The Prepubis of Pterosaurs (and Fenestrasaurs)

Formerly the Prepubis was Found Only in Pterosaurs
Only a few years ago it seemed that pterosaurs had three bones not found in other tetrapods: the prepubis, the pteroid and the preaxial carpal. Now these bones have been found in three fenestrasaur sister taxa, with Peters (2009) reporting on the latter two. Unfortunately, manuscripts reporting the appearance of the prepubis in Longisquama, Sharovipteryx and Cosesaurus were blackballed during the review process on claims that I used photographs. Well, they’re still the best way to share data (Figure 1) and review old errors (see below).

Cosesaurus prepubis

Figure 1. Cosesaurus prepubis in situ and reconstructed.

In tiny Cosesaurus (Figure 1), the first known prepubis measured only 3 mm in length. In Peters (2000) I didn’t recognize the prepubes when I first saw them, opting instead to describe the ilium anterior process with its own anterior process, following Ellenberger (1978, 1993), who did the same. That “process” was really the stem of the prepubis with the fan portion coincident with the broad, flat anterior process of the ilium. The other matching prepubis was largely beneath the right femur.

The pelvis and prepubes of Sharovipteryx.

Figure 2. The pelvis and prepubes of Sharovipteryx.

I presented the prepubes of Sharovipteryx during a podium session at the meeting of the Society of Vertebrate Paleontology in 2003. In Sharovipteryx both prepubes are readily visible. Unfortunately no one bothered to look for them, or map them, before. Click here to see a rollover image of the fossil and its interpretation.

In Longisquama I only found one prepubis and it was right at the edge of the rock break. The other one may have been a wee bit beyond. Click here to see the in situ fossil and its interpretation.

By convergence, basal mammals, like the opossum and platypus, have prepubes (known as marsupial or epipubic bones), which are used to support the marsupium, or pouch. There is no reason to believe that pterosaurs had marsupial pouches since they were lizards that layed full term eggs. Certain ornithischian dinosaurs had a prepubic process and crocodilians have a hinged pubis, otherwise similar in appearance to the prepubis of pterosaurs.

Why Develop a Prepubis?
In many respects the prepubis of fenestrasaurs resembled a miniaturized version of the booted pubis in Postosuchus, Herrerasaurus and Tyrannosaurus and likely served the same purpose, as an aid in locomotion. In Cosesaurus the prepubis was a novel ossification, extending ventrally from the pubis and no doubt anchored muscles of adduction, pulling the femora in toward the midline. In Pteranodon the prepubis fused to its counterpart and the posterior gastralia, but such fusion is not found in most other pterosaurs. Even so, the prepubis does appear to also enhance the function of the gastralia in support of the lower torso.

prepubes, prepubis

Figure 3. The pelvis (in tints of gray) and prepubis (in orange) of several tritosaurs, fenestrasaurs and pterosaurs. Arrow points to the anterior.

The Prepubis as Part of the Pelvis
The anterior and posterior expansion of the ilium and the infilling of the thyroid fenestra are gradually evolving traits in basal tritosaurs. The prepubis appeared when the anterior process of the ilium became hyper-elongated in Cosesaurus, and so was likely related to bipedalism. The feet of Cosesaurus have been matched to occasionally bipedal and narrow-gauge Triassic tracks within the ichnogenus, Rotodactylus (Peters 2000) and the anterior extension of the ilium is a hallmark of bipedalism in lizards (Snyder 1954) and dinosaurs.

Sister taxa more primitive than Cosesaurus had only two sacral vertebrae, an unfused ventral pelvis and short ilia. The fenestrasaurs, including Cosesaurus and basal pterosaurs, had four or more sacral vertebrae, a fused puboischiadic plate and elongated ilia, plus a prepubis — all of which served to increase and elongate the lateral surfaces of the pelvis.

The prepubes were more or less aligned with the femora when standing bipedally. In this configuration they served as anchors for femoral adduction (keeping the knees from sprawling too much). In addition, these pelvic surfaces likely anchored more extensive rotator femoral muscles because the femoral retractors were concurrently shrinking, judging by the reduction of the transverse processes and chevrons of the increasingly attenuated tail.

The Prepubis as Part of the Gastralia
Gastralia are not observable in Huehuecuetzpalli. They are barely visible to lightly present in Macrocnemus and Langobardisaurus. In Cosesaurus and pterosaurs robust gastralia span the gap between the sternal complex  and prepubis. As in other tetrapods, gastralia would have stiffened the belly adding ventral support in a bipedal configuration. Such support would have had been useful to counter the long moment arm that would have developed at the posterior dorsal vertebrae with the fulcrum at the acetabulum whenever the hind limbs supported all the weight.

Prepubis Orientation and Movement
Claessens et al. (2009) imagined the prepubis oriented in line with the gastralia, but that is false (Figure 3). They imagined it able to rotate at its base to facilitate respiration, but that is also false. They mistakenly compared pterosaurs to archosaurs (birds and crocs). They imagined a prepubis for Anhanguera, which is probably true, but none was preserved.

In their best prepubis example, a Rhamphorhynchus specimen (MB-R. 3633.1-2), Claessens et al. (2009) considered the prepubis to be articulated to the pubis with a moveable joint and with its major axis in line with the gastralia. The prepubis was correctly identified, but Claessens et al. (2009) failed to notice it had been rotated more than 90 degrees posteriorly during taphonomy such that the hollow cylindrical stem moved into the plane of the gastralia with its pubic articulation open anteriorly. The actual prong-like anterior process of the prepubis is visible ventral to the pubis. The ventral prong continues largely hidden beneath the pelvis. Properly rotated and configured like that of other fenestrasaurs, including other pterosaurs, the prepubis actually deepens the torso.

A Respiration Function for Prepubes?
Claessens et al. (2009) sought to demonstrate ventral expansion of the pterosaur abdomen to facilitate respiration via “caudoventral rotation of the prepubis.”  Unfortunately they misoriented the prepubis. Correctly configured, the proposed “caudoventral rotation at the pubis prepubis joint” could only stretch the gastralia away from the sternal complex (IF it was mobile), not ventrally expand the torso.

Perhaps more importantly, the actual pubis-prepubis joint is actually flat, or slightly, expanded, preventing caudoventral rotation. It’s a butt joint. To that point, in Claessens et al. (2009, fig. 3b) the prepubis of Pteranodon was correctly figured (extending ventrally from the pubis) and their own figure demonstrates torso expansion during respiration without prepubis rotation.

Variation in Prepubes
The shape and size of the prepubis is fenestrasaurs varied greatly. In certain Campylognathoides the prepubes are relatively large, fan-like and perforated. In Dorygnathus and basal ornithocheirids the prepubes were larger and longer than the shortened pubes. In a derived ornithocheird the prepubes were quite tiny. In Nyctosaurus and several other pterosaurs, like Rhamphorhyrnchus, the perforation expanded beyond the anterior margin of the prepubis, creating a “fork” ventrally with one prong contacting the posterior gastralium and the other prong articulating and sometimes fusing with its symmetrical counterpart.

In Summary
The prepubis has been largely ignored in pterosaur studies. It first appeared in basal fenestrasaurs. It was immobile, acting like an extension of the pubis.  It’s use was in locomotion and ventral support during bipedal excursions. The shape of the prepubis, while difficult to quantify, is distinct for every genus.

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.

References:
Claessens LPAM, O’Connor,PM, Unwin DM 2009. Respiratory Evolution Facilitated the Origin of Pterosaur Flight and Aerial Gigantism. PLoS ONE 4(2):e4497. online PLOS paper.
Ellenberger P 1978. L’Origine des Oiseaux. Historique et méthodes nouvelles. Les problémes des Archaeornithes. La venue au jour de Cosesaurus aviceps (Muschelkalk supérieur) in Aspects Modernes des Recherches sur l’Evolution. In Bons, J. (ed.) Compt Ren. Coll. Montpellier12-16 Sept. 1977. Vol. 1. Montpellier, Mém. Trav. Ecole Prat. Hautes Etudes, De l’Institut de Montpellier 4: 89-117.
Ellenberger P 1993. Cosesaurus aviceps . Vertébré aviforme du Trias Moyen de Catalogne. Étude descriptive et comparative. Mémoire Avec le concours de l’École Pratique des Hautes Etudes. Laboratorie de Paléontologie des Vertébrés. Univ. Sci. Tech. Languedoc, Montpellier (France). Pp. 1-664.
Peters D 2000a. Description and Interpretation of Interphalangeal Lines in Tetrapods.  Ichnos 7:11-41.
Peters D 2000b. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Peters D 2009. A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29: 1327-1330.
Snyder RC 1954. The anatomy and function of the pelvic girdle and hind limb in lizard locomotion. American Journal of Anatomy 95:1-46.

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