Rhynchosaur

Rhynchosaurs Temporal range: Olenekian–Norian PreЄ Є O S D C P T J K Pg N
life restoration of Hyperodapedon
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Archosauromorpha
Order:	†Rhynchosauria Osborn, 1903
Subgroups
See text.

Rhynchosaurs were a group of Triassic diapsid reptiles related to the archosaurs.

Description

File:Estauricossauro rincossauro.JPG

Staurikosaurus and rhynchosaur.

Rhynchosaurs were herbivores, and at times abundant (in some fossil localities accounting for 40 to 60% of specimens found), with stocky bodies and a powerful beak. Early primitive forms, like Mesosuchus and Howesia, were generally small and more typically lizard-like in build, and had skulls rather similar to the early diapsid Youngina, except for the beak and a few other features. Later and more advanced genera grew to medium to medium large size, up to two meters in length. The skull in these forms were short, broad, and triangular, becoming much wider than long in the most advanced forms like Hyperodapedon (= Scaphonyx), with a deep cheek region, and the premaxilla extending outwards and downwards to form the upper beak. The broad skull would have accommodated powerful jaw muscles. The lower jaw was also deep, and when the mouth was closed it clamped firmly into the maxilla (upper jaw), like the blade of a penknife closing into its handle. This scissors-like action would have enabled rhynchosaurs to cut up tough plant material.

The teeth were unusual; those in the maxilla and palate were modified into broad tooth plates. The hind feet were equipped with massive claws, presumably for digging up roots and tubers by backwards scratching of the hind limbs.

Like many animals of this time, they had a worldwide distribution, being found across Pangea. These abundant animals might have died out suddenly at the end of the Carnian (Middle of the Late Triassic period), perhaps as a result of the extinction of the Dicroidium flora on which they may have fed. On the other hand, Spielmann, Lucas and Hunt (2013) described three distal ends of humeri from early-mid Norian Bull Canyon Formation in New Mexico, which they interpreted as bones of rhynchosaurs belonging to the species Otischalkia elderae; thus, the fossils might indicate that rhynchosaurs survived until the Norian.

Classification

Taxonomy

Genera
Genus	Species	Age	Location	Unit	Notes	Images
Ammorhynchus	A. navajoi	Anisian	USA (Arizona)	Moenkopi Formation		Hyperodapedon huxleyi (=Paradapedon) Mesosuchus browni Skull of Rhynchosaurus articeps
Bentonyx	B. sidensis	late Anisian	England	Otter Sandstone Formation
Eohyosaurus	E. wolvaardti	early Anisian	South Africa	Burgersdorp Formation
Fodonyx	F. spenceri	late Anisian	England	Otter Sandstone Formation
Howesia	H. browni	early Anisian	South Africa	Burgersdorp Formation
Hyperodapedon	H. gordoni	Carnian	Scotland	Lossiemouth Sandstone Formation	Five valid species are known, the most of any rhynchosaur.
	H. huenei	Carnian	Brazil	Santa Maria Formation
	H. huxleyi	Carnian	India	Lower Maleri Formation
	H. mariensis	Carnian	Brazil Argentina	Santa Maria Formation Ischigualasto Formation
	H. sanjuanensis	Carnian	Argentina Brazil	Ischigualasto Formation Santa Maria Formation
Isalorhynchus	I. genovefae	Carnian	Madagascar	Makay Formation (Isalo II)
Mesodapedon	M. kuttyi	Anisian	India	Yerrapalli Formation
Mesosuchus	M. browni	early Anisian	South Africa	Burgersdorp Formation
Noteosuchus	N. colletti	early Induan	South Africa	Katberg Formation	The earliest known species, and the only Early Triassic representative.
Otischalkia	O. elderae	late Carnian	USA (Texas)	Dockum Group
Rhynchosaurus	R. articeps	Anisian	England	Tarporley Siltstone Formation
	R. brodiei	Anisian	England	Bromsgrove Sandstone Formation
Stenaulorhynchus	S. stockleyi	late Anisian	Tanzania	Manda Formation
Supradapedon	S. stockleyi	Middle - Late Triassic	Tanzania	Tunduru district
Teyumbaita	T. sulcognathus	early Norian	Brazil	Caturrita Formation	The latest surviving species, and the only Norian rhynchosaur.

Phylogeny

Skull of a rhynchosaur, in Educational Museum Gama D'Eça.

Illustration of the ventral surface of a tooth plate of Hyperodapedon.

Traditionally, the Rhynchosauria included a single family, named Rhynchosauridae. Hyperodapedontidae, which was named by Lydekker (1885), was considered its junior synonym. All rhynchosaurs, apart from the three Early and Middle Triassic monospecific genera, Mesosuchus, Howesia and Noteosuchus, were included in this family. According to Dilkes (1998), Noteosuchus colletti is most probably a junior synonym of Mesosuchus browni. Langer et al. (2000) noted that the family Hyperodapedontidae was erected by Lydekker to include Hyperodapedon gordoni and H. huxleyi, clearly excluding Rhynchosaurus articeps, which was the only other rhynchosaur known at that time. Thus, they defined it as the stem-based taxon that includes all rhynchosaurs more closely related to Hyperodapedon than to Rhynchosaurus. Using this definition, Rhynchosauridae must include all rhynchosaurs more closely related to Rhynchosaurus than to Hyperodapedon (or a smaller group), as a family can't be included in another family.

The cladogram below follows Langer et al. (2000) definitions.

Rhynchosauria	Mesosuchus browni Howesia browni Stenaulorhynchus stockleyi Rhynchosauridae Rhynchosaurus articeps Rhynchosaurus brodiei Hyperodapedontidae Fodonyx spenceri Hyperodapedontinae Ammorhynchus navajoi Mariante rhynchosaur Teyumbaita sulcognathus Hyperodapedon huenei Hyperodapedon huxleyi Hyperodapedon gordoni Hyperodapedon mariensis Hyperodapedon sanjuanensis

The cladogram below is based mostly on Langer et al. (2010). The relations between species of Hyperodapedon follow Langer et al. (2000), as Langer et al. (2010) performed a genus-level phylogenetic analysis.

Rhynchosauria	Mesosuchus browni Howesia browni Rhynchosaurus spp. Hyperodapedontidae Mariante rhynchosaur Stenaulorhynchus stockleyi Bentonyx sidensis Fodonyx spenceri Hyperodapedontinae Isalorhynchus genovefae Teyumbaita sulcognathus Hyperodapedon huenei Hyperodapedon mariensis Hyperodapedon sanjuanensis Hyperodapedon gordoni Hyperodapedon huxleyi

See also

• Paleorrota

References

Notes

Bibliography

• Benton, M. J. (2000), Vertebrate Paleontology, 2nd ed. Blackwell Science Ltd
• Carroll, R. L. (1988), Vertebrate Paleontology and Evolution, W.H. Freeman & Co.
• Dilkes, D. W. 1998. The Early Triassic rhynchosaur Mesosuchus browni and the interrelationships of basal archosauromorph reptiles. Philosophical Transactions of the Royal Society of London: Biological Sciences, 353:501-541.

External links

• Dinosaurs of Rio grande do Sul, Brasil.