Megapnosaurus

Megapnosaurus rhodesiensis is an extinct species of coelophysid theropod dinosaur that lived approximately 188 million years ago during the early part of the Jurassic Period in what is now Africa. The species was a small to medium-sized, lightly built, ground-dwelling, bipedal carnivore, that could grow up to long. It was originally given the genus name Syntarsus, but that name was later determined to be preoccupied by a beetle. The species was subsequently given a new genus name, Megapnosaurus, by Ivie, Ślipiński & Węgrzynowicz in 2001. Many subsequent studies have classified it as a species within the genus "Coelophysis."
Description.
"Megapnosaurus rhodesiensis" measured up to 3 meters (10 ft) long from nose to tail and weighed about 32 kilograms (70 lb). The bones of 30 "M. rhodesiensis" individuals were found together in a fossil bed in Zimbabwe, so paleontologists think it may have hunted in packs. The various fossils attributed to this species have been dated over a relatively large time span – the Hettangian, Sinemurian, and Pliensbachian stages of the Early Jurassic – meaning the fossils represent either a highly successful genus or a few closely related animals all currently assigned to "Coelophysis".
"M. rhodesiensis" was a lean, elongated species of theropod dinosaur with an S shaped neck, long hind limbs that resembled the legs of large birds such as the secretarybird, shorter forelimbs with four digits on each hand unlike most later theropods, and a long tail. While still lean, it sported a more robust frame than other members of Coelophysoidea. Its lithe and superifically bird-like body lead to "M. rhodesiensis" being one of the first dinosaurs to be portrayed with feathers, though there is no direct evidence that it actually had feathers.
Specimen UCMP V128659 was discovered in 1982 and referred to "Megapnosaurus kayentakatae" by Rowe (1989), as a subadult gracile individual and later, Tykoski (2005) agreed. Gay (2010) described the specimen as the new tetanurine taxon "Kayentavenator elysiae", but Mortimer (2010) pointed out that there was no published evidence that "Kayentavenator" is the same taxon as "M. kayentakatae".
Classification.
""Syntarsus" rhodesiensis" was first described by Raath (1969) and assigned to Podokesauridae. The taxon "Podokesauridae", was abandoned since its type specimen was destroyed in a fire and can no longer be compared to new finds. Over the years paleontologists assigned this genus to Ceratosauridae (Welles, 1984), Procompsognathidae (Parrish and Carpenter, 1986) and Ceratosauria (Gauthier, 1986). Most recently, it has been assigned to Coelophysidae by Tykoski and Rowe (2004), Ezcurra and Novas (2007) and Ezcurra (2007), which is the current scientific consensus.
According to Tykoski and Rowe (2004) "Coelophysis rhodesiensis" can be distinguished based on the following characteristics: it differs from "Coelophysis bauri" in the pit at the base of the nasal process of the premaxilla; it differs from "C.? kayentakatae" because the promaxillary fenestra is absent and the nasal crests are absent; the frontal bones on the skull are not separated by a midline anterior extension of the parietal bones; the anterior astragalar surface is flat; metacarpal I has a reduced distal medial condyle (noted by Ezcurra, 2006); the anterior margin of antorbital fossa is blunt and squared (noted by Carrano "et al"., 2012); the base of lacrimal vertical ramus width is less than 30% its height (noted by Carrano "et al"., 2012); the maxillary and dentary tooth rows end posteriorly at the anterior rim of the lacrimal bone (noted by Carrano "et al"., 2012)
Marsh and Rowe (2020) retain the generic name "Syntarsus" for both QG 1 and MNA V2623, and the respective specimens assigned to these taxa, as opposed to "Coelophysis" or "Megapnosaurus", due to systematic relationships within Coelophysoidea in flux. As such, congenericity or the need for "Megapnosaurus" would not be supported if "Coelophysis bauri", "Syntarsus rhodesiensis", and "Syntarsus kayentakatae" do not form respective clades, as evidenced by their phylogenetic analyses.
Ezcurra "et al". (2021) found "Megapnosaurus rhodesiensis" to have been quite distant from both "Coelophysis bauri" (now the only species in genus "Coelophysis") and "Syntarsus" "kayentakatae" (not currently classified in a valid genus). In this analysis, the closest relatives of "M. rhodesiensis" are "Camposaurus", "Segisaurus" and "Lucianovenator".
Paleoecology.
Provenance and occurrence.
The holotype of "M. rhodesiensis" (QG1) has been recovered in Upper Elliot Formation in South Africa, as well as the Chitake River bonebed quarry at the Forest Sandstone Formation in Rhodesia (now known as Zimbabwe). In South Africa, several individuals were collected in 1985 from mudstone deposited during the Hettangian stage of the Jurassic period, approximately 201 to 199 million years ago. In Zimbabwe, twenty-six individuals were collected in 1963, 1968 and 1972 from yellow sandstone deposited during the Hettangian stage of the Jurassic period, approximately 201 to 199 million years ago.
Fauna and habitat.
The Upper Elliot Formation is thought to have been an ancient floodplain. Fossils of the prosauropod dinosaur "Massospondylus" and "Ignavusaurus" have been recovered from the Upper Elliot Formation, which boasts the world's most diverse fauna of early Jurassic ornithischian dinosaurs, including "Abrictosaurus", "Fabrosaurus", "Heterodontosaurus", and "Lesothosaurus", among others. The Forest Sandstone Formation was the paleoenvironment of protosuchid crocodiles, sphenodonts, the dinosaur "Massospondylus" and indeterminate remains of a prosauropod. Paul (1988) argued that members of the species lived among desert dunes and oases and hunted juvenile and adult prosauropods.
Paleobiology.
Growth.
Age determination studies using growth ring counts suggest that the longevity of "M. rhodesiensis" was approximately seven years. Recent research has found that "M. rhodesiensis" had highly variable growth between individuals, with some specimens being larger in their immature phase than smaller adults were when completely mature; this indicates that the supposed presence of distinct morphs is simply the result of individual variation. This highly variable growth was likely ancestral to dinosaurs but later lost, and may have given such early dinosaurs an evolutionary advantage in surviving harsh environmental challenges.
Feeding and diet.
The supposed "weak joint" in the jaw, led to the early hypothesis that dinosaurs such as these were scavengers, as the front teeth and bone structure of the jaw were thought to be too weak to take down and hold struggling prey. "M. rhodesiensis" was one of the first dinosaurs to be portrayed with feathers, though there is no direct evidence that it actually had feathers. Paul (1988) suggested that members of the species may have hunted in packs, preying upon "prosauropods" (basal sauropodomorphs) and early lizards.
Comparisons between the scleral rings of "M. rhodesiensis" and modern birds and non-avian reptiles indicate that it may have been nocturnal.
Paleopathology.
In "M. rhodesiensis", healed fractures of the tibia and metatarsus have been observed, but are very rare. "[T]he supporting butresses of the second sacral rib" in one specimen of "Syntarsus rhodesiensis" showed signs of fluctuating asymmetry. Fluctuating asymmetry results from developmental disturbances and is more common in populations under stress and can therefore be informative about the quality of conditions a dinosaur lived under.
Ichnology.
Dinosaur footprints that were later attributed to "M. rhodesiensis" were discovered in Rhodesia in 1915. These tracks were discovered at the Nyamandhlovu Sandstones Formation, in eolian red sandstone that was deposited in the Late Triassic, approximately 235 to 201 million years ago.