| Megapnosaurus | |
|---|---|
 | |
| Life restoration | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Family: | †Coelophysidae |
| Genus: | †Megapnosaurus Ivieet al., 2001[1] |
| Type species | |
| †Megapnosaurus rhodesiensis | |
| Synonyms | |
| |
Megapnosaurus (meaning "big dead lizard", from Greek μέγα = "big", ἄπνοος = "not breathing", "dead", σαῦρος = "lizard"[1]) is anextinctgenus ofcoelophysidtheropoddinosaur that lived approximately 188million years ago during the early part of theJurassicPeriod in what is now Africa. The species was a small to medium-sized, lightly built, ground-dwelling,bipedalcarnivore, that could grow up to 2.2 m (7.2 ft) long and weigh up to 13 kg (29 lb).
It was originally given the genus nameSyntarsus,[2] but that name was later determined to be preoccupied by abeetle.[1] The species was subsequently given a new genus name,Megapnosaurus, byIvie, Ślipiński & Węgrzynowicz in 2001. Some studies have classified it as a species within the genusCoelophysis,[3] but this interpretation has been challenged by more subsequent studies and the genusMegapnosaurus is now considered valid.[4][5][6]
The first fossils ofMegapnosaurus were found in 1963 by a group of students from Northlea School on Southcote Farm inNyamandhlovu,Zimbabwe (thenRhodesia). Michael A. Raath, the describer, was shown the fossils by school staff in 1964 and over several weeks, was excavated from theForest Sandstone, the layers dating to theearly Jurassic.[2] The type specimen (QG 1) consisted of a well preserved postcranial skeleton, missing only the skull and cervical vertebrae.[7][2] In another sandstone block, a few fossils of another specimen intermixed with the bones of a prosauropod, likelyMassospondylus. Later in 1968, Raath and D. F. Lovemore discovered additional Jurassic rock layers northeast of the type locality of Southcote Farm.[7] These rock layers were then known as the Maura River Beds, but due to the strata bearing fossils ofMassospondylus, the beds were determined to be the same age as those of the Forest Sandstone.[7] This second locality produced many articulated partial skeletons ofMassospondylus, but only fragmentary postcranial remains ofMegapnosaurus.[7] Raath would name the animal in 1969, dubbing itSyntarsus rhodesiensis, after the fused tarsal bones in its foot.[2]
Still in search of complete skeletons, Raath continued searching in the Jurassic rocks of Zimbabwe until finding what would become the most productiveS. rhodesiensis-bearing locality near theChitake River in 1972.[7] 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.[7] The fossils were described in detail by Raath in his thesis in 1977, including skeletal and musculoskeletal reconstructions ofS. rhodesiensis. All specimens collected from Southcote, Maura River, andChitake River now reside at the Queen Victoria Museum.[7]
In 1989, a second species of "Syntarsus" was proposed asSyntarsus kayentakatae, a description byTimothy Rowe of a well preserved skull and partial remains of postcranial skeleton.[8] The fossils came from the early Jurassic strata of theKayenta Formation in Arizona, USA. The phylogenetic position of "Syntarsus" kayentakatae is debated, with a position inMegapnosaurus,[9][8]Coelophysis,[10] or a making a new genus being proposed.[5][11]
The next year Darlington Munyikwa and Raath described a partial snout of"S." rhodesiensis from theElliot Formation in South Africa,[12] but the material has been referred toDracovenator.[13] A "Syntarsus" specimen was discovered in the United Kingdom in the 1950s and consisted of several postcranial elements. The specimen have now been referred to a new genus and species,Pendraig milnerae in 2021.[4] A partial coelophysoid sacrum and several additional elements from the Early Jurassic of Mexico were described as a new species of "Syntarsus","Syntarsus" "mexicanum", in 2004.[14] The remains were not given proper description in their naming and are likely from an indeterminate coelophysoid.[15] Fragmentary coelophysid specimens (FMNH CUP 2089 and FMNH CUP 2090) from theLufeng Formation of southern China have been identified ascf.Megapnosaurus, though phylogenetic analyses cannot be conducted due to poor preservation.[16][17] A partial skeleton described from theMoenave Formation was listed with the nameSyntarsus "moenavensis" by Tykoski in 2005, attributing the name to the describers of the specimen,[18] though the name was not used in their study.
Megapnosaurus rhodesiensis measured up to 2.2 m (7.2 ft) long from nose to tail and weighed up to 13 kg (29 lb).[19] 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 thesecretarybird, 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 ofCoelophysoidea. Its lithe and superifically bird-like body lead toM. rhodesiensis being one of the first dinosaurs to be portrayed with feathers, though there is no direct evidence that it actually had feathers.[20]
The bones of at least 30M. rhodesiensis individuals were found together in a fossil bed inZimbabwe, 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 – theHettangian,Sinemurian, andPliensbachian stages of theEarly Jurassic – meaning the fossils represent either a highly successful genus or a few closely related animals all currently assigned toCoelophysis.[21]
Specimen UCMP V128659 was discovered in 1982 and referred toMegapnosaurus kayentakatae by Rowe (1989),[22] as a subadult gracile individual and later, Tykoski (1998)[23] agreed. Gay (2010) described the specimen as the new tetanurine taxonKayentavenator elysiae,[24] but Mortimer (2010) pointed out that there was no published evidence thatKayentavenator is the same taxon asM. kayentakatae.[25]
The cladogram below was recovered in a study by Ezcurraet al. (2021).[5]
| Coelophysoidea |
| |||||||||||||||||||||||||||||||||||||||||||||||||||
"Syntarsus" rhodesiensis was first described by Raath (1969) and assigned toPodokesauridae.[2] The taxon "Podokesauridae", was abandoned since itstype specimen was destroyed in a fire and can no longer be compared to new finds. Over the years paleontologists assigned this genus toCeratosauridae (Welles, 1984), Procompsognathidae (Parrish and Carpenter, 1986) andCeratosauria (Gauthier, 1986). Most recently, it has been assigned toCoelophysidae by Tykoski and Rowe (2004), Ezcurra and Novas (2007) and Ezcurra (2007), which is the current scientific consensus.[21][5][26]
According to Tykoski and Rowe (2004)Coelophysis rhodesiensis can be distinguished based on the following characteristics:[21] it differs fromCoelophysis bauri in the pit at the base of the nasal process of thepremaxilla; it differs fromC.? kayentakatae because the promaxillaryfenestra is absent and the nasal crests are absent; thefrontal bones on the skull are not separated by a midline anterior extension of theparietal bones; the anteriorastragalar surface is flat;metacarpal I has a reduced distal medial condyle (noted by Ezcurra, 2006); the anterior margin ofantorbital fossa is blunt and squared (noted by Carranoet al., 2012); the base oflacrimal vertical ramus width is less than 30% its height (noted by Carranoet al., 2012); the maxillary and dentary tooth rows end posteriorly at the anterior rim of thelacrimal bone (noted by Carranoet al., 2012)
Marsh and Rowe (2020) retain the generic nameSyntarsus for both QG 1 and MNA V2623, and the respective specimens assigned to these taxa, as opposed toCoelophysis orMegapnosaurus, due to systematic relationships within Coelophysoidea in flux. As such, congenericity or the need forMegapnosaurus would not be supported ifCoelophysis bauri,Syntarsus rhodesiensis, andSyntarsus kayentakatae do not form respective clades, as evidenced by their phylogenetic analyses.[27]
Ezcurraet al. (2021) foundMegapnosaurus rhodesiensis to have been quite distant from bothCoelophysis bauri (currently the only undisputed species in genusCoelophysis) and "Syntarsus"kayentakatae (not currently classified in a valid genus). In this analysis, the closest relatives ofM. rhodesiensis areCamposaurus,Segisaurus andLucianovenator.[5] Similar results were found in analyses years before, supporting this position.[26][28]
The holotype ofM. rhodesiensis (QG1) has been recovered inUpper Elliot Formation inSouth Africa, as well as the Chitake River bonebed quarry at theForest Sandstone Formation in Rhodesia (now known as Zimbabwe). In South Africa, several individuals were collected in 1985 from mudstone deposited during theHettangian stage of theJurassic period, approximately 201 to 199 million years ago.[29] 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.[2][30][7]
TheUpper Elliot Formation is thought to have been an ancient floodplain. Fossils of the prosauropod dinosaurMassospondylus andIgnavusaurus have been recovered from the Upper Elliot Formation, which boasts the world's most diverse fauna of early Jurassic ornithischian dinosaurs, includingAbrictosaurus,Fabrosaurus,Heterodontosaurus, andLesothosaurus, among others. TheForest Sandstone Formation was the paleoenvironment of protosuchid crocodiles, sphenodonts, the dinosaurMassospondylus and indeterminate remains of aprosauropod. Paul (1988) argued that members of the species lived among desert dunes and oases and hunted juvenile and adult prosauropods.[31]
Age determination studies using growth ring counts suggest that the longevity ofM. rhodesiensis was approximately seven years.[32] Recent research has found thatM. 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.[33]
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" (basalsauropodomorphs) and early lizards.[31]
Comparisons between thescleral rings ofM. rhodesiensis and modern birds and non-avian reptiles indicate that it may have beennocturnal.[34]
InM. rhodesiensis, healed fractures of thetibia andmetatarsus have been observed, but are very rare. "[T]he supporting butresses of the second sacral rib" in one specimen ofSyntarsus rhodesiensis showed signs of fluctuating asymmetry. Fluctuatingasymmetry 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.[35]
Dinosaur footprints that were later attributed toM. rhodesiensis were discovered in Rhodesia in 1915. These tracks were discovered at the Nyamandhlovu Sandstones Formation, in eolian red sandstone that was deposited in theLate Triassic, approximately 235 to 201 million years ago.[36]
{{cite book}}: CS1 maint: location missing publisher (link).jpg)
.jpg)
.png)
.jpg)
