Rhynchosaur

Rhynchosaurs
Temporal range: Induan-early Norian
Mounted skeleton model of Hyperodapedon
Scientific classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Archosauromorpha
Clade: Crocopoda
Order: Rhynchosauria
Osborn 1903
Subgroups

Rhynchosaurs are a group of extinct herbivorous Triassic archosauromorph reptiles, belonging to the order Rhynchosauria. Members of the group are distinguished by their triangular skulls and elongated, beak like premaxillary bones. Rhynchosaurs first appeared in the Early Triassic, reaching their broadest abundance and a global distribution during the Carnian stage of the Late Triassic.

Description

Life restoration of Hyperodapedon sanjuanensis.

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. Rhynchosaur teeth had a unique condition known as ankylothecodonty, similar to the acrodonty of modern tuataras and some lizards but differing in the presence of deep roots.

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. Similar to elephants they had a fixed number of teeth where those further back in the jaws replaced those who were worn out as the animal grew in size and the teeth was worn out because of a diet of very tough plants. In the end they probably starved to death.

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

List of genera

List of rhynchosaur genera
Genus Species Age Location Unit Notes

Ammorhynchus

A. navajoi

Anisian

 US (Arizona)

Moenkopi Formation

A stenaulorhynchine.

Beesiiwo

B. cooowuse

Carnian

 US (Wyoming)

Popo Agie Formation

A hyperodapedontine previously referred to Hyperodapedon.

Brasinorhynchus

B. mariantensis

Ladinian

 Brazil

Santa Maria Formation

A stenaulorhynchine, previously known as the "Mariante Rhynchosaur".

Bentonyx

B. sidensis

late Anisian

 UK (England)

Otter Sandstone Formation

A basal hyperodapedontid.
Elorhynchus E. carrolli late Ladinian? - earliest Carnian?  Argentina Chañares Formation (Tarjadia Assemblage Zone) A stenaulorhynchine.

Eohyosaurus

E. wolvaardti

early Anisian

 South Africa

Burgersdorp Formation (Cynognathus Assemblage Zone)

A basal (non-rhynchosaurid) rhynchosaur.

Fodonyx

F. spenceri

late Anisian

 UK (England)

Otter Sandstone Formation

A basal hyperodapedontid.

Howesia

H. browni

early Anisian

 South Africa

Burgersdorp Formation (Cynognathus Assemblage Zone)

A basal (non-rhynchosaurid) rhynchosaur.

Hyperodapedon

H. gordoni

Carnian

 UK (Scotland)

Lossiemouth Sandstone Formation

A hyperodapedontine, one of the most abundant and speciose rhynchosaur genera. Six valid species has been named, 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

H. tikiensis

Carnian

 India

Tiki Formation

Isalorhynchus

I. genovefae

Carnian

 Madagascar

Makay Formation (Isalo II)

A hyperodapedontine occasionally referred to Hyperodapedon.

Mesodapedon

M. kuttyi

Anisian

 India

Yerrapalli Formation

A stenaulorhynchine.

Langeronyx

L. brodiei

Anisian

 UK (England)

Bromsgrove Sandstone Formation

A basal hyperodapedontid.

Mesosuchus

M. browni

early Anisian

 South Africa

Burgersdorp Formation (Cynognathus Assemblage Zone)

A basal (non-rhynchosaurid) rhynchosaur.

Noteosuchus

N. colletti

early Induan

 South Africa

Katberg Formation (Lystrosaurus Assemblage Zone)

A basal (non-rhynchosaurid) rhynchosaur. The earliest known species of rhynchosaur, and the only known Early Triassic representative.

Oryctorhynchus

O. bairdi

latest Carnian?-earliest Norian?

 Canada (Nova Scotia)

Wolfville Formation

A hyperodapedontine previously referred to Hyperodapedon.

Rhynchosaurus

R. articeps

Anisian

 UK (England)

Tarporley Siltstone Formation

A basal rhynchosaurid.

Stenaulorhynchus

S. stockleyi

late Anisian

 Tanzania

Manda Formation

A stenaulorhynchine.

Supradapedon

S. stockleyi

Middle - Late Triassic

 Tanzania

Tunduru district

A hyperodapedontine previously referred to Hyperodapedon.

Teyumbaita

T. sulcognathus

late Carnian - early Norian

 Brazil
 Argentina

Caturrita Formation
Ischigualasto Formation

The latest surviving species, and the only rhynchosaur known with confidence to have survived into the Norian stage.

Hyperodapedon huxleyi (=Paradapedon)
Mesosuchus browni
Skull of Rhynchosaurus articeps

Phylogeny

Skull of a rhynchosaur, in Educational Museum Gama D'Eça.
Illustration of the ventral surface of a tooth plate of Hyperodapedon.

The Rhynchosauria included a single family, named Rhynchosauridae. All rhynchosaurs, apart from the four Early and Middle Triassic monospecific genera, Eohyosaurus, Mesosuchus, Howesia and Noteosuchus, are included in this family. Hyperodapedontidae named by Lydekker (1885) was considered its junior synonym. However, Langer et al. (2000) noted that 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.

Within Hyperodapedontidae, which is now a subgroup of Rhynchosauridae, two subfamilies have been named. Stenaulorhynchinae named by Kuhn (1933) is defined sensu Langer and Schultz (2000) to include all species more closely related to Stenaulorhynchus than to Hyperodapedon. Hyperodapedontinae named by Chatterjee (1969) was redefined by Langer et al. (2000) to include "all rhynchosaurs closer to Hyperodapedon than to "Rhynchosaurus" spenceri" (now Fodonyx).

The cladogram below is based on Schultz et al. (2016) which is the most genera inclusive rhynchosaur phylogenetic analysis to date, with the position of Noteosuchus taken from other recent analyses (since it was removed in Schultz et al. (2016)), all in consensus with one another.

Rhynchosauria 

Noteosuchus colletti

Mesosuchus browni

Howesia browni

Eohyosaurus wolvaardti

 Rhynchosauridae 

Rhynchosaurus articeps

 Hyperodapedontidae 
 Stenaulorhynchinae 

Ammorhynchus navajoi

Mesodapedon kuttyi

Brasinorhynchus mariantensis

Stenaulorhynchus stockleyi

Bentonyx sidensis

Langeronyx brodiei

Fodonyx spenceri

Hyperodapedontinae

Isalorhynchus genovefae

Teyumbaita sulcognathus

Hyperodapedon spp.


This page was last updated at 2023-12-30 13:14 UTC. Update now. View original page.

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