Megapnosaurus

Megapnosaurus Temporal range: Early Jurassic, 199–188Ma PreЄ Є O S D C P T J K Pg N
Profile of the head of M. rhodesiensis
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Theropoda
Family:	†Coelophysidae
Subfamily:	†Coelophysinae
Genus:	†Megapnosaurus Ivie, Ślipiński & Węgrzynowicz, 2001
Species
†M. rhodesiensis (Raath, 1969) (type) †M. kayentakatae? (Rowe, 1989)
Synonyms
Syntarsus Raath, 1969 (preoccupied by Fairmaire, 1869)

Megapnosaurus (pron.: meg-APP-no-SORE-us) is an extinct genus of coelophysid theropod dinosaur that lived approximately 199 to 188 million years ago during the early part of the Jurassic Period in what is now Africa and North America. Megapnosaurus was a small to medium-sized, lightly built, ground-dwelling, bipedal carnivore, that could grow up to 3 m (9.8 ft) long. It was formerly called Syntarsus but that name was already taken by a beetle, and was subsequently given its current name by Ivie, Ślipiński & Węgrzynowicz, in 2001. It is notable for being one of the most specimen-rich dinosaur genera, with dozens of individuals recovered across two continents.

Etymology

The genus name Megapnosaurus comes from the Greek words μεγα/mega (meaning 'big') and απνοος/apnos (meaning 'not breathing' but implying 'dead'), thus "big dead lizard" which was named facetiously.

History of the naming controversy

Megapnosaurus was originally named Syntarsus by Raath in 1969.^[1] It was renamed by American entomologist Dr. Michael Ivie (Montana State University of Bozeman), Polish Australian Dr. Adam Ślipiński, and Polish Dr. Piotr Węgrzynowicz (Muzeum Ewolucji Instytutu Zoologii PAN of Warsaw), the three scientists who discovered that the genus name Syntarsus was already taken by a colydiine beetle described in 1869.^[2] Many paleontologists did not like the naming of Megapnosaurus, partially because taxonomists are generally expected to allow original authors of a name to correct any mistakes in their work. Raath was aware of the homonymy between the dinosaur Syntarsus and beetle Syntarsus, but the group who published Megapnosaurus were led to believe Raath was deceased and therefore unable to correct his mistake, and proceeded accordingly. Megapnosaurus remains the valid name for the fossil material, although many paleontologists still refer to this genus as Syntarsus. Mortimer (2012) points out that "Paleontologists might have reacted more positively if the replacement name (Megapnosaurus) hadn't been facetious, translating to "big dead lizard".^[3]

Description

Megapnosaurus measured up to 3 meters (10 ft) long from nose to tail and weighed about 32 kilograms (70 lb). The bones of 30 Megapnosaurus 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 genus 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 Megapnosaurus.

The African species (M. rhodesiensis) is known from almost 30 specimens. The North American species (M. kayentakatae) had small crests and may show an evolutionary step toward later and larger coelophysoids, such as the more derived Dilophosaurus. Both possess a "weak joint" between the premaxillary and the maxillary bones, creating a hooked premaxillary jaw.

Specimen UCMP V128659 was discovered in 1982 and referred to Megapnosaurus kayentakatae by Rowe (1989),^[4] as a subadult gracile individual and later, Tykoski (2005)^[5] agreed. Gay (2010) described the specimen as the new tetanurine taxon Kayentavenator elysiae,^[6] but Mortimer (2010) pointed out that there was no published evidence that Kayentavenator is the same taxon as M. kayentakatae.^[7]

Classification

Megapnosaurus was first described by Raath (1969) and assigned to Podokesauridae.^[1] 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, is has been assigned to Coelophysidae by Tykoski and Rowe (2004), Ezcurra and Novas (2007) and Ezcurra (2007), which is the current scientific consensus.^[9]

Megapnosaurus is almost identical to Coelophysis, and Yates (2005) suggested that Megapnosaurus was possibly synonymous with Coelophysis.^[10] In 2004, Raath co-authored two papers in which he argued that Syntarsus was a junior synonym of Coelophysis.^[11][12]

Tykoski and Rowe (2004) and Irmis (2004) have argued that Megapnosaurus is not the same as Coelophysis and is therefore a valid genus.^[9][13]

Distinguishing anatomical features

A diagnosis is a statement of the anatomical features of an organism (or group) that collectively distinguish it from all other organisms. Some, but not all, of the features in a diagnosis are also autapomorphies. An autapomorphy is a distinctive anatomical feature that is unique to a given organism or group.

According to Tykoski and Rowe (2004) Megapnosaurus kayentakatae can be distinguished based on the following characteristics:

• the presence of nasal crests on the skull
• the frontal bones on the skull are separated by a midline anterior extension of the parietal bones

According to Tykoski and Rowe (2004) Megapnosaurus 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 Megapnosaurus 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)

Paleoecology

Migration

Megapnosaurus is a good example of how dinosaurs spread across the globe from their ancestral habitats (which was possibly South America). This small predator had the same basic features found in early dinosaurs, and its appearance in both Africa and the southwestern U.S. indicates that it migrated through the continents, which at the time were joined together as Pangaea. There is species-level differentiation between the African and U.S. specimens, again supporting the migration and adaptation theories.

Provenance and occurrence

The holotype of Megapnosaurus 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 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.^[14] 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.^[15][16][17]

The holotype of Megapnosaurus kayentakatae (MNA V2623) was recovered in the Silty Facies Member of the Kayenta Formation, in Arizona. This material was collected in 1977 from carbonaceous sandstone deposited during the Sinemurian and Pliensbachian stages of the Jurassic period, approximately 196 to 183 million years ago.

Fauna and habitat

M. kayentakatae feeding on a Scutellosaurus

The Upper Elliot Formation is thought to have been an ancient floodplain. Fossils of the prosauropod dinosaur Massospondylus and Plateosaurus 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) noted that Megapnosaurus rhodesiensis lived among desert dunes and oases and hunted juvenile and adult prosauropods.^[18]

The Kayenta Formation was primarily deposited by rivers, with the silty facies as the slower, more sluggish part of the river system. A definitive radiometric dating of this formation has not yet been made, and the available stratigraphic correlation has been based on a combination of radiometric dates from vertebrate fossils, magnetostratigraphy and pollen evidence.^[19] It is been surmised that the Kayenta Formation was deposited during the Sinemurian and Pliensbachian stages of the Early Jurassic Period or approximately 199 to 182 million years ago.^[20] The Kayenta Formation is part of the Glen Canyon Group that includes formations not only in northern Arizona but also parts of southeastern Utah, western Colorado, and northwestern New Mexico. The formation was primarily deposited by rivers. During the Early Jurassic period, the land that is now the Kayenta Formation experienced rainy summers and dry winters. By the Middle Jurassic period it was being encroached upon from the north by a sandy dune field that would become the Navajo Sandstone.^[21] The animals here were adapted to a seasonal climate and abundant water could be found in streams, ponds and lakes. Megapnosaurus shared its paleoenvironment with other dinosaurs, such as several theropods including Dilophosaurus, Kayentavenator^[22] "Syntarsus" kayentakatae, the "Shake N Bake" theropod, the basal sauropodomorph Sarahsaurus,^[23]heterodontosaurids, and the armored dinosaurs Scelidosaurus and Scutellosaurus. The Kayenta Formation has produced that remains of three coelophysoid taxa of different body size, which represents the most diverse ceratosaur fauna yet known.^[24] The Kayenta Formation has yielded a small but growing assemblage of organisms.^[25] Vertebrates present in the Kayenta Formation at the time of Megapnosaurus included hybodont sharks, bony fish known as osteichthyes, lungfish, salamanders, the frog Prosalirus, the caecilian Eocaecilia, the turtle Kayentachelys, a sphenodontian reptile, various lizards. Also present were the synapsids Dinnebiton, Kayentatherium, and Oligokyphus.,^[26] several early crocodylomorphs including Calsoyasuchus, Eopneumatosuchus, Kayentasuchus, and Protosuchus, and the pterosaur Rhamphinion.^{[25][26][27][28]} The possible presence of the early true mammal Dinnetherium, and a haramyid mammal has also been proposed, based on fossil finds. Vertebrate trace fossils from this area included coprolites^[29] and the tracks of therapsids, lizard-like animals, and dinosaurs, which provided evidence that these animals were also present.^[30] Non-vertebrates in this ecosystem included microbial or "algal" limestone,^[29] freshwater bivalves, freshwater mussels and snails,^[21] and ostracods.^[31] The plant life known from this area included trees that became preserved as petrified wood.^[27]

Paleobiology

Age determination studies using growth ring counts suggest that the longevity of Megapnosaurus was approximately seven years.^[32] Comparisons between the scleral rings of Megapnosaurus and modern birds and reptiles indicate that it may have been nocturnal.^[33]

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. Megapnosaurus was one of the first dinosaurs to be portrayed with feathers, though there is no direct evidence that it actually had feathers. Paul (1988) noted that this genus, which may have hunted in packs, preyed upon prosauropods and early lizards.^[18]

Paleopathology

In Syntarsus 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.^[34]

Taphonomy

The holotype (MNA V2623) of Megapnosaurus kayentakatae likely came to rest on its left side and was partially buried. This served to stabile the bones on the left side of the skull, however the right side of the skull was likely exposed to water currents as it began to decompose. Some of the loosely connected bones on this side drifted out of place before the entire skull was finally buried. Later, the great weight of overlying sediment served to distort the specimen.^[35]

Ichnology

Dinosaur footprints that were later attributed to Megapnosaurus were discovered in Zimbabwe 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]

References