The Elongation of the Lower Pelvis

Recent posts on Diandongosuchus brought up the elongation of the lower pelvic elements, the pubis and ischium, and what those mean in terms of phylogeny and ability. Here we’ll look at where the elongation is and isn’t, how it began, where it began and if there appear to be any reversals according to the large reptile tree. Please check out the links where noted to see illustrations.

Basal reptiles on both branches (archosauromorph and lepidosauromorph) had a rather short pubis and ischium sutured or fused together creating a large pelvic bowl. Such a shape is typically correlated with a sprawling posture, as seen on modern lizards and turtles.

The new Lepidosauromorpha
Within the Lepidosauromorpha this shape and size was largely maintained with some narrowing of the pubis and ischium  following the development of the thyroid fenestra in basal lepidosaurs like Gephyrosaurus. Most lizards retained the thyroid fenestra, but higher tritosaurs (descendants of a sister to Huehuecuetzpalli, including drepanosaurs and fenestrasaurs) developed a solid lower pelvis (ignoring the prepubis for the moment). Pterosaurs turned that around again when they redeveloped the thyroid fenestra with some taxa going back and redeveloping a solid lower pelvis. Trilophosaurus also lost the thyroid fenestra.

In no Lepidisauromorpha do the pubis and ischium elongate past their plesiomorphic lengths.

The new Archosauromorpha
The bowl-shaped solid lower pelvis was retained by basal archosauromorphs. Some, like Procynosuchus up to Homo developed a thyroid fenestra that did not extend to the medial rim. Basal enaliosaurs, like Claudiosaurus separated the pubis from the ischium, but others re-solidified this connection. Basal archosauriforms had a solid lower pelvis, but many develop a thyroid fenestra separating the elements. In Lazarussuchus the lower elements are medial in orientation. In parasuchians + proterochampsids the pubis develops a lateral flange, but the elements remain short in all taxa, including Lagerpeton, a taxon often wrongly linked to the origin of the Dinosauria.

The new Archosauriformes
Proterosuchus, a basal archosauriform, has a primitive sort of pelvis with short lower elments and no thyroid fenestra, but the pubis also includes lateral reinforcements with a flange. The basal erythrosuchid, Garjainia, developed a deep thyroid fenestra separating the pubis from the ischium. In Euparkeria the narrow ischium elongated. The ornithosuchids, Ornithosuchus and Riojasuchus developed a longer pubis, rivaling the length of the femur.

 

Vjushkovia.

Figure 4. Vjushkovia. The last of the short pubis archosauriforms.  

Vjushkovia lagged behind with a short, but well-separated pubis and ischium. Apparently independent of the ornithosuchids, the rauisuchids arising from a sister to Vjushkovia developed an elongated pubis and ischium. Arizonasaurus was one of these and it had a short foot at the ventral tip of its pubis.  The lower pelvis of Revueltosaurus appears shorter than its sisters, but it retains its length relative to the shorter femur. The lower pelvis of aetosaurs like Stagonolepis, appear to have short lower elements, but here the elements remain elongated, just filled in.

Archosaurs
Vjushkovia
also gave rise to the proto-archosaur Decuriasuchus, with its very narrow lower pelvic elements. These shorten with Gracilisuchus and much more so with Scleromochlus, but become much more elongated in Turfanosuchus,  Terrestrisuchus, Sphenosuchus and Protosuchus.

Beginning with Herrerasaurus, all dinosaurs (the other archosaurs) had elongate lower pelvic elements. Some, like Archaeopteryx, Mononykus and Scelidosaurus rotated the pubis backward, closing in on and contacting the ischium. Most poposaurs had elongated lower pelvic elements, reflecting their bipedal habits, but quadrupedal Lotosaurus is an exception (in many ways!)

Insights
Pterosaur pelves do not at all resemble any sort of archosaur pelvis. The pelvis of Lagerpeton does not resemble those of any sort of archosaur. The Diandongosuchus pelvis does not resemble those of Qianosuchus and poposaurs.

The Gradual Accumulation of Character Traits
The large reptile tree demonstrates the gradual accumulation of character traits in the pelvis and elsewhere. That’s where the authority comes from when I correct the results of smaller studies that do not demonstrate gradual accumulations of character traits, but continue to create “strange bedfellows.”

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.

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