Tangasaurus, the great, great diapsid grandfather of birds and crocs
Tangasaurus mennelli (Haughton 1924) was a small aquatic(?) diapsid of the Late Permian. It is known from only two specimens collected in 1922 by F. P. Mennell in Tanzania. Previous analyses by Bickelmann et al. (2009) and Reisz et al. (2011) nested Tangasaurus with Acerosodontosaurus and Hovasaurus and also with Thadeosaurus and Claudiosaurus, all basal diapsids. The large reptile tree found a similar nesting.
Figure 1. Tangasaurus mennelli reconstructed. This long-necked, wide-torso, long-legged, big-footed reptile is known from two specimens. The skull is known only in the smaller specimen. An image of better resolution is requested for both specimens. The pectoral girdle may be set slightly more forward. Seems like this morphology follows that of Araeoscelis (Fig. 4) more than that of any aquatic relative. The gastralia indicate a wide torso.
Reptiles like Tangasaurus don’t get all the press they deserve. Tangasaurus turns out to be a key taxon at the base of all archosauriforms + protorosauria + younginiforms. A sister to Tangasaurus was the great, great grandfather of birds and crocs (plus a host of other very cool archosauriforms). Tangasaurus nests at the very base of the terrestrial diapsids, distinct from the marine diapsids, the Enaliosauria.
Figure 2. Thadeosaurus, another taxon at the base of the terrestrial diapsids with a poorly known skull. Here the pectoral girdle is fused. The sternae are ossified. There were fewer cervicals and more dorsals here than in Tangasaurus.
Thadeosaurus used to nest in this spot. Now it is slightly more derived than Tangasaurus.
Haughton (1924) described a long, powerful, flattened tail and presumed an aquatic existence. The specimens I’ve seen do not include very much of the tail and what is present does not seem particularly flattened or aquatic in design in the manner of Hovasaurus, for instance. To me, this seems like a powerful terrestrial reptile. The size of the transverse processes at the base of the tail are notable. So is there anterior curvature. These may reflect the size of the caudofemoralis muscles driving the large hind limbs.
Figure 3. Tangasaurus juvenile. Scale unknown. Here the skull is present. A foot is reconstructed with PILs. The ribs indicate a narrower deeper torso. Please provide a high resolution image if you have one. I’d like to reexamine the skull, which is slightly exploded here, apparently with a leg sticking through the jaws at the orbits.
The large reptile tree divided lizards and turtles (and kin) from mammals and archosaurs (and kin). That makes traditional diapsids diphyletic. Among the true diapsids (Petrolacosaurus and kin, derived from synapsid protodiapsids) the branches split into the Enaliosauria (marine diapsids) and the terrestrial diapsids led by Thadeosaurus.
Figure 4. Basal diapsids including Araeoscelis, Hovasaurus, Adelosaurus, Acerosodontosaurus, Tangasaurus and Thadeosaurus, all to scale.
Now, having added Tangasaurus to the large reptile tree we have a new terrestrial diapsid taxon more basal than Thadeosaurus, not far from Adelosaurus (Fig. 4) at the base of the Enaliosauria.
There are two specimens of Tangasaurus. The smaller juvenile includes skull parts.
Tangasaurus is traditionally included within the Tangasauridae. I propose a new clade, the Tangasauria for Tangasaurus, Passer, their most recent common ancestor and all of its descendants. That’s a big important monophyletic clade that needs a name.
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
Bickelmann C, Müller J and Reisz RR 2009. The enigmatic diapsid Acerosodontosaurus piveteaui (Reptilia: Neodiapsida) from the Upper Permian of Madagascar and the paraphyly of ‘‘younginiform’’ reptiles”. Canadian Journal of Earth Sciences 49: 651–661.
Haughton SH 1924. On Reptilian Remains from the Karroo Beds of East Africa. Quarterly Journal of the Geological Society 80 (317): 1–11.
Reisz RR, Modesto SP and Scott DM 2011. A new Early Permian reptile and its significance in early diapsid evolution. Proceedings of the Royal Society B 278 (1725): 3731–3737.
2012-11-14 21:01:19
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