Megapnosaurus

Credit: Wikipteryx on the Wikimedia Commons.

*Megapnosaurus* is an extinct genus of coelophysid theropod dinosaur that lived in North America and Africa during the Early Jurassic.

## History

*Megapnosaurus* was first named as a species *Syntarsus* by Raath (1969) which was later discredited as the name is preoccupied by a genus of extant beetle. Although it is considered by entomologists to be synonymous with *Cerchanotus* and the original naming complies with the ICZN (which remains uncontested), the name must be replaced. Ivie et al. (2001) did this and coined *Megapnosaurus* ("big dead lizard" (intended); a literally "big lizard without breath"). They comply with the ICZN, though later workers considered this unethical because they could not contact Raath and was published in an entomology journal. Typically, the original namer would be contacted to correct their work, and though the team tried, they never received a reply and proceeded after Jack Horner had incorrectly notified them that Raath had died. This received backlash, being described as "facetious" by Bristowe and Raath due to having a humourous etymology. In the original publication, they stated that "to the scale of an entomologist, [it] look[ed] like a big dead lizard", and that this manner is commonplace for entomologists and ethical. Regardless, it is thus considered valid by McDavid and Bugos (2022).

*Coelophysis* was considered by some (such as Paul (1993)) to be a senior synonym of *Megapnosaurus* and a nomen dubium . A diagnostic neotype was then select to validate the former. Phylogenetic analysis suggests distinction, which is supported by McDavid and Bugos (2022), who state that that notion is supported but requires further clarification. Rauhut and Hungerbühler (1998) assign NHMUK PV R 37561 to *S.* sp., which was moved into *Pendraig* in 2021 and supported by McDavid and Bugos. A snout tip BP/1/5278 was described by Munyikwa and Raath (1999) and assigned *"S." rhodesiensis*, but Yates (2005) reassigned it to *Dracovenator*. McDavid and Bugos state that this is tentative, but support it nonetheless.

## Description

30 *Megapnosaurus* were found in a fossil bed from Zimbabwe, suggesting it may have hunted in packs. Many fossils referred to *Megapnosaurus* range a broad geologic timespan, from the Hettangian, Sinemurian and Pliensbachian of the Early Jurassic. This either means *Megapnosaurus* was a very successful genus or several closely-related fossils assigned *Coelophysis*. *Megapnosaurus* was a lean and elongate species, having an S-shaped neck and long, stilt-like forelimbs and a bird-like body. All of these lead *Megapnosaurus* to being one of the first non-avian dinosaur given feathers in paleoart. However, there is no direct evidence of feathery integument in *Megapnosaurus*, Each hand has 4 digits. *"Syntarsus" rhodesiensis*, assigned Podokesauridae (later discarded since its holotype was destroyed in a fire) and named by Raath (1969), has been assigned to many groups (Podokesauridae, Ceratosauridae, Procompsognathidae, Ceratosauria and Coelophysidae) and recently the genus *Megapnosaurus*. It is distinguished from a pit at the nasal processes' base, absent nasal crests and premaxillary fenestra, frontals unfused from the midline anterior extension of the parietal, a flat anterior astragalar surface, a reduced distal medial condyle of the metacarpal I, the blunt and squared anterior margin of the antorbital fossa, the lacrimal vertical ramus' width is ~30% the height in the base and maxillary and dentary tooth rows that posteriorly end at the anterior rim of the lacrimal.

## Classification

Marsh and Rowe (2020) retained *Syntarsus* for QC 1 and MNA V2623 instead of using *Coelophysis* or *Megapnosaurus* since systematics in Coelophysoidea are in constant flux. Congenericity for *Megapnosaurus* would not be supported if *Coelophysis bauri*, *"S." rhodesiensis* and *"S." kayentakatae* do not form clades, further suggested by phylogenetic analyses. Ezcurra et al. (2021) found *M. rhodesiensis* to have been distant from *C. bauri* and *"S." kayentakatae*, who found *Camposaurus*, *Segisaurus* and *Lucianovenator* in their analysis. Ezcurra et al. (2020) conclude:

## Paleobiology

Comparison of the sclerotic rings to modern birds and reptiles indicate *Megapnosaurus* may have been nocturnal. Determining age using growth rings find *Megapnosaurus* lived ~7 years. Recent studies place *M. rhodesiensis* as having variable growth, with some immature specimens being larger and other adults being smaller, which indicates morphs are actually individual variation. This is likely something that was ancestral to Dinosauria, but lost due to early dinosaurs having the advantage of surviving harsh environmental challenges. The assumed "weak joint" in the jaw led to an early hypothesis that *Megapnosaurus* were scavengers, as the front teeth and mandibular bone structure led scientists to believe it was too weak to take prey. Paul (1988) suggested they hunted in packs and preyed upon basal sauropodomorphs and early lizards. Healed fractures in *M. rhodesiensis* tibia and metatarsus have been reported, but are very rare. "[T]he supporting butresses of the second sacral rib" in a specimen of *"S." rhodesiensis* shows fluctuated asymmetry, which results from developmental disturbances, being more common in stressed populations; this may gain insight into what the quality of life for *Megapnosaurus* was like.

## Paleoecology

Holotype QG 1 was found from the Upper Elliot Formation, South Africa and from the Chitake River bonebed quarry, Forest Sandstone Formation, Zimbabwe (prev. Rhodesia) Several individuals from South Africa were collected from Hettangian-aged mudstone in 1985, 201-199 million years old. In Zimbabwe, 26 individuals were collected through 1963, '68 and '72 from Hettangian-aged yellow sandstone, 201-199 million years old. The Upper Elliot is thought to be an ancient floodplain, with *Massospondylus* and *Ignavusaurus* recovered. *Abrictosaurus*, *Lesothosaurus* (including *Fabrosaurus*) and *Heterodontosaurus* also recovered. The Forest Sandstone brought protosuchid crocodiles, sphenodonts, Massospondylus and indeterminate sauropodomorphs. Pail (1988) argued *Megapnosaurus* prowled desert dunes and oases to hunt mature and immature sauropodomorphs. Footprints assigned to *M. rhodesiensis* were found in Zimbabwe in 1915 from the Nyamandhlovu Sandstones Formation, in eolian red sandstone, deposited ~235-201 million years ago.

## References

* Ezcurra, M.D. (2017). A new early coelophysoid neotheropod from the Late Triassic of northwestern Argentina. *Ameghiniana, 54*(5), 506-538.
* Ivie, M.A., Slipinski, S.A., & Wegrzynowicz, P. (2001). Generic homonyms in the Colydiinae (Coleoptera: Zopheridae). Insecta Mundi, 15(1), 63-64.
* Raath, M.A. (1969). A new coelurosaurian dinosaur from the Forest Sandstone of Rhodesia. *Arnoldia, 4*(28), 1-25.
* Paul, G.S. (1988). Predatory Dinosaurs of the World. *Simon and Schuster*.
* https://incertaesedisblog.wordpress.com/2019/12/29/in-defense-of-megapnosaurus/
* https://en.wikipedia.org/wiki/Megapnosaurus
* McDavid SN & Bugos JE (2022) Taxonomic notes on *Megapnosaurus* and '*Syntarsus*' (Theropoda: Coelophysidae) *The Mosasaur* 12: 1-5