Xianglong – A Glider, But Not an Agamid Lizard

Updated August 27, 2015 with new reconstructions of the skull. 

Figure 1. Xianglong zhaoi, a late-surviving sister to Kuehneosaurus and Icarosaurus. What appear to be ribs framing the gliding membrane are in fact dermal ossifications as in Coelurosauravus.

Figure 1. Xianglong zhaoi, a late-surviving sister to Kuehneosaurus and Icarosaurus. What appear to be ribs framing the gliding membrane are in fact dermal ossifications as in Coelurosauravus.

Xianglong zhaoi (Li et al. 2007, Fig. 1) Yixian Formation, Early Cretaceous, 15.5 cm in length was originally considered an agamid lizard with elongated transverse processes and hyperelongated ribs, like the extant Draco volans (Fig. 2). Well those aren’t exactly transverse processes. They’re fused ribs. That makes the rib-like frames for the gliding membranes actually dermal ossifications, as in Coelurosauravus and other Triassic rib gliders. Xianglong shares a suite of traits with Kuehneosaurus and Icarosaurus, but it had fewer membrane supports. Xianglong was a Triassic rib-gliding kuehneosaur that survived into the Cretaceous.

Key distinctions include: Xianglong had what appear to be elongated transverse processes, but no agamid nor Triassic rib glider has elongated transverse processes. These are actually ribs fused to the neural spines and centra, as in Icarosaurus and Kuehneosaurus. This is not the pattern seen in Draco (Fig. 2). Xianglong had a pes in which metatarsal 2 was longer than mt 4, as in the Triassic rib gliders, not lizards, in which metatarsal 4 (or 3 and 4) is generally the longest.

Draco volans

Figure 2. Draco volans in dorsal view based on an X-ray. Note the lack of transverse processes. Metatarsal 3 is subequal to mt 4 and mt 2 is shorter. Click for more info.

Distinct from Icarosaurus, the skull of Xianglong had a larger lacrimal and a more robust jugal and postorbital. The anterior cervicals were taller. The fused ribs were relatively shorter. As in Icarosaurus, posterior ribs did not carry pseudoribs. Like Kuehneosaurus, the tail was longer than the presacral series. The forelimb was relatively short, especially in the forearm. The carpus was poorly ossified, a trait shared with Kuehneosaurus. Metacarpal 2 was reduced relative to mc 3. The hind limbs were gracile, as in Kuehneosaurus. Metatarsal 2 was  longer than mt 3 and mt 4 was short as in Icarosaurus and Kuehneosaurus. Xianglong was a sister to Icarosaurus. Moving it to Kuehneosaurus adds 5 steps. Moving it to a sisterhood with Draco adds 36 steps.

Original Xianglong tree

Figure 3. Original Xianglong tree (modified with color). Click to enlarge. Taking a look at this tree makes one think that kuehneosaurids were not given a “fair shake” because individual and outgroup taxa were not provided for them. Note the lack of resolution within the Iguania, which raises red flags.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

Li P-P, Gao K-Q, Hou L-H and Xu X. 2007. A gliding lizard from the Early Cretaceous of China. PNAS 104(13): 5507-5509. doi: 10.1073/pnas.0609552104 online pdf
Robinson PL 1962.
 Gliding lizards from the Upper Keuper of Great Britain. Proceedings of the Geological Society London 1601:137–146.
Stein K, Palmer C, Gill PG and Benton MJ 2008. The aerodynamics of the British Late Triassic Kuehneosauridae. Palaeontology, 51(4): 967-981. DOI: 10.1111/j.1475-4983.2008.00783.x


The “Headless” Langobardisaurus and the DGS Method

Langobardisaurus was just discussed less than a week ago with regard to Pteromimus. Langobardisaurus was a sister to other long-necked tritosaurs, like Tanytrachelos and Tanystropheus, and the shorter necked Cosesaurus.

Back in the day (the mid 1990s), when I was still tracing 8x10s with a pen on acetate (instead of a digital mouse after scanning) Dr. Silvio Renesto was kind enough to send  a photo of his recently discovered Langobardisaurus pandolfi (Renesto 1994). Apparently it was missing a neck and skull (Fig. 1) but this taxon was of particular interest due to its elongated, pterosaur-like, tanystropheus-like pedal digit 5.

Langobardisaurus pandolfi

Figure 1. Langobardisaurus pandolfi. The apparently "headless" langobardisaur. The neck and skull are in black, discovered by tracing the elements without seeing the specimen. To the right is a restoration of the skull.

The Thrill of Discovery
As I continued tracing the specimen, I realized there were some extra parts present behind the dorsal ribs. These turned out to be the apparently “missing” neck and skull. Unfortunately much of the skull was hidden beneath the vertebrae, so the details were beyond recovery, but the general outline and anterior jaws were clear. The orbit was much larger than originally anticipated and the rostrum much smaller with more derived teeth.

That was One of my First Contributions to Paleontology
That discovery was later supported by the discovery of Langobardisaurus tonneloi (Muscio 1997), which clearly exposed a very similar skull and neck. I have been employing the DGS (digital graphic segregation) method ever since much to the chagrin of my colleagues. I have been ignored and vilified ever since in print and in private for announcing discoveries based on interpreting photograph evidence. Well, this promising start is how it all began. It’s been an uphill struggle ever since.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

Muscio G 1997. Preliminary note on a specimen of Prolacertiformes (Reptilia) from the Norian (Late Triassic) of Preone (Udine, north-eastern Italy). Gortania – Atti del Museo Friulano di Storia Naturale 18:33-40
Renesto S 1994. A new prolacertiform reptile from the Late Triassic of Northern Italy. Rivista di Paleontologia e Stratigrafia 100(2): 285-306.
Renesto S and Dalla Vecchia FM 2000. The unusual dentition and feeding habits of the Prolacertiform reptile Langobardisaurus (Late Triassic, Northern Italy). Journal of Vertebrate Paleontology 20: 3. 622-627.


What is Apsisaurus?

Today’s blog will focus on Apsisaurus witteri (Laurin 1991, Fig. 1), a small Early Permian reptile of questionable affinity. The skull is incomplete. The post-crania is difficult to reassemble with available data. It needs to be seen from several views.


Figure 1. Apsisaurus witteri

Reisz, Laurin and Marjanovic (2010) in their report on Apsisaurus stated, “Paleozoic varanopid synapsids and diapsids, rare members of the terrestrial fossil assemblages, are not closely related to each other but appear to have acquired a number of interesting similarities that have resulted in their frequent misidentification.” These workers would benefit from a larger study. Here basal members of the varanopid synapsids and the protodiapsids ARE related to each other. The second evolved from the first.

Reisz, Laurin and Marjanovic (2010) also stated, Archaeovenator, based on a single small skeleton from the Upper Carboniferous of Kansas, was first identified as a diapsid reptile, but a restudy of the material clearly showed that it was a basal varanopid (Reisz and Dilkes, 2003). Perhaps the most striking examples are those of Mesenosaurus and Heleosaurus, two Middle Permian varanopid synapsids from Russia and South Africa that were previously misidentified as archosauromorph and eosuchian diapsids, respectively (Reisz and Berman, 2001; Reisz and Modesto, 2007).” Here Mesenosaurus and Heleosaurus are two basal protodiapsids outside of the clade of varanopid synapsids, but descended from them. Due to their limited gamut of taxa, Reisz, Laurin and Marjanovic (2010) were not aware of the protodiapsid grade arising from the varanopids.

Traditional Confusion Based on a Reduced Inclusion Set
A larger taxon list sheds light on earlier confusion. These new heretical nestings expose and illustrate the origin of the proto-diapsid taxon list, which was overlooked or ignored in traditional studies. Here Apsisaurus nested at the base of the non-varanopid synapsids, more primitive than Archaeothyris, the oldest known synapsid. So, Apsisaurus was not far from the base of the varanopids and not far from the protodiapsids. No wonder it was difficult to nest. More data in the future could move the lines of division.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

Laurin M 1991. The osteology of a Lower permian eosuchian from Texas and a review of diapsid phylogeny. Zoological Journal of the Linnean Society 101 (1): 59–95. doi:10.1111/j.1096-3642.1991.tb00886.x.
Reisz RR,  Laurin M and Marjanovic D 2010. Apsisaurus witteri from the Lower Permian of Texas: yet another small varanopid synapsid, not a diapsid. Journal of Vertebrate Paleontology 30 (5): 1628–1631. doi:10.1080/02724634.2010.501441.

The Enigmatic Teraterpeton

Updated November 16, 2014 with new illustrations and text reflected a relationship with Diandongosuchus, which was published after this blog post was originally presented.

Teraterpeton hrynewichorum (Sues 2003) Late Triassic, ~215 mya, Figs. 1, 2), was described as euryapsid (lacking a lateral temporal fenestra) and possibly related to the rhynchocephalian, Trilophosaurus, on that basis. Here Teraterpeton nests as a sister to Diandongosuchus (Fig. 2), not far from parasuchians, with a stretched out rostrum and far fewer, smaller teeth. The lateral temporal fenestra was largely blocked by the large quadrate. The antorbital fenestra was much larger here than in sister taxa due to the confluence of the naris.

Figure 2. Teraterpeton skull. Note the confluent naris/antorbital fenestra.

Figure 2. Teraterpeton skull. Note the confluent naris/antorbital fenestra.

Distinct from Diandongosuchus, the skull had a narrower configuration in dorsal view. The pedal(?) unguals were robust, but note the disparate sizes, perhaps used for digging.

Figure 1. Diandongosuchus (above) compared to Teraterpeton (below). Note the similar scapula shapes and the way the posterior dorsal ribs terminate in a line. Both lack the flaring cheeks of parasuchians and Youngina. Teraterpeton, with so few teeth, could well have been a plant eater or anything but a carnivore. Hopefully we'll find more of this genus someday.

Figure 1. Diandongosuchus (above) compared to Teraterpeton (below). Note the similar scapula shapes and the way the posterior dorsal ribs terminate in a line. Both lack the flaring cheeks of parasuchians and Youngina. Teraterpeton, with so few teeth, could well have been a plant eater or anything but a carnivore. Hopefully we’ll find more of this genus someday.

There is not much to go on with this specimen, but the few clues we do have indicate that Teraterpeton was a strange sort of quasi-parasuchid and with so few teeth, likely an herbivore.

Sues H-D 2003. 
An unusual new archosauromorph reptile from the Upper Triassic Wolfville Formation of Nova Scotia. Canadian. Journal of Earth Science 40(4): 635-649.

Cerritosaurus – A Key Overlooked Taxon in the Pararchosauriformes

Cerritosaurus binsfeldi,

Figure 1. Cerritosaurus binsfeldi, Late Triassic, known only from a skull. Such a taxon was basal to Chanaresuchus and the chanaresuchids. It also would have been morphologically close to the ancestor of the phytosaurs (parasuchians) and not far from Proterochampsa given its resemblance to the RC 91 specimen of Youngoides.

Where are the Phytosaur and Chanaresuchid Ancestors?
There has been relatively little interest in finding ancestral taxa to the phytosaurs and chanaresuchids. Prior efforts have recovered questionable candidates. Nesbitt’s (2011) tome on archosaurs recovered Euparkeria nesting at the base of the Phytosauria.  He also recovered Vancleavea nesting at the base of the Proterochampsia (= Tropidosuchus + Chanaresuchus). Erythrosuchus nested basal to all the above taxa.

These Nestings Raise Red Flags
Phytosaurs and chanaresuchids were flat-headed archosauriformes with skulls wider than tall and nares located dorsally on the skull. The orbits were located high on the skull. The rostrum was narrow in dorsal view and the “cheeks” flared widely. The antorbital fenestra was small. By contrast the skulls of VancleaveaEuparkeria and Erythrosuchus were taller than wide, with narrow cheeks, lateral nares and the latter two had a large antorbital fenestra. Vancleavea did not have an antorbital, mandibular or upper temporal fenestra because indeed it was not related to archosaurs. Vancleavea was a thalattosaur as reported earlier. Nesbitt (2011) did not include other thalattosaurs in his analysis, so Vancleavea nested by default within the Archosauriformes. The large reptile study solves that shortcoming.

Cerritosaurus binsfeldi (Price 1946, Fig. 1) Late Triassic, ~210 mya, nests here between the Parasuchia and the base of the Chanaresuchidae within the Pararchosauriformes. Nesbitt (2011) briefly mentioned Cerritosaurus as a member of the Proterochampsia [a paraphyletic taxon]. With its short snout and generally primitive characters Cerritosaurus likely also resembled the common ancestor of the Choristodera, Parasuchia and Proterochampsa. It was also not far from the RC 91 specimen of Youngoides (Fig. 1).

Distinct from RC91Cerritosaurus had a skull with a downturned rostrum. The skull was box-like with distinct rims both anterior and posterior to the orbits. The nares opened dorsally. An antorbital fenestra appeared with a deep fossa. The dorsal squamosal flared posteriorly. The mandibular fenestra was enlarged. The retroarticular process ascended. The teeth were extremely long, which is an autapomorphy.

With its wide flat skull, dorsal nares and elevated orbits Cerritosaurus provides a nearly ideal transitional taxon linking the RC91 specimen of Youngoides to basal phytosaurs and chanaresuchids. It is certainly a superior candidate compared to the taller narrow skulls of Euparkeria and Erythrosuchus. Exclusion of Cerritosaurus by Nesbitt (2011) and others before him impaired those earlier studies.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

Bonaparte JF 1971Cerritosaurus binsfeldi Price, tipo de uma nova família de tecodontes (Pseudosuchia-Proterochampsia). Anais da Academia Brasileira de Ciências, 43(Supl.): 417-422.
Kischlat E-E and Schultz CL 1999. Phylogenetic analysis of Proterochampsia (Thecodontia: Archosauriformes): Ameghiniana, v. 36, p. 13R.
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.
Price LI 1946. Sôbre um novo pseudosuquio do Triássico superior do Rio Grande do Sul: Boletim da Divisão de Geologia e Paleontologia, DNPM, v. 120, p. 7-38.


Where is the Ulna on Drepanosaurus?


Figure 1. The unusual lepidosaur, Drepanosaurus with forelimb and elbow elements re-identfied. The carpals were poorly ossified.

Drepanosaurus unguicaudatus (Pinna 1980, 1986) Norian, Late Triassic ~210 mya was originally considered an unusual lizard. It had a fused astragalus/calcaneum and sprawling limbs.

Drepanosaurus was the first, and one of the most unusual, of all the drepanosaurs, those hook-tailed, bird-headed, arboreal, chamaeleon-like reptiles of the Triassic.

Renesto’s Reinvestigation of Pinna’s Misidentifications
In 1994 Dr. Silvio Renesto reexamined the skeleton of Drepanosaurus and clarified certain earlier errors (Fig. 2). Those plate-like bones at the elbows were originally identified as coracoids by Pinna — because they looked like coracoids. Renesto (1994) tentatively considered them ulnae.

The forelimbs of Drepanosaurus.

Figure 2. The forelimbs of Drepanosaurus. Left: According to Pinna (1986). Right: Re-identified by Renesto (1994).

Pinna (1986) considered the ulna-like bones the ulna + radius. Renesto (1994) considered them the ulnare + intermedium, essentially wrist bones replacing the ulna.

Pinna (1986) considered the medial forearm bone the scapula. Renesto (1994) identified it as the radius.

Pinna (1986) considered the tall narrow bone the interclavicle. Renesto (1994) identified it as the scapula. Pinna (1986) considered the bone between the humeri a clavicle). Renesto (1994) reidentified it as a coracoid.

Renesto (1994) correctly identified many of the strange bones of Drepanosaurus, but the result created a most unusual three-part (rather than two-part) forearm in which the tubular ulna became a plate-like disc at the elbow and the tiny disc-like ulnare became elongated and tube-like. Very unusual, but this identification was widely accepted.

The fore limb of Hypuronector

Figure 3. The fore limb of Hypuronector (from Colbert and Olsen 2001). Here the humerus is much more robust than the ulna and radius. Around the elbow there are a number of ossified elements and breaks, so the positive identification of the separate olecranon ossification, as found in sister taxa, is more difficult to ascertain.

The Evidence from Sister Taxa
Curious about the homologies of the large plate-like “elbow” bone, I looked at sister taxa recovered by the large reptile tree (Vallesaurus, Huehuecuetzpalli and Cosesaurus) to see what clues they might offer. Notably, all had an olecranon sesamoid, a distinct and separate elbow bone (Fig. 3) that typically would have been fused to the ulna, as in Sphenodon (Fig. 3).

Thus, if homologous, the bone identified as the “coracoid” by Pinna (1986) and the tentative “ulna” by Renesto (1994) was actually a greatly enlarged olecranon sesamoid that articulated with the humerus, radius and ulna. In turn, that makes the tube-like “ulnare + intermedium” tentatively identified by Renesto (1994) the ulna, located parallel to the radius as in all other tetrapods. The actual ulnare + intermedium is a small wrist bone, essentially the only bones that were ossified in the wrist.

megalancosaur elbows and wrists

Figure 3. Click to enlarge. Most sister taxa of Drepanosaurus had an olecranon sesamoid. Drepanosaurus simply had a larger one. See the Megalancosaurus olecranon below.

So what looks like the ulna is the ulna. What looks like the wrist bones are wrist bones. The big elbow bone is an elbow bone (the olecranon sesamoid). All that makes more sense, yet takes away none of the wonder from this incredible arboreal reptile.

The huge olecranon sesamoid anchored a huge muscle to drive digit 2. The ulna was “dished out” to make more room for this forearm/finger muscle complex.

The elbow of Megalancosaurus.

Figure 5. The elbow of Megalancosaurus. (UPDATED BELOW) The perfect alignment of the olecranon sesamoid with the ulna masked the separation of these two bones. Note the ulna no longer articulates with the humerus as in Drepanosaurus. Here the DGS (digital graphic segregation using Photoshop) method uncovered an overlooked trait. Personal communication from S. Renesto identifies this intriguing break as a taphonomic artifact. More on this later as the details emerge!

An Olecranon Sesamoid in Megalancosaurus
The olecranon bone was overlooked in Megalancosaurus, probably due to its perfect alignment with the ulna. Larger than in outgroup taxa, the olecranon bone separated the humerus from the ulna as in its sister taxon, Drepanosaurus.

Megalacosaurus elbow

Figure 6. The break and the broken pieces of the Megalancosaurus ulna are reidentified here. The sesamoid is prominent and crescent-shaped as in Drepanosaurus. Note that the broken part of the ulna would have stood straight up from the matrix if similar to that of Drepanosaurus, hence its destruction during crushing.

A New Interpretation of the Sesamoid in Megalancosaurus
Here the various broken pieces of the ulna are reidentified using DGS (digital graphic segregation). The results are more similar to the situation in Drepanosaurus.

A Lepidosaur?
Renesto (1994) considered the taxonomic assignment of Drepanosaurus “quite difficult,” and labeled it a Neodiapsid (all diapsids other than Araeoscelidae under the old paradigm). “Neodiapsida” is here considered a diphyletic taxon since lepidosaurs and archosaurs nest on separate reptile branches. Therefore this clade label has lost its utility.

An Atypical Tritosaur with a Fused Ankle
As Pinna (1980) surmised, Drepanosaurus indeed nested with the lepidosaurs, but it did not nest with either the Iguania or the Scleroglossa. Here Drepanosaurus nested within the Tritosauria, a third clade of squamates. And yes, the fusion of the astragalus and calcaneum came about by convergence with other members of the Lepidosauria.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

Colbert EH and Olsen PE 2001. A New and Unusual Aquatic Reptile from the Lockatong Formation of New Jersey (Late Triassic, Newark Supergroup) American Museum Novitates, 3334: 15pp.
Olsen PE 1979. A new aquatic eosuchian from the Newark Supergroup LateTriassic-Early Jurassic) of North Carolina and Virginia. Postilla 176: 1-14.
Pinna G 1980. Drepanosaurus unguicaudatus, nuovo genere e nuova specie di Lepidosauro del trias alpino. atti Soc. It. Sc.Nat. 121:181-192.
Pinna G 1986. On Drepanosaurus unguicaudatus, an upper Triassic lepidosaurian from the Italian Alps. Journal of Paleontology 50(5):1127-1132.
Renesto S 1994. The shoulder girdle and anterior limb of Drepanosaurus unguicaudatus(Reptilia, Neodiapsida) from the upper Triassic (Norian of Northern Italy. Zoological Journal of the Linnean Society 111(3):247-264


Mutual Sexual Selection in Pterosaurs and Dinosaurs. Yes!

In a recent paper by Hone, Naish and Cuthill (2011) the authors reviewed the available evidence for the functions of “ornithodiran” [a paraphyletic taxon] cranial crests. They concluded that mutual sexual selection presents a valid hypothesis for their presence and distribution.

Why Did They Feature Pterosaurs?
In their section on pterosaurs Hone, Naish and Cuthill (2011) noted that the majority of pterosaur taxa are known from single specimens (Unwin 2005) “and as a result it cannot generally be determined if crests were present in both sexes.”

Fair enough.

Then they went on to reference Bennett’s work (1992, 1994) promoting sexual dimorphism, but that has been falsified. And it doesn’t support their hypothesis.

They referenced the crestless Darwinopterus with egg (Lu et al. 2011), and reported that it was identical in size to conspecific crested individuals, but actually differences abound and the two are not conspecific. And it doesn’t support their hypothesis.

Hone, Naish and Cuthill (2011) reference adolescent development of a bony crest in thalassodromids (Martill and Naish 2006), but this example indicates that crests developed long before half adult size had been reached and therefore long before the individual had become interested in sex.

Hone, Naish and Cuthill (2011) referenced strong allometric growth of the crest in Pteranodon (Tomkins et al. 2010), suggesting a role that only becomes relevant after maturity, but that has been falsified as noted earlier. All tritosaurs, including Pteranodon, developed isometrically.

Hone, Naish and Cuthill (2011) reported that the coincident appearance of a structure with maturity is a hallmark of a role in sexual selection. True enough. But the authors failed to show that the appearance of crests in pterosaurs was ontogenetic, rather than phylogenetic. Moreover, they failed to show that both genders sported crests, which was their hypothesis of mutual sexual selection. To support that hypothesiss, I would have reported that every known Dsungaripterus sports the same crest, for instance. Then add in all the Tapejara, Tupuxuara and Thallasodromeus skulls. They could all be male, but the odds are stacked against that.

I Don’t Have any Problem with Mutual Sexual Selection in Pterosaurs
All the present evidence indicates that crests developed in certain pterosaur species only, without regard for age or gender. That indicates mutual sexual selection. So why, then, did Hone, Naish and Cuthill (2011) reference those several cases of sexual dimorphism? It doesn’t make sense given their headline and hypothesis.

Does one wonder how the crestless pterosaurs found each other for mating?
No. Every species had its own identifying marks, whether crest or vane or color or wattle.

Nits and Picks
Hone, Naish and Cuthill (2011, fig. 1) reported that no birds had crests. Actually the hornbill and cassowary have them (not counting roosters and cockatiels).

Nesting pterosaurs with crocodiles and dinosaurs is not valid. Pterosaurs are lizards.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

Bennett SC 1992. Sexual dimorphism of Pteranodon and other pterosaurs, with comments on cranial crests. Journal of Vertebrate Paleontology 12: 422–434.
Bennett SC 1994. Taxonomy and systematics of the Late Cretaceous pterosaur Pteranodon (Pterosauria, Pterodactyloidea). Occassional Papers of the Natural History Museum University of Kansas 169: 1–70.
Hone DWE Naish D and Cuthill IC 2011. Does mutual sexual selection explain the evolution of head crests in pterosaurs and dinosaurs? Lethaia, DOI: 10.1111/j.1502-3931.2011.00300.x
Lü J, Unwin DM, Deeming DC, Jin X, Liu Y and Ji Q 2011. An egg-adult association, gender, and reproduction in pterosaurs. Science, 331(6015): 321-324. doi:10.1126/science.1197323
Martill DM and Naish D 2006. Cranial crest development in the azhdarchoid pterosaur Tupuxuara, with a review of the genus and tapejarid monophyly. Palaeontology 49, 925-941.
Tomkins JL, Lebas NR, Witton MP, Martill DM and Humphries S 2010. Positive allometry and the prehistory of sexual selection. The American Naturalist 176, 141–148.
Unwin DM 2005. The Pterosaurs: From Deep Time. Pi Press, New York.