Athena Review Vol. 5, no. 1
Records of Life: Fossils as Original Sources
26. Archosaurian reptiles
Archosaurs
Archosauria ("ruling reptiles") were diapsids from the Late Permian through Mesozoic periods. Descendants of archosaurs, including dinosaurs, dominated the realm of the terrestrial vertebrates for a majority of the Mesozoic Era. Today, only the birds and c
rocodilians remain. Basic traits of archosaurs included single openings in each side of the skull in front of the eyes; a skull adapted and simplified for carnivore existence, with fusion of multiple bones and increased openings for muscle attachments; teeth set in sockets (called thecodont teeth); a modified ankle joint, leading to increases in bipedalism; and an increase in skeletal armor for herbivores hunted by archosauran carnivores.
Fig.1: Phylogeny of the archosaurian reptiles (after Romer 1966, fig. 207).
Romer (1966) adapted the order Thecodontia ("socket teeth") which Owen had originated, as an all-inclusive group of early archosaurs showing these traits. While the term is currently out of favor in cladistics,, Romer's graph (fig.1) of the groupings of archosaurs remains directly useful to grasp their relations. These include dinosaurs (Sauropoda, Theropoda, Ornithopoda, Ceratopsia, Ankylosauria, and Stegosauria), pterosaurs (winged reptiles or pterodactyloidea), birds, and crocodiles. The latter include Archaeosuchia and Proterosuchia, both from the Triassic; Mesosuchia, which died out in the early Cenezoic; and Eusuchia, including present-day crocodilians (all today grouped by by cladistics into the clade called "pseudosuchians").
Euparkeria ("Parker's good animal") was a small archosauriform reptile from the Middle Triassic of South Africa's Karoo Valley, dating from 245-230 mya. It is considered close to the ancestry of Archosauria and typical of the traits of archosaurs (Romer 1966),
Euparkeria (figs.2-4) had hind limbs that were slightly longer than its forelimbs, taken as evidence that it may have been able to rear up on its hind legs as a part-time biped. Euparkeria's more normal movement may have been analogous that of crocodilians. Euparkeria
had rows of osteoderms along its back that could have stabilized it,
and a long tail that could act as a counterbalance to the rest of the body, including its large head. Fig.2: Skeleton of Euparkeria (after Romer 1966, fig. 211).
Some specimens of Euparkeria (fig.3) preserve sclerotic rings or bony rings in the eye sockets simil
ar to those of modern birds and reptiles
that are nocturnal, suggesting that Euparkeria may have have also been
noctural. Placement of Euparkeria and other euparkeriids within Archosauriformes remains controversial. Benton & Clark (1988) consider Euparkeria as a member of Archosauriformes in a position outside the lineages of both crocodilians and birds. Benton (1999) and (Ezcurra) place the long-snouted proterochampsians as more closely related to archosaurs than euparkeriids.
Figs.3,4: Skulls of Euparkeria (above right: drawing after Romer 1966, fig. 211; below right: cast after Ezcurra 2016).
Proterosuchus ("early crocodile") was an Early Triassic archosaur. Two species are known in South Africa and another in China. The type species, Proterosuchus fergosi (fig.5), was identified by Robert Broom in 1903 from Tarkastad, in the Eastern Cape in South Africa. It is associated with the Lystrosaurus Assemblage Zone of the Beaufort Group in the South African Karoo, dating from 252-247 mya. The Chinese genus Chasmatosaurus (fig.6) is considered a junior (i.e., later disc
overed) synonym of Proterosuchus. Proterosuchus was one of the largest land reptiles during the Early Triassic period, reaching a length of 3.5 m. Its habits were mainly riverine, and its hunting and predatory practices were probably similar to those of crocodiles.
Fig.5: Skull of Proterosuchus fergosi (after Foth et al. 2016).
Its skull was large, with a downward curve in the premaxilla, and a formidable array of teeth used in hunting and tearing prey. The upper jaw or palatal structure of Proterosuchus is considered primitive (Romer 1966). The premaxilla had up to nine teeth on each side, with 59 more teeth on each side in the maxilla and dentary (
upper and lower jaws). As
in most archosauriforms, all the teeth of Proterosuchus were curved and
serrated, and of the same pointed shape.Fig.6: Skull of Chasmatasaurus, a Chinese variant of the Lower Triassic archosaur Proterosuchus (after Romer 1966, fig. 212)
Erythrosuchus ("red crocodile") is an archosauriform reptile from the Triassic of South Africa. The first remains of the type species E. africanus were found by Broom (1905) in the Cynognathus Assemblage Zone of the Beaufort Group in the Karoo of South Africa, dating from 250-230 mya. The first complete description of the genus was given by Friedrich von Huene in 1911.
Erythrosuchus (figs.7-8), measuring some 5 meters in length, was the largest predator of its time. Unlike the more sprawling gait of earlier reptiles, it walked on all fours with limbs positioned semi-vertically under its body. Some features of the ankle of Erythrosuchus suggest that it was beginning to adapt toward walking on toes rather than having the entire foot placed on the ground. The ankle, similar to that of Euparkeria, is considered more advanced in terms of movement and possible bipedalism than those of other archosauriformes (Cruikshank 1978). In the Late Triassic, the ecological niche left by Erythrosuchus was filled
by large reptiles including Saurosuchus and Postosuchus.A complete skull of Erythrosuchus was first described in 1963, showing this to be less tall than previously realized, with a somewhat pointed snout.
The large head was about 1 meter long, on a short neck. It had sharp, conical teeth (Foth et al. 2016).
Fig.7: Skeleton of Erythrosuchus (after Romer 1966, fig. 213).
Erythrosuchus shares various traits with the
hypothetical last common ancestor of archosaurs, including some
characters which relate to the region of the ear. The inner part
of the otic capsule which surrounds the inner ear is not entirely
ossified, nor is the duct for the lagena, the portion of the inner
ear responsible for hearing (known as the cochlea in mammals). Erythrosuchus has a short lagena, a trait expected in the last common ancestor of all archosaurs (Gower 1997)Fig.8: Skull of Erythrosuchus (after Foth et al. 2016).
Stagonolepis (" scale") (fig.9) was an aetosaur from the Late Triassic (Carnian stage) found in Europe in the Hassberge Formation of Germany, the Drawno Beds of Poland, and the Lossiemouth Sandstone of Scotland. Aetosaurs ("eagle lizards") were heavily armoured Late Triassic
herbivorous archosaurs,
now all placed within the family Stagonolepididae. Two related genera are Aetosauroides in southern Brazil, and Calyptosuchus in the Chinle Formation of Arizona and Utah and the Bluewater Creek Formation of New Mexico.Fig.9: Skeleton of Stagonolepis (after Fraser and Sues 2011).
The first examples of Stagonolepis were identified by Louis Agassiz (1844) from fossils in Scotland. He named the genus, but (mistakenly) attributed the scales to gar-like fish. Thomas Henry Huxley (1859, 1875), based on more detailed information from geologists Charles Lyell and Hugh Miller, first correctly identified the fossil scales of Stagnolepis and other Aetosaurians as belonging to a crocodilian-like reptile.
Stagonolepis robertsoni was a quadruped about 3 meters long, covered in thick armoured scales. A slow-moving browser, its heavy body armour was used for protections against comtemporary carnivores. Stagonolepis
had a relatively small skull about 25 cm long (fig.10), less than one
tenth of its body length. It lacked teeth in the front of its jaws,
which had a beak-like tip arching upwards, used to uproot plants. The
peg-like teeth at the back of its mouth were suitable for chewing riverine vegetion, including ferns, cycads, and horsetails.Fig.10: Skull of Stagonolepis (after Romer 1966, fig. 215).
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