Acerosodontosaurus – a Key Taxon at the Base of the Diapsida

The “Big Belly” Basal Diapsid
Acerosodontosaurus (Currie 1980, Bickelmann, Müller and Reisz 2009) Late Permian, was the fat kid on the block (=clade). Even so, this largely ignored and otherwise unspectacular taxon is key to our understanding of all higher diapsids, both marine and terrestrial. Hard to believe a sister to Acerosodontosaurus was ancestral to both hummingbirds and elasmosaurs, for instance. That’s the beauty of reptile evolution.


Figure 1. Acerosodontosaurus in lateral view with a closeup of the hand and a cross section of the rib cage. This was the most robust member of the basal Diapsida. It had the largest dorsal vertebrae and deepest rib cage among its sisters. Such characters typically indicate an herbivorous diet, distinct from all sister taxa.

Acerosodontosaurus piveteaui was originally considered a younginiform. A later analysis by Bickelmann et al. (2009) nested Acerosodontosaurus with Hovasaurus and the same sisters recovered in the large reptile family tree seen here (Fig. 2). Always good to report when phylogenetic testing supports the literature.

The Questionable Quadratojugal/Rib Fragment
Bickelmann, Müller and Reisz (2009) determined that a bone considered to be quadratojugal (Fig. 1 in red) was instead a portion of a rib. Because of that new identification the authors considered Acerosodontosaurus a phylogenetic enigma in which the phylogenetic position was poorly understood, so they retested it. Bickelmann, Müller and Reisz (2009) then nested Acerosodontosaurus with Hovasaurus.

Ironically that questionable “rib fragment” fit perfectly into a reconstruction as a quadratojugal – BUT – the quadratojugal was the first bone to disappear in sister taxa – AND – the quadratojugal was considered missing in both Hovasaurus and Claudiosaurus, (which is disputed here). In any case and either way, one error in coding doesn’t change things much in a study of this size. Phylogenetically it just doesn’t matter. It just shifts the disappearance of the quadratojugal up or down the line.

Supporting the hypothesis that the quadratojugal was still present in Acerosodontosaurus and Claudiosaurus is the fact that successor taxa, including Stereosternum, Mesosaurus and ichthyosauriformes like Hupesuchus and Utatsusaurus all have a quadratojugal. Moreover, there are no other genuine rib fragments anywhere near the skull.

Basal diapsids.

Figure 2. Basal diapsids. Basal terrestrial forms in white. Younginiforms in green. Enaliosauria in blue. Mesosauria + Thalattosauria + Ichthyosauriformes in deeper blue.

Phylogenetic Nesting
Here Acerosodontosaurus nested at the base of two major clades, the marine Enaliosauria (= (Mesosauria + (Ichthyosauriformes + Thalattosauriformes)) + Sauropterygia) and the terrestrial Younginiformes, here represented by Thadeosaurus. So it’s a key taxon that should be used as an outgroup in focused studies of both clades.

In Acerosodontosaurus the large size of the dorsal vertebrae, coupled with the deep and wide dorsal ribs, coupled with the short robust limbs and lack of a canine tooth all point toward an herbivorous diet. No succeeding taxa had a canine tooth, but no succeeding taxon had the hallmark “big belly” of this possible herbivore. Alternatively, Acerosodontosaurus may have fed on small invertebrates and other slow-moving prey.

Acerosodontosaurus was considered aquatic in niche, like its sister taxa, the deep-tailed Hovasaurus and the less deep-tailed Thadeosaurus. The descendants of the former became increasingly marine. Descendants of the latter became increasingly terrestrial.

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.

Currie PJ 1980. A new younginid (Reptilia: Eosuchia) from the Upper Permian of Madagascar. Canadian Journal of Earth Sciences 17(4):500-51.
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 46:651-661.


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