur morphotypes. A, non-pterodactyloid morphotype with short skull and neck (1a), short metacarpals (2a), prominent, medially directed fifth toe (3a), and long tail (4a); B, pterodactyloid morphotype with long skull and neck (1b), long metacarpal (2b), reduced fifth toe (3b) and short tail (4b). Further abbreviations: bp, brachiopatagium; pp, propatagium; up, uropatagium. Figure modified after Unwin, 2005. b) Soemmering’s (1817) bat-like pterosaur reconstruction, that later became known as the ‘traditional broad-winged model’ of pterosaurs based on the presumed attachment areas of the wing membranes.

Until the discovery of Darwinopterus (Lü et al. 2010) and related forms, pterosaurs had been divided into two distinct groups: 1) the paraphyletic and earlier non-pterodactyloid morphotype (previously known as “rhamphorhynchoids”) with a head, neck, and metacarpal IV proportionally similar in length to other, contemporary terrestrial archosauromorphs, a long tail (except for anurognathids) and long, medially directed fifth toes; and 2) the monophyletic and more derived pterodactyloid morphotype with an elongated head, neck, and metacarpal IV, a short tail and a reduced fifth toe. Darwinopterus and kin have reduced the gap between the two forms as they possess a pterodactyloid-like skull and neck, whereas the postcranium shows features of non-pterodactyloids (Lü et al. 2009, Wang et al. 2009), although recent finds have shown a succession of forms among the darwinopterans with successive changes in features including the tail and fifth toe (Hone et al. 2024).

Although pterosaurs have undergone something of a recent renaissance in research (Hone 2012), many basic features of their biology including their origins, functional morphology and palaeoecology are still strongly debated. Few issues could demonstrate this better than the constant disagreement on the general appearance of pterosaurs regarding the extent, structure or even the existence of different parts of their flight membranes. However, while there have been disagreements about the shape and structure of the brachiopatagium and the propatagium, it is only the uropatagium whose very existence has been questioned, with some authors suggesting that it was not present in pterosaurs (see below).

The word uropatagium (i.e. tail membrane) was originally coined for the part of the flight apparatus which extends between the hind limbs in bats (Yalden & Morris 1975; = ‘interfemoral membrane’). This term has been adopted for the pterosaurian membrane positioned between the legs, although Unwin (1999), doubting the integration of the tail in the pterosaurian wing apparatus, used the term ‘cruropatagium’ (meaning ‘lower leg membrane’) instead, which has been adopted by some other authors (e.g., Wilkinson et al. 2006, Wilkinson 2007, Stein et al. 2008). We prefer to use the term uropatagium in pterosaurs and define it as a flight membrane attaching to the hind limbs and to the fifth toe of the pes (in forms where it is present) and, additionally, most probably the tail (or at least the base of the tail). The four main points of debate we discuss in this review related to the pterosaurian uropatagium: 1) the existence, 2) the overall shape and extent, 3) the structure, and 4) the function of the uropatagium.

Unfortunately, only very few pterosaur fossils preserving uropatagia, or their external moulds, have been recovered, and detailed descriptions are largely lacking in the literature. As noted by Elgin et al. (2011), without understanding the basic morphology of the pterosaur flight apparatus, the reconstruction of their flight abilities and by extension much of their basic biology, becomes difficult if not impossible. By assessing all known specimens with a preserved uropatagium, here we provide a historical overview and a detailed evaluation of the information available to date, including the most reasonable interpretations concerning the shape, extent, structure and function of the uropatagium in pterosaurs and how it has evolved over time.

Institutional abbreviations:

BSPG: Bayerische Staatsammlung für Paläontologie und Geologie, München, Germany.

JME SOS: Jura Museum, Eichstätt, Germany.

HLMD: Hessisches Landesmuseum Darmstadt, Germany.

MCSNB: Museo Civico di Scienze Naturali “E. Caffi” di Bergamo, Bergamo, Italy.

NHMW: Natural History Museum, Vienna, Austria.

PIN: Palaeontological Institute, Russian Academy of Sciences, Moscow, Russia.

IVPP: Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, China.

TMP: Royal Tyrrell Museum of Paleontology, Drumheller, Alberta, Canada.

JZMP: Jinzhou Museum of Paleontology, Liaoning Province, China.

THE PRESENCE-ABSENCE DEBATE

The history of reconstructing the flight membranes of pterosaurs began with the drawings of the German anatomist, surgeon and palaeontologist Samuel Thomas von Soemmerring in the year 1817. He became interested in pterosaurs while working on the first Pterodactylus antiquus specimen, already recognized by Cuvier (1809) as a flying reptile. V. Soemmerring (1817) concluded that it was a hitherto unknown kind of bat and accordingly, depicted the pterosaur as having a narrow propatagium, a broad brachiopatagium attaching to the tarsus, and a bat-like uropatagium stretching between the hind limbs and the short tail (Figure 1b). His aim was apparently to give the creature as much of a bat-like appearance as he could (Wellnhofer 1991). More recent workers referred to the bat-like membrane configuration as the ‘traditional broad-winged model’ of pterosaurs (e.g., Unwin 1999), though this often refers solely, or primarily to the broad attachment of the brachiopataium (Elgin et al. 2011).

Indirect fossil evidence for the presence of a uropatagium was first published by Owen (1870) in the form of the long but clawless fifth toe of non-pterodactyloid pterosaurs, the general appearance and orientation of which resembled that of the calcar of some bats. The chiropteran calcar is a long, medially pointing, cartilaginous or osseous structure which articulates with the calcaneum as a support of the uropatagium, and the calcar lies subparallel to the trailing edge (Hill & Smith 1984, Stanchak & Santana 2018). Based on these analogies in relative position and form, Owen (1870) proposed that the fifth toe in pterosaurs might have had the same function as the calcar in bats, namely, to provide control and structural support for the uropatagium. One of the consequences of this, again based on observations of extant bats, is that the integration of the hind limb within the flight membranes might have made terrestrial locomotion more complicated for pterosaurs (Abel 1925).

During the 19th century different attempts of life restorations of pterosaurs were put forward both with (Owen [see fig. 8. in Padian 1979], Marsh 1882) and without (Von Zittel 1882) a uropatagium, but all of them showing the brachiopatagium attaching to the hind limbs. Although this reconstruction was widely accepted at that time, in the early 20th century some workers, such as Stromer (1913), contested this model saying that the hind limbs were free of the flight membranes and that pterosaurs lacked a uropatagium.

The renewed interest in a bird-like bipedal cursorial model of pterosaurs first proposed by Cuvier (1809) and Seeley (1901) challenged the idea of a bat-like wing membrane configuration and, as a consequence, the existence of a uropatagium, (although Seeley [1901 p 164] illustrated pterosaurs both with and without this structure). Although there were some minor differences in the interpretation of the bird-like pterosaur model among different authors, the best known is probably that advocated by Padian (1979, 1983a, b, 1985, 1991, 2003), who reconstructed pterosaurs as bipedal, digitigrade, cursorial archosaurs with the hind limbs free of both the brachiopatagium and uropatagium. This bipedal reconstruction also challenged the notion that the fifth toe controlled a uropatagium, and rather suggested that the toe was a pivot during walking. In addition, the fact that more derived pterodactyloids have only a reduced fifth toe was used as an argument against the uropatagium being a universal feature among pterosaurs.

Since the first appearance of Padian’s model several lines of evidence have been laid out against the idea of bipedal, digitigrade pterosaurs, and were instead quadrupeds. Most notably, trackway data for both pterodactyloids (Mazin et al. 2003, Lockley et al. 2008) and non-pterodactyloids (Mazin & Pouech 2020) support a quadrupedal stance, although the nature of the latter tracks is more controversial (see Wroblewski 2023). Despite disagreements over the origin of tracks, terrestrial habitats were occupied by both early and late pterosaurs with the latter apparently being more adapted to move on the ground (Witton 2015, Smyth et al. 2024). Although Elgin et al. (2011) noted that the ankle attachment of the brachiopatagium, which is most likely a universal feature in pterosaurs, does not necessarily affect the bipedal model, an extensive uropatagium would have arrested bipedal locomotion (Unwin & Bakhurina 1994, Unwin 1996), though this alone does not demonstrate that the uropatagium was present.

The unsettled situation concerning the reconstruction of the various flight membranes has also been reflected in the variable reconstructions published on presumed flight abilities, aerodynamics and biomechanics of pterosaurs. Whereas both Hankin & Watson (1914) and Bramwell & Whitfield (1974) involved the uropatagium in their calculations, other workers have favoured a model without uropatagium either assuming its complete absence (Padian 1979, 1983a, 1985, 1991, Brower 1980, 1982, 1983, Brower & Veinus 1981) or considering its aerodynamic effect insignificant (Hazlehurst & Rayner 1992 – which seems unlikely for a feature that persisted for at least 80 million years). In addition to these simple presence/absence interpretations, a unique model was proposed by Bennett (1987). He suggested that the brachiopatagium in Pteranodon attached directly onto the rod-like tail with no uropatagium being present, and thus the hind limbs were free of any flight membrane. However, in his subsequent work he quickly abandoned this reconstruction (Bennett 1991, 2001).

The reinvestigation of the holotype of Sordes pilosus (Unwin & Bakhurina 1994) found in the Upper Jurassic Karabastau Formation of Karatau, Kazakhstan (Sharov 1971) heralded a breakthrough. Although Sharov had described this specimen in 1971 explicitly stating the presence of a uropatagium, this work – being in Russian – had been largely unknown outside of the Eastern Block. Unwin & Bakhurina (1994) redescribed the specimen and showed that the holotype of Sordes (PIN 2585/3, Sharov 1971) not only preserves an extensive uropatagium with highly detailed mineralized soft tissues, but it also demonstrates the role of the fifth toe supporting the uropatagium. This well-preserved specimen became widely known for confirming the presence of a large uropatagium in at least one pterosaur species.

Despite this clear evidence, Peters (1995) interpreted the uropatagium in Sordes as part of the right brachiopatagium displaced post-mortem. In their reply to this note, Unwin & Bakhurina (1995) dismissed this assertion for a number of reasons: the uropatagium is symmetrically positioned between the hind limbs, the internal fibres of the uropatagium (that are different to those found in the brachiopatagium) were oriented parallel to the hind limb, the right brachiopatagium remained intact, and finally that both the fore and hind limbs lie almost perfectly symmetrical with respect to each other precluding post-mortem disturbance.

More recently, further pterosaur specimens (both non-pterodactyloid and pterodactyloid) have been either uncovered or re-examined that show physical evidence of a uropatagium. These include the exquisitely preserved holotype specimen of the anurognathid Jeholopterus ningchengensis IVPP V12705 (Wang et al. 2002, Kellner et al. 2009), another anurognathid, Luopterus mutoudengensis (JZMP-04-07-3) with a poorly preserved uropatagium was also subsequently described (Lü & Hone 2012). A three dimensionally preserved specimen of Rhamphorhynchus muensteri, was described (Frey et al. 2003 – no specimen number) with a partial uropatagium, and this structure is also visible in another specimen of the same species (Hone et al. 2015 – TMP 2008.41.0001) (Figure 2).

Figure 2
Close ups of the hindlimbs region of pterosaur specimens that preserve at least a partial uropatagium. a) Sordes (PIN 2585/3), b) Jeholopterus (IVPP V12705), c) Rhamphorhynchus (the ‘Dark Wing’ specimen), d) Pterodactylus.

Revisions of previously described material also yielded further evidence. A juvenile specimen referred to Eudimorphodon ranzii (Wild 1994, Dalla Vecchia 1995 – MCSNB 8950) represents the only Triassic pterosaur showing possible traces of a uropatagium (Bakhurina & Unwin 2003).

Evidence for a uropatagium is also present in pterodactyloids. The “Munich specimen” (Broili 1938 - BSPG 1937. I. 18) of Pterodactylus shows one, and a second one was noted by Wellnhofer (1970) though not illustrated or any specifics given. More recently, a privately held specimen of Pterodactylus was described as having mineralised preservation of the uropatagium with well-defined outline (Frey & Martill 1998), where the uropatagium attaches to the fifth toe.

In summary, there are now a number of pterosaur fossils that show evidence of a uropatagium, and its existence can no longer be denied. However, its preservation is uncommon with just a handful of specimens showing physical traces of a uropatagium. The lack of a preserved uropatagium even in otherwise complete and well-preserved fossils is more likely an indication of its delicate nature rather than of its absence in vivo. The presence of an elongate fifth toe in all non-pterodactyloids (except the taxa closest to the pterodactyloids – Spindler 2024, Hone et al. 2024), combined with the direct evidence of the uropatagium in the eudimorphodontids, anurognathids, and rhamphorhynchids suggests that it was probably present in all non-pterodactyloid pterosaurs (except perhaps the most derived forms with a reduced tail and fifth toe e.g., Propterodactylus, Spindler 2024). However, with the lack of an osteological correlate in the reduced fifth toe, the situation in pterodactyloids is still unclear: while present in Pterodactylus at least, it could perhaps have been lost in other lineages. It seems that the uropatagium was present in the earliest pterosaurs, although the exact shape and structure and the potential for variation among groups requires further discussion.

THE SHAPE AND EXTENT OF THE UROPATAGIUM

Based on the available fossil evidence, Unwin (2005) suggested that pterosaurs exhibited two main types of uropatagium. First, an extensive, confluent uropatagium stretching between the hind limbs with its trailing edge supported by the prominent fifth toe and thus reaching as far as the ankles, which was present in the non-pterodactyloid pterosaurs. Second, a smaller, split uropatagium extending from the ankles to the base of the reduced tail (or not even to the tail) is found in the pterodactyloids. However, as seen in extant bats where the size and shape of the uropatagium is highly variable, as is the degree of involvement of the tail and the calcar (Hill & Smith 1984), it is highly likely that pterosaurs also evolved a variety of uropatagium phenotypes. This notion is also supported by fossils showing intermediate and/or modular combination of morphologies, such as a reduced fifth toe and a intermediate-length tail found in the derived darwinopterid Skiphosoura (Hone et al. 2024).

Direct evidence

Direct evidence for the extensive type of uropatagium were first provided by the holotype of Sordes pilosus, where the tail membrane attaches along the medial side of the hind limbs as far as the ankle and is supported along its reverse V-shaped trailing edge by the clawless, medially directed fifth toe (Unwin & Bakhurina 1994). Other Sordes specimens also confirm the idea of an extensive uropatagium linked to the fifth toe in this taxon (Bakhurina & Unwin 1995) (Figure 3).

Figure 3
Interpretative line drawing of a, the counter slab of Sordes specimen MCSNB 8950 and b, c, close-ups of its highlighted regions comparing three different hypotheses (W: Wild, 1994; BU: Bakhurina & Unwin, 2003; CI: current interpretation) on the origin of various soft tissue remains (1-7). 1, Left seam interpreted by both W and BU as the trailing edge of the right brachiopatagium, by CI as the trailing edge of the uropatagium; 2, right seam identified by W as the trailing edge of the left brachiopatagium, by CI as the trailing edge of the uropatagium, and not interpreted by BU; 3, a stripe considered by W the trailing edge of the right brachiopatagium, by BU as uropatagial trailing edge, by CI as the trailing edge of the left or right brachiopatagium; 4, stripes interpreted by W as „wrinkles”, and both by BU and CI as structural fibres of the right brachiopatagium; 5, stripes considered folds of the right brachiopatagium by W, folds of the uropatagium by CI, and not interpreted by BU; 6, striae identified as pycnofibres by W, as uropatagial structural fibres by BU, as structural fibres of the right brachiopatagium by CI, 7, curved striation considered structural fibres of the right brachiopatagium by W, uropatagial structural fibres by BU, structural fibres of the left or right brachiopatagium by CI. Figure modified after Wild (1994).

Although Wang et al. (2002) stated that the uropatagium, along with the other flight membranes, are preserved in the holotype of Jeholopterus and attach to the fifth toe, its exact outline cannot be determined (Kellner et al. 2009). Despite the presumed short tail of Jeholopterus (there are no caudal vertebrae preserved in this specimen, but anurognathids have short tails), the soft part remains between the hind limbs suggest an anteroposteriorly deep uropatagium (Figure 2), probably similar to that of Sordes. Thus, the presence of a long tail is not indicative for the extent of the uropatagium in pterosaurs. Again, this is similar to bats where the extent of the uropatagium and the length of the tail are not correlated (Hill & Smith 1984). However, the lack of both the tail and a well-defined outline of the uropatagium in Jeholopterus mean that this conclusion awaits confirmation.

Another pterosaur specimen that provides further tentative evidence for a large uropatagium is the “Eudimorphodon” specimen MCSNB 8950 (see above). The line drawings provided by Wild (1994), which have also been reproduced by Bakhurina and Unwin (2003), show two ‘seams’ between the hind limbs that converge medially in a very similar fashion to the reverse, broad V-shaped trailing edge of the uropatagium in Sordes (Figure 3). The left of the two seams has been interpreted by both papers as the trailing edge of the right brachiopatagium. According to Wild (1994), the right seam could represent the trailing edge of the left brachiopatagium, whereas Bakhurina & Unwin (2003) did not specify which structure they considered this to belong to. There is a very short but clear ‘stripe’ posterior to the left seam and running to the proximal end of the left fifth toe which has been suggested by Wild (1994) to be part of the trailing edge of the right brachiopatagium. Bakhurina & Unwin (2003), again, did not indicate the nature of this structure in their line drawing, however, judging by the context of their description, they seem to refer to it as the trailing edge of the uropatagium. Based on the line drawings of Wild (1994) and Bakhurina & Unwin (2003) and comparing them with the soft tissue traces in Sordes pilosus (Sharov 1971, Bakhurina & Unwin 2003), our new interpretation of the two, converging seams is that they represent the inverted V-shaped trailing edge of a uropatagium, whereas the short stripe would belong to the trailing edge of the left or right brachiopatagium, as was suggested by Wild (1994). This would mean that the right brachiopatagium and the uropatagium overlap each other in this specimen. If so, the uropatagium of MCSNB 8950 was very similar in shape and extent to that of Sordes. Although the medially directed fifth toe in MCSNB 8950 has no direct contact with the proposed uropatagium traces, the membranous structures may have separated from the fifth toe during decay, as sometimes seen with the proximal part of the brachiopatagium that lack aktinofibrils (Elgin et al. 2011).

Pterosaurs having this extensive type of uropatagium have been referred to as “rhamphorhynchoid” constructions by Unwin (2005 p. 172-175) and “Sordes-type” constructions by Frey et al. (2003). These terms have been considered suitable to describe the generalized uropatagium in almost all non-pterodactyloids pterosaurs. Although this is far less clear in other specimens that often preserve only partial patches of soft tissues inferred to be uropatagia (e.g., Lü & Hone, 2012, Frey et al. 2003, Hone et al. 2015), these traces are found in parts of the specimen that imply a large tail membrane.

The derived non-monofenestratan Rhamphorhynchus is known from a huge number of specimens, many of which preserve soft parts, although only two preserve parts of a uropatagium. The “Dark Wing” specimen shows soft tissues preserved between the hindlimbs apparently connecting the ankle to the distal ischium on one side and the acetabulum on the other, which implies something of a split uropatagium similar to that seen in pterodactyloids (see below). Indeed, such a pattern has been depicted in Rhamphorhynchus reconstructions both prior to, and after the discovery of the Dark Wing specimen. For example, Wellnhofer shows Rhamphorhynchus with a pterodactyloid-like split uropatagium (1975) although without a reference to a source for this interpretation (and after having noted the shape in Sordes). Similarly, Bennett (2015) illustrates the same profile (his figure 9) without specifically commenting on it, since the paper was primarily discussing the brachiopatagium.

However, with the strongly retracted legs and femora directed anteriorly in the Dark Wing specimen, even a Sordes-like deep uropatagium could appear to be shallow as the trailing edge would be brought anteriorly and would then appear to reach the base of the tail. Furthermore, although the Dark Wing specimen has superbly preserved brachiopatagium, the uropatagium is less-well defined and is present as soft tissue patches between the legs as well as lateral to the right leg (Frey et al. 2003). These traces suggest that the uropatagium is not preserved in situ but got separated and displaced from its original attachment structures before fossilization, or even that some of these traces might not represent pterosaur soft tissues. The patches of soft tissues lateral to the right hind limb could also be explained by the rotation of the right foot that would have brought the fifth toe into a laterally pointing position, and along with it, the broad uropatagium. In fact, the medially directed terminal phalanx of the fifth toe in several other Rhamphorhynchus specimens (e.g., BSPG 1964 XXIV 321 and 1877 X 1) is a strong indicator of their function in supporting the trailing edge of the uropatagium.

Even though the actual outline of soft tissue traces in the Dark Wing may suggest a narrow and split uropatagium, the integration of the 5th toe, the presence of tissue traces lateral to the retracted legs all mean that this likely still has a deep-chord uropatagium (Figure 2). This is supported by a second specimen of Rhamphorhynchus (TMP 2008.41.001 - Hone et al. 2015) with uropatagium and similarly retracted legs. However, in TMP 2008.41.001, the uropatagium trailing edge appears to be distinctly broad extending medially from the distal end of the tibia which clearly implies a deep rather than a split or narrow uropatagium. We therefore favour the interpretation that Rhamphorhynchus is characterized by the more typical, “Sordes-like” non-pterodactyloid tail membrane.

A second uropatagium morphotype is reported from two specimens of Pterodactylus kochi (Figure 2) In both cases the uropatagium is split in the midline, forming a pair of smaller patagia. The soft tissues in the dorsally exposed Munich specimen (BSPG 1937. I. 18 - Broili 1938, see also Pittman et al. 2021) are visible along the right leg as extending from the distal half of the tibia and terminating at the pelvis / base of the tail. The trailing edge is concave converging to a point at the tarsus posteriorly and to the caudal terminus of the ilium anteriorly. The uropatagium is widest at the level of the knee joint. The second, privately held, Pterodactylus specimen described by Frey & Martill (1998) lies on its left side, though most of its trunk skeleton is missing. Its uropatagium has a well-defined, concave outline similar to that seen in the Munich specimen. Although its proximal part is partially obscured, the authors described it as extending from metatarsal V to the tip of the tail and increasing in width proximally. Frey & Martill (1998) also describe soft tissues along the cranial face of the tibia, however, it is not clear whether these tissues belong to the uropatagium or to the lower leg muscles and tendons. Other Pterodactylus specimens from Solnhofen may also support this, such as the Vienna specimen of Pterodactylus (NHMW 1975/1756) apparently showing patches of a small and split uropatagium under UV light. Similarly, Wellnhofer (1970) illustrates the hindlimb of a specimen of Pterodactylus (referred to as Exemplar 21) with a split uropatagium that is distinctly widest at the knee and tapering at both ends (referred to as Exemplar 21). Wellnhofer (1970) notes that this specimen was described by Meyer in 1862, unfortunately, we do not have access to this work.

This split, reduced uropatagium morphotype is referred to as the “pterodactyloid” construction (Unwin 2005, Frey et al. 2003), and has been considered a characteristic of nearly all pterodactyloids.

Indirect evidence

Osteological correlates as well as pterosaurian trackways provide indirect clues to the shape and extent of the pterosaurian uropatagium. The reduction and split of the uropatagium into a pair of patagia in pterodactyloids is likely linked to the reduction of the 5th toe (Unwin 1999, see also Hone et al. 2024). On the other hand, the anurognathids with a short tail may have still likely possessed an extensive uropatagium.

Pterosaur ichnites can also provide indirect evidence for the presence of an uropatagium. The first pterosaur trackway Pteraichnus saltwashensis was described by Stokes (1957). Stokes suggested that the track-maker was a quadrupedal pterodactyloid pterosaur. Since then, numerous pterosaur trackways have been described, which all support Stokes’ interpretation (see Unwin 1996, Mazin et al. 2003, Lockley et al. 2008 and references therein) and argue against Padian’s bird-like, bipedal model with freely moving hindlimbs or use of the fifth toe when walking (Figure 4a).

Figure 4
a) A typical example of pterodactyloid ichnite showing the tridactyl digitigrade manus prints on the outside, and the plantigrade tetradactyl feet in the centre. Figure taken from Mazin et al. (2003). b) Interpretative drawing on the variability in the shape and extent of the uropatagium in different extant bats. Note that if there is an extensive uropatagium as well as a tail, the tail is always integrated within the tail membrane. A) Phyllostomidae B) Phyllostomidae C) Nyctimene, Paranyctimene D) Phyllostomidae E) Craseonycertis F) Rhinopomatidae G) Molossidae H) Vespertilionidae I) Nycteridae J) Emballonuridae, Noctilionidae, Mormoopidae. Figure taken from Hill & Smith (1984).

However, all but one of the currently described tracks show the same morphology lacking any trace of pedal digit V or tail, which implies a pterodactyloid trackmaker (Unwin 1996, 2005, Mazin et al. 2003). The temporal range in which these trackways have been found also corresponds with this previous interpretation, as the earliest ichnological evidence comes from the late Middle Jurassic of the Sundance Formation of Alcova, Wyoming (Logur 1994); around the time when the pterodactyloids likely first arose. There is no pterosaur trackway record from the Late Triassic up to the Middle Jurassic; a time period from which non-pterodactyloids are the only known pterosaurs (see also Unwin 2005). Since non-pterodactyloids were diverse and numerous in this period, one would expect at least some ichnites from them, although if they were predominantly arboreal (e.g., see Smyth et al. 2024) this could explain the discrepancy.

As nearly all pterosaur ichnotaxa to date refer to pterodactyloids, could probably be explained by differences in the construction of the uropatagium in non-pterodactyloids that may have influenced the terrestrial locomotion abilities of the latter (Unwin 1996, 2005). Early pterosaurs with an extensive uropatagium would have had their hindlimbs shackled together by the membranes and with the forelimbs and hindlimbs also linked by the brachiopatagia, locomotion on the ground would have been awkward (Unwin 1999). They most probably have spent most of their time in the air or in the trees leaving no trackways behind. With the advent of the more derived “pterodactyloid construction” with their reduced, split tail membranes the hindlimbs became independent from each other, and only the respective limbs on the left and right sides would be joined by the wing membranes. As such the individual limbs were able to move with greater independence and terrestrial locomotion corresponding with the occurrence of pterodactyloid tracks in the Late Jurassic onwards (Unwin 2005).

Although Witton (2015) has strongly challenged the idea that early pterosaurs were not adept at terrestrial locomotion, the absence of trackways for early taxa remains striking with a single example of a non-pterodactyloid track described by Mazin & Pouech (2020), but subsequently challenged by Wroblewski (2023). Despite the large number of taxa occupying marine and coastal environments, no tracks are known to date that could be unequivocally referred to non-pterodactyloids. It is therefore still possible that the size, shape and extent of the uropatagium will influence the presence, or the exact shape and pattern, of pterosaur trackways. In the absence of further trackways of non-pterodactyloids this will remain an area of contention, but one that should be considered in the future.

Unwin & Bakhurina (1994) also proposed taphonomy as a useful tool to infer the presence and extent of a uropatagium suggesting that the position of hind limb bones in many articulated pterosaur specimens may reflect their association with the brachio- and uropatagium. The position of the femur lying almost perpendicular to the spinal column and pronated forward 90°, a laterally directed proximal face of the tibia and the clawless fifth toe deflected medially in early forms are all proposed to be taphonomical indicators of this association. However, the posture of the femur and tibia described above is also true of several non-avian and avian dinosaur fossils that show the characteristic opisthotonic posture (Faux & Padian 2007). Thus, the described position of the femur and tibia in pterosaurs may not reflect the effect of their integration within the flight membranes.

The tail and the uropatagium

One of the most intriguing questions concerning the pterosaurian uropatagium is whether or not the tail (or part of it) was integrated with the uropatagium. Based on the holotype of Sordes it was suggested by both Sharov (1971), and Unwin and Bakhurina (1994), that the tail lay dorsal to the uropatagium and was not integrated with it. Thus, they argued, the tail could be moved independently from the hind limbs and the uropatagium. This idea was based on the displaced position of the tail and the apparent undisturbed nature of the uropatagium below it.

However, among extant bats, which provide a useful analogy for the variation in uropatagial shape, such an arrangement where the tail lies above a broad uropatagium is unknown (Figure 4b). Some extant bats do not possess an externally visible tail at all (e.g., some New World leaf-nosed bats [Phyllostomidae]), in others the tail membrane is attached at the base of the tail (e.g. Tube-nosed fruit bats in the genera Nyctimene and Paranyctimene), but if they have an extensive uropatagium as well as a tail, the tail is always incorporated into the membrane, (e.g. free-tailed bats – Molossidae) and never lies above it (Hill & Smith 1984, p. 17). Furthermore, in fossil bats, (e.g., Palaeochiropteryx tupaiodon HLMD-Be 201) with superb soft part preservation from Messel, the caudal vertebrae are displaced and even disarticulated, whereas the uropatagium beneath is preserved intact; a condition similar to that found in Sordes. Clearly, there is no reason to assume that the arrangement of this anatomical region was essentially different in fossil and extant bats to explain this phenomenon, and likewise the dissociation of the tail from the uropatagium seen in Sordes (and the complete loss of a tail in Jeholopterus) is likely a taphonomical effect rather than a natural arrangement. A similar effect has also been described for a young specimen of the fish Tharsis dubius from the Solnhofen limestones where the vertebral column has become detached from the tail fin (Viohl 1990).

Further support for the integration of the tail comes from the preserved shape of the uropatagium in Sordes. This has a clearly defined sharp and angular trailing edge that meets in the midline (Figure 2). As the tail membrane must have had some degree of elasticity to allow leg movements during flight or walking, it could not have maintained this angular shape without being integrated with the tail, but would have contracted resulting in a U-shaped posterior margin. However, if the membrane was originally attached to the tail, then the membrane would have been under tension giving the sharp edge tailing edge and angular midpoint to the membrane, prior to the separation and displacement of the tail. This would be an example of the ‘stick and peel’ phenomenon (Orr et al. 2016) where soft tissues may stick to substrates, while even articulated elements can move prior to preservation leaving the two intact, but separated.

Such an integration of the tail with the uropatagium would provide both a functional role for the tail and a pathway for the transition to the pterodactyloid condition. Functionally, the tail would have provided axial support and tension to the uropatagium turning it into an effective control surface for directing air flow and thereby increasing manoeuvrability. A membrane attached to the tail would also allow the two sides of the uropatagium to operate at least partly independently – one leg could be raised or lowered or bent at the knee to change the airflow on one side of the body separately to the other side, providing options in roll and yaw during flight. Reconstructing the evolutionary transition from the broad to the narrow and split uropatagium morphotype also makes more sense with a tail membrane incorporating the tail. A broad uropatagium could have been reduced in area, retreating in the midline and still being supported by the tail, the legs and fifth toe, before as well as during the reduction in the tail and toe length. Such a scenario may be supported by the existence of the derived non-pterodactyloid darwinopteran Skiphosoura which has a reduced, but stiff, tail and a reduced fifth toe (Hone et al. 2024). Such an arrangement implies that the tail and toe were integrated with the membrane and both reduced as this was shorter in the midline and then may have split. A membrane unsupported by a long tail (as is possible in some anurognathids – see below) could be reduced in depth but would be difficult to hold under tension (and therefore would flutter and provide little control of the air while adding drag), if it was still attached to a long fifth toe and was not supported by the tail.

Although there is still a low degree of uncertainty in the inferred integration of the tail with the uropatagium in non-pterodactyloids, it is strongly supported by multiple independent lines of arguments, including taphonomical, comparative anatomical, mechanical and evolutionary considerations. Therefore, we conclude here that the tail was incorporated within the uropatagium in early pterosaurs. In more derived taxa, the reduced uropatagium appears to separate from and attach to the base of the tail, thereby splitting up into a pair of membranes. This notion is further supported by the inferred lack of stiffness in the tail of derived pterosaurs to withstand considerable aerodynamic stress, and by other aerodynamic considerations (Koroljov 2017).

THE STRUCTURE OF THE UROPATAGIUM

With the possible exception of the propatagium, the uropatagium is the most rarely preserved integumentary structure among pterosaurs. It appears only in the most exceptionally preserved specimens in which generally all other types of epidermal structures are preserved (brachiopatagia, foot webs, pycnofibres etc.) and even then, the uropatagia are not often well-preserved themselves (e.g., “Dark Wing” Rhamphorhynchus, Jeholopterus holotype). This suggests that the pterosaurian uropatagium is generally a thin and delicate structure compared to the brachiopatagium. Furthermore, in some specimens where soft parts like the uropatagium are indistinct, some traces may have been lost to preparation procedures. Today, this can be at least partly alleviated by preparation under UV light, which in some cases has revealed previously indistinct or even unknown soft tissues (e.g., see Tischlinger & Frey 2001, Bennett 2007b).

Unwin & Bakhurina (1994) described the uropatagium of Sordes as containing short, sinuous, loosely packed fibres that are different from the long, straight, closely packed stiff fibres, the aktinofibrils, found across all but the proximal part of the brachiopatagium. Based on these differences in the structural fibres, Unwin and Bakhurina suggested that the proximal area of the brachiopatagium (also referred to as tenopatagium, Schaller 1985) as well as the uropatagium had significantly more flexibility than the distal parts of the brachiopatagium stiffened by aktinofibrils.

However, the structures described in Sordes interpreted as uropatagial fibres appear to be very similar to the pycnofibres that are present on the body and even the wings of many pterosaurs, including Sordes (Frey & Martill 1998, Wang et al. 2002, Kellner et al. 2009). These could also correspond to the ‘B’ type aktinofibrils of the proximal part of the brachiopatagium in Jeholopterus (Kellner et al. 2009), which appear somewhat intermediate between the main aktinofibrils of the wing and the pycnofibres. Sharov (1971) also described “hair” coverage at the base of the tail also seen in Jeholopterus (Lü & Hone 2012), which might correspond to the “uropatagial fibres” mentioned by Unwin & Bakhurina (1994). Depending on the mode of soft tissue preservation (Anderson 2023), modern imaging techniques and analytical tools, such as Raman spectroscopy, scanning and backscatter electron microscopy, reflectance transformation imaging or computed tomography and synchrotron X-ray-based approaches (Gueriau et al. 2014, Cunningham et al. 2017, Henkemeier et al. 2023, Falk et al. 2024) could provide more details to separate the various types of pterosaurian fibres.

In “Eudimorphodon” MCSNB 8950, several striae are seen in the area between the hindlimbs but their origin is difficult to determine. According to the reconstruction of Wild (1994), there are two different features seen proximal to a prominent trace on the left side that Wild interpreted as the trailing edge of the right wing: 1, undulating wrinkles that run almost perpendicular to the proposed trailing edge and that are spaced between 0.3 to 1.5 mm; 2, folds, which appear as 0.1 mm thick stripes and are parallel and adjacent to the trailing edge. Distal to the trailing edge of the brachiopatagium Wild (1994) described very thin (0.01-0.02 mm), and short (0.4 mm) striae running parallel to each other and being separated by 0.08-0.1 mm. Wild (1994) compared these striae with those described by Broili (1938) in Rhamphorhynchus and Pterodactylus and interpreted them as “hair-like” structures (i.e., pycnofibres). However, the parallel orientation of these contradicts this referral because pycnofibres generally lack such organization, are somewhat sinusoidal and thicker than aktinofibrils, taper distally, and usually form clusters (e.g., see Lü 2002, Wang et al. 2002, Kellner et al. 2009). Distal to the left seam identified by Wild (1994) as the trailing edge of the left wing, there are patches of curved striations, which have been interpreted as structural fibres of the right wing. Wild (1994) indicated however, that these structural fibres are not synonymous with the aktinofibrils found in the distal part of the brachiopatagium, but they correspond to the proximal, very short fibres that diminish and finally disappear towards the body (tenopatagium; the Type B fibres of Kellner et al. 2009).

Bakhurina & Unwin (2003) suggested that the “wrinkles” of Wild (1994) represent aktinofibrils of the right brachiopatagium, and the short parallel stripes identified by Wild as “hair-like” structures are actually structural fibres of the uropatagium corresponding to the fibres described in the tail membrane of Sordes. The striations, which Wild (1994) considered to consist of loose, proximal structural fibres of the right tenopatagium, have also been referred to as uropatagial fibres by Bakhurina & Unwin (2003). Based on this interpretation they presented a new reconstruction of the arrangement of structural uropatagial fibres for basal pterosaurs (Bakhurina & Unwin 2003) where the uropatagial fibres lie roughly parallel to the tibiae (Bakhurina & Unwin 2003, Unwin 2005 p. 177).

Uropatagial fibres were briefly mentioned in the original description of Jeholopterus ningchengensis IVPP V12705 by Wang et al. (2002), but only with respect to their length. Kellner et al. (2009) stated that in the uropatagium of this specimen the fibres run in two different directions: parallel to the longitudinal body axis and perpendicular to the tibiae. They argued that the thickness of these fibres corresponds to that of aktinofibrils not pycnofibres (though the latter were also present in the area around and over the uropatagium that were likely in association with the tail – Lü & Hone 2012). Furthermore, Kellner et al. (2009) concluded that the uropatagium, along with the also rarely preserved propatagium, is structurally similar to the aktinopatagium, although probably more tensile than the latter. Again, detailed images and analyses of these structures has yet to occur, and greater detail is required to compare these fibres with others preserved on the specimen.

However, specimens of Rhamphorhynchus (Frey et al. 2003, Hone et al. 2015) do suggest that visible fibres are a genuine component of the uropatagium and therefore at least potentially support their presence in Sordes, Jeholopterus and Eudimorphodon. In both of the specimens of Rhamphorhynchus preserving a uropatagium, the wing membranes – especially the aktinopatagia – are well preserved, and distinct and separate from the uropatagium. Furthermore, neither specimen had any evidence of pycnofibres preserved (either because they were lost in decay or during preservation) meaning that the fibres associated with the uropatagium cannot be attributed to either of these sources. In the case of Rhamphorhynchus (TMP 2008.41.0001, Hone et al. 2015), the fibres are very clear and simple, straight and unbranched structures in tight parallel rows contrasting the pattern seen in pycnofibres, and as such can be confidently attributed to the uropatagium (Figure 5).

Figure 5
Close up of uropatagial fibers in Rhamphorhynchus (TMP 2008.41.001). Image modified from Hone et al. 2015. Scale bar is 2 mm.

The uropatagial fibres in TMP 2008.41.0001 are approximately 0.06-0.1 mm in diameter and can reach 3 mm or more in length (Hone et al. 2015). These are much thinner than aktinofibrils in the wings of this specimen (as much as 0.3 mm in diameter) but are close to values reported by Bennett (2000) for Rhamphorhnchus at 0.05 mm in diameter and the Type A fibres of Jeholopterus (c. 0.1 mm, but thinner than the type B fibres in this taxon - Kellner et al. 2009). They are simple, straight and unbranched, sit in rows (parallel to the long axis of the body) and are present throughout the preserved area, but are more densely packed – up to 12 fibres per mm – closer to the tailing edge of the uropatagium (Hone et al. 2015). These fibres therefore clearly have some features in common with the described aktinofibrils in other parts of pterosaur wing membranes but appear to be uniquely short.

In summary it can be assumed that even if we exclude the possible loss of some material through preparation, the preservational potential of the uropatagium is very low. We suggest that this may be due to the lower structural integrity (lower stiffness and higher elasticity) of the uropatagium which could have resulted in a more rapid decomposition / dissociation. Low preservational potential could also be the result of post-mortem shrinkage of the uropatagium that would have especially affected the pterodactyloids with their small, split uropatagia (for which there is also, so far, no evidence of supporting fibres).

THE FUNCTION OF THE UROPATAGIUM

Given the prevalence of a uropatagium of some form in bats as well as numerous gliding lineages (e.g., Dermoptera, Pteromyini, Sharovipteryx), it is reasonable to assume that pterosaur uropatagia operated to improve flight performance in some way. It could have played a role in increasing lift and degree of control and manoeuvrability of flight, act as a brake, or any combination of these depending on the exact shape, extent, structure and position of the uropatagium.

Whatever the function, tension control in the uropatagium would be essential. A loose membrane that fluttered uncontrollably would add drag and not be able to effectively control airflow, making it a hinderance and not a benefit in flight. Pennycuick (1988) noted if the brachiopatagium attached to the fifth toe (as in some bats), then moving the toe would, in particular, control the trailing margin of a brachiopatagium to alter its position, but the same function would be true of a broad uropatagium. Compared to a brachiopatagium, given the proportionally smaller size and increase in support from the legs, toes, and tail, a uropatagium would still be moved by the toe, and these various bony supports, combined with the stiffening fibres would ensure that the membrane retained under tension but could also be manipulated.

Unwin & Bakhurina (1994) gave a brief suggestion of the function stating that the uropatagium was probably used for manoeuvring and braking, and was manipulated by the fifth toe semi-independently of the brachiopatagium. They also indicated that an anteroposteriorly deep uropatagium that was effectively forming a continuous surface with the brachiopatagium (a point echoed by Koroljov 2017, and incidentally supports an ankle attachment for the brachiopatagium) and would have significantly increased the surface area of the flight membrane. This construction would result in low wing loading (i.e. a larger surface area generating more lift relative to the weight of the animal), which generally correlates with slow, manoeuvrable flight. In contrast to this, the split uropatagium of pterodactyloids implies not only a reduced effect during flight manoeuvres, but also a higher wing loading and faster flight speed due to the decreased surface area. This conclusion, however, must be tested in the context of the overall changes during the evolution of the pterosaurian ‘body plan’. Evolutionary changes relevant to uropatagial functional considerations include the shape and proportion of the brachiopatagium associated with changes in wing and leg proportions, and increased pneumaticity that would reduce relative body mass to wing area in later taxa. Even though Sharov suggested that a tail independent from the uropatagium in Sordes would have been useful to separate the tail as a functioning rudder or elevator (Sharov 1971), these proposed tail functions could have been equally well performed with a tail incorporated within an elastic uropatagium that was actively controlled by the position of the hind limbs, including the fifth toe. Tail vanes are now known to have been held vertically and so this would not function as an elevator, but even if the tail was integrated and was moved laterally to induce yaw, the elasticity of the uropatagium means that the tail could move while the membrane held tension and so retained functional airflow.

Tischlinger & Frey (2002) and Frey et al. (2003) discussed in more detail how the uropatagium could have contributed to flight control. They suggested that with the synchronised dorsal or ventral rotation of the hind limbs the pterosaur could potentially control the pitch axis, whereas moving one leg ventrally and one dorsally could result in roll. If the hind limbs were simultaneously rotated ventrally, the camber of the proximal part of the wing membrane increased allowing slow flight and further reducing flight for landing, while the uropatagium itself could act as an air-brake if the legs were lowered. Comparing the flight apparatus of a Sordes-type and that of a pterodactyloid construction, the authors concluded that in the non-pterodactyloid construction the hind limbs could not be moved independently but they had a rather bat-like wing beat cycle with synchronized hind limb movements during up- and downstroke, whereas the pterodactyloid construction with the reduced uropatagium indicates its decreased aerodynamical role and a highly independent mobility of the hind limbs.

Koroljov (2017) noted that the effective integration of the uropatagium with the brachiopatagia to form a ‘monowing’ would benefit in general flight, but meant that the left and right wings could not operate independently, and that actions such as lowering the legs would cause aerodynamic problems. However, it is not clear how this is problematic with such a configuration existing for millions of years in bats that are known to be perfectly capable of manoeuvres that require differential folding or extension of the two wings or and lowering or lifting of the legs with the uropatagium and brachiopatagia moving accordingly. Although Koroljov considered that the tail would be integrated with the uropatagium generally, he also suggested that the apparent lack of a tail in the anurognathids would limit the function of the uropatagium in flight, and instead suggested that it may have helped the animal to shelter itself from rain as in some bats. This was provided without any real explanation and with the obvious issue, that there is no reason to think anurognathids were truly tail-less or had an inverted resting posture. This idea also overlooks the variation seen in anurognathid tails: although some do have a short and simple pterodactyloid-like tail, early forms have longer tails (though still shorter than other early pterosaurs) and notably retains supporting elongate zygapophyses (see Hone 2020).

In birds, while structured very differently, the tail can provide both extra lift and reduced drag for the bird in flight (Maybury & Rayner 2001, Maybury et al. 2001) and a similar effect might be expected in pterosaurs by providing an extension to the body in the uropatagium. Returning to bats, the free-tailed bats (Molossidea) have an uropatagial configuration similar to the construction of Sordes. Molossidae have a calcar, a deep uropatagium that stretches between the ankles, and a long tail at least half of which protrudes from the trailing edge of the uropatagium. The calcar of bats supports the trailing edge of the tail membrane, and having a muscular control, it can alter the camber in the uropatagium during flight (Hill & Smith 1984, p .16, Stanchak & Santana 2018) which confers differing lift on the two sides and so potentially induce roll or yaw.

Finally, purely aerodynamical considerations have implications on the functional alteration of different wing configurations, and thus on the evolution of pterosaurian flight. Hence, it is likely that the uropatagium – especially the broad type integrating the tail – assisted with steering and braking and perhaps provided some lift or reduction of drag.

EVOLUTION OF THE UROPATAGIUM

Gliding is a likely intermediate stage in the origins of powered flight (Simmons et al. 2008). Small gliding animals with limited wing area would have high wing loading resulting in poor glide angles, so any additional wing area and subsequent increase lift would be advantageous for them (Alexander 2003, p. 198). Gliders therefore generally have extensive wing membranes and will increase their area in any form that is possible with things like toe-webbing in frogs and geckoes, as well as flaps around the base of the tail in some, and even a uropatagium of some form is very common in mammalian gliders (Khandelwal et al. 2023). As such, a uropatagium was most likely present in the ancestral “proto-pterosaur” that could parachute or glide, but was not yet capable of powered flight. This would probably be extensive and stretch between the two hindlimbs and incorporated the tail, as often seen in modern gliders and some ancient ones (e.g., Sharovipteryx - Dyke et al. 2006).

This is supported by the presence of the well-developed and specialised 5th toe of non-pterodactyloids in even the earliest members of the clade (Dalla Vecchia 2014). The support and additional potential airflow control provided by the fifth toe could have been further enhanced by the evolution of supporting fibres within the uropatagium that would have decreased its undesired flutter. Integumentary fibers of some form may predate the origin of pterosaurs if these are ancestral to ornithodirans (Benton et al. 2019), but since these are not common in gliders, this may be a feature that developed as a result of the origins of aktinofibrils in the main wings and so were coopted into the uropatagium. The apparent absence of fibres in the propatagia of various pterosaurs (including Jeholopterus) does suggest that these are not a universal feature of pterosaur wing membranes. So far, physical and theoretical evidence favours the hypothesis that early pterosaurs bore an anteroposteriorly deep-chorded uropatagium that stretched between the legs terminating at the ankles and extending onto the fifth toe of each foot.

The presence of a uropatagium increased lift in these early pterosaur lineages, but perhaps also set limitations in terrestrial locomotion (Unwin 1999), and limited their ability in landing (Koroljov 2017). The non-pterodactyloid monofenestratans show a limited amount of tail reduction (Lü & Hone 2012), with the tail retaining elongate supporting zygapophyses and chevrons. Later taxa reduced their tail further in length (Hone et al. 2024) eventually to a tail stub with no apparent supporting role in the more derived non-pterodactyloids (Tischlinger & Frey 2013). The fifth toe progressively reduces in these later non-pterodactyloid monofenestratan taxa (Hone et al. 2024) indicating that the tail shortening preceded that of the fifth toe which started to reduce in size only when the tail got shorter than the long axis of the hind limbs in flight position. It is likely that the tail became progressively less important for the uropatagium. We suggest that by taxa such as Propterodactylus (Spindler 2024) the uropatagium has split and the tail is providing little support, while the toes are giving some limited movement to the two independent sides.

The supposed increasing axial reduction of the membrane until reaching the base of the tail resulted in a deeply split uropatagium, yet still representing a considerable surface area with a now longer trailing edge but still an extensive membrane that may be subject to flutter. With limited or no support from the tail, this transitional construction might have necessitated additional support by an increased amount of or stiffer uropatagial fibres. Once the uropatagium was fully split the trailing edge was primarily perpendicular to the airflow and with a narrow membrane less support would be needed.

This morphological transition occurs alongside evidence for increasing occupation of and adaptations to terrestrial environments (Hone et al. 2024) supporting that terrestrial locomotion was a major drive for changing the uropatagial configuration in pterosaurs (see Unwin 1999). In the early pterodactyloids, and presumably the whole lineage, the uropatagium was fully split and present as two membranes that sat in the crux of the legs, with tail were not involved in the membrane, or had the most token of connections. At whichever point the uropatagium split, there would be no need for supporting fibres or support from the fifth toe, as there would be no long trailing edge of the membrane perpendicular to the direction of travel that could be subject to flutter, being well-supported and controlled by the legs alone. In later pterodactyloids this condition presumably held, though to date uropatagia are unknown outside of Pterodactylus.

However, evidence from the anurognathids and some pterodactyloids suggests that other configurations of the uropatagium might have been present. In the anurognathids there is successive reduction in the tail so that it is initially shortened but with retention of extended zygopophyses, and only later is this reduced further to a short and simple pterodactyloid-like tail (Hone 2020). Interestingly, however, they seem to have retained a broad and unsplit uropatagium throughout this evolutionary process. Although at best loose and indirect evidence for a changing uropatagium, among the pterodactyloids there is some variation in tail length with long tails seen in Pterodaustro especially (Lü & Hone 2012) which are unsupported but of similar length to some early monofenestratans, and in Pteranodon the tail is notably relatively long and also stiffened (Bennett 2001). Thus, it is possible that larger uropatagia existed in some pterodactyloids or perhaps partially split membranes were present in some non-pterodactyloids (Figure 6). It seems unlikely that such a few configurations were present across a clade that was around for c. 180 million years and had representatives capable of flight across an order of magnitude (or perhaps even two) in wingspan and across three in mass, and that lived in every environment from forests to open ocean.

Figure 6
Simplified phylogeny of pterosaurs based on Hone et al. (2024) with those genera known to have a uropatagium marked with the general form seen as illustrated on the right A) typical non-pterodactyloid morphotype, B) anurognathid morphotype, and C) pterodactyloid morphotype.

SUMMARY

The debate on the presence-absence of an uropatagium in pterosaurs is irrevocably decided in favour of its presence. The direct and indirect evidence demonstrated by soft tissue preservation, osteological and ichnological features, add to a comprehensive and mutually supporting picture of the shape and extent of the pterosaurian uropatagium. Plesiomorphically, a prominent, anteroposteriorly deep uropatagium integrated with the tail was supported and controlled by the elongate, medially directed fifth toes and by hindlimb movements: a condition found in Sordes, Jeholopterus and likely in other non-pterodactyloids including the derived Rhamphorhynchus. In at least some early pterodactyloids had a split uropatagium being reduced along the longitudinal axis of, and eventually separating from, the tail. Although lacking direct fossil evidence yet, several intermediate conditions from a broad to a strongly reduced, vestigial uropatagium and other possible combinations have likely occurred throughout the evolution of the pterosaurian body plan.

The internal structure of the tail membrane remains equivocal. Uropatagial fibres have been reported for Sordes, MCSNB 8950, Jeholopterus and Rhamphorhynchus. No pterodactyloid specimen has been found so far that could confirm the presence of internal uropatagial fibres in this more derived clade. The poor preservational potential of the pterodactyloid uropatagium indicates the absence of stiff, resistant structural fibres in uropatagium suggesting that it was a thin membrane.

The uropatagium is likely to have assisted in a variety of flight and landing manoeuvres controlling movements about the pitch and roll axes during flight and could have been used for braking or generating higher lift during slow flight and landing. Uropatagial efficacy – and hence functional importance – at these manoeuvres must have depended on its shape, extent and structure, which is in line with the inferred differences in flight capabilities and flight styles among basal, derived non-pterodactyloid and pterodactyloid pterosaurs.

Acknowledgements

We would like to thank N. Micklich for providing additional information on bats Wang X. for access to Jeholopterus, O. Rauhut for access to Pterodactylus, H. Tischlinger for UV photos, Liu Jun for photos of Sordes, E. Frey and R. Elgin for discussions of pterosaur wings.

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Publication Dates

• Publication in this collection
  15 Sept 2025
• Date of issue
  2025

History

• Received
  5 Feb 2025
• Accepted
  27 May 2025