Erpetonyx, not a bolosaurid, but a milleropsid (protodiapsid)

Updated February 9, 2022
with new reconstruction and skull.

A new paper
by Modesto et al. (2014) describes a new Late Carboniferous reptile, Erpetonyx arsenaultorum (Fig. 1, ROM 55402, 303.7 to 298.9 Ma).

First in 20 years!
Erpetonyx arsenaultorum is the first Carboniferous reptile to be described in nearly two decades.

It made the news!
From the CBC website: “A fossil of a lizard-like creature found by a boy on a Prince Edward Island beach is a new species and the only reptile in the world ever found from its time, 300 million years ago, a new study shows.”

Complete but crushed,
with a disarticulated skull and extremities, this protodiapsid was considered a bolosaurid by Modesto et al, likely because it nests very close to Eudibamus, a basal diapsid they consider to be a bolosaurid. We looked at Eudibamus earlier here.

One mistake makes the next mistake easier to make.
Bolosaurids are turtle and pareiasaur sisters with crushing teeth. Note the long, narrow teeth of Erpetonyx (Fig. 1) more closely resemble those of Milleropsis (Fig. 2). This is an on-going issue.

Modesto et al. also support the paraphyletic clade Parareptilia and list Erpetonyx among them (Fig. 1). According to Modesto, et al., parareptilia (Fig. 1) is a clade nesting between Synapsida and Eureptilia (all the rest of the Amniota).

adding taxa, as in the large reptile tree, splits up most of the ‘parareptiles’. Mesosaurs nest with ichthyosaurs and thalattosaurs. The conventional millerosaurs, Milleroposis (Fig. 2) and Milleretta, are not related to each other. Procolophonids are derived from diadectids, not pareiasaurs. Australothyris is related to Milleretta and caseasaurs, not Lanthanosuchus. However, Lanthanosuchus, Microleter and Nyctiphruretus are related, not to far from bolosaurids, pareiasaurs and turtles.

Figure 1. Skull of Erpetonyx in situ and reconstructed.

Figure 1. Skull of Erpetonyx in situ and reconstructed.

The Late Carboniferous age
of Erpetonyx does precede that of its relatives, Broomia (Middle Permian) and Milleropsis (Early Permian), but a more derived taxon, Petrolacosaurus, is also Late Carboniferous in age.

Milleropsis, a largely forgotten taxon that displays possible bipedal traits at the base of the Diapsida.

Figure 1. Milleropsis, a largely forgotten taxon that displays possible bipedal traits at the base of the Diapsida. Note the long stiff tail without chevrons, as in Erpetonyx.

The carpus of Erpetonyx
is distinctive and quite similar to that of Broomia (Fig. 3). The original diagnosis of Erpetonyx makes note of the carpus. From the text: “A small, basal parareptile that possesses 29 pre sacral vertebrae (viz. five cervicals and 24 dorsals), relatively small carpal bones (the radiale and the pisiform are ca one-half the size of the ulnare and the fifth distal carpal, respectively), a femoral distal end with an epicondylar axis at 45º to the shaft, a fourth metatarsal with a relatively broad distal end, and well-developed unguals with prominent flexor tubercles.”

Figure 1. Broomia. A long-recognized sister to Milleropsis, an early possible biped. Check out those thighs!

Figure 3. Broomia. Another long-recognized sister to Milleropsis, an early possible biped close to Erpetonyx. Note the similarities in the carpus and tarsus.

The authors resurrect the name Bolosauria, which was erected as an ordinal name by Kuhn to contain the family Bolosauridae Cope, 1878, and define it as a branch-based group: Bolosaurus striatus Cope, 1878 and all species related more closely to it than to Procolophon trigoniceps Owen, 1876. Unfortunately the last common ancestor of these two taxa in the large reptile tree is Saurorictus and Erpetonyx does not nest in this clade. The authors did not provide reconstructions.

Figure 2. Erpetonyx in situ.

Figure 2. Erpetonyx in situ.

Modesto SP, Scott DM, MacDougall MJ, Sues H-D, Evans DC and Reisz RR 2014. The oldest parareptile and the early diversification of reptiles. Proc. R. Soc. B 282:

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