Megapnosaurus

The first fossils of Megapnosaurus were found in 1963 by a group of students from Northlea School on Southcote Farm in Nyamandhlovu, Zimbabwe (then Rhodesia). Michael A. Raath, the describer, was shown the fossils by school staff in 1964 and over several weeks, was excavated from the Forest Sandstone, the layers dating to the early Jurassic.[2] The type specimen (QG 1) consisted of a well preserved postcranial skeleton, missing only the skull and cervical vertebrae.[10][2] In another sandstone block, a few fossils of another specimen intermixed with the bones of a prosauropod, likely Massospondylus. Later in 1968, Raath and D. F. Lovemore discovered additional Jurassic rock layers northeast of the type locality of Southcote Farm.[10] These rock layers were then known as the Maura River Beds, but due to the strata bearing fossils of Massospondylus, the beds were determined to be the same age as those of the Forest Sandstone.[10] This second locality produced many articulated partial skeletons of Massospondylus, but only fragmentary postcranial remains of Megapnosaurus.[10] Raath would name Megapnosaurus in 1969, dubbing it Syntarsus rhodesiensis, after the fused tarsal bones in its foot.[2]

Still in search of complete Megapnosaurus skeletons, Raath continued searching in the Jurassic rocks of Zimbabwe until finding what would become the most productive Megapnosaurus-bearing locality near the Chitake River in 1972.[10] The quarry contained hundreds of bones of at least 26 individuals from many growth stages, making it one of the most productive quarries for African Theropods. The quarry contained several skulls and cervical vertebrae, elements missing in previously collected specimens, and some specimens even preserved gastralia, sexual dimorphism, and gut contents.[10] The fossils were described in detail by Raath in his thesis in 1977, including skeletal and musculoskeletal reconstructions of Megapnosaurus. All specimens collected from Southcote, Maura River, and Chitake River now reside at the Queen Victoria Museum.[10]

Life restoration

Megapnosaurus rhodesiensis measured up to 2.2 m (7.2 ft) long from nose to tail and weighed up to 13 kg (29 lb).[20] It 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.[21]

The bones of at least 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.[22]

Specimen UCMP V128659 was discovered in 1982 and referred to Megapnosaurus kayentakatae by Rowe (1989),[23] as a subadult gracile individual and later, Tykoski (1998)[24] agreed. Gay (2010) described the specimen as the new tetanurine taxon Kayentavenator elysiae,[25] but Mortimer (2010) pointed out that there was no published evidence that Kayentavenator is the same taxon as M. kayentakatae.[26]

The cladogram below was recovered in a study by Ezcurra et al. (2021).[5]

"Syntarsus" rhodesiensis was first described by Raath (1969) and assigned to Podokesauridae.[2] 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.[22][5][6]

According to Tykoski and Rowe (2004) Coelophysis rhodesiensis can be distinguished based on the following characteristics:[22] 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.[27]

Ezcurra et al. (2021) found Megapnosaurus rhodesiensis to have been quite distant from both Coelophysis bauri (currently the only undisputed 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.[5] Similar results were found in analyses years before, supporting this position.[6][7]

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.[36]