Desmatosuchus

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Desmatosuchus
Temporal range: Late Triassic
Desmatosuchus mount.jpg
Desmatosuchus from Petrified Forest National Park, Arizona
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Aetosauria
Family: Stagonolepididae
Subfamily: Desmatosuchinae
Genus: Desmatosuchus
Case, 1920
Type species
D. spurensis
Case, 1921
Species
  • D. haplocerus (Cope, 1892)
  • D. smalli Parker, 2005[1]

Desmatosuchus (/dɛzmætsʌs/, from Greek δεσμός desmos 'link' + σοῦχος soûkhos 'crocodile') is an extinct genus of archosaur belonging to the Order Aetosauria. It lived during the Late Triassic.

Description[edit]

D. spurensis compared to a human

Desmatosuchus were large quadrupeds that were upwards of 3 meters in length.[2] Their spines had amphicoelus centra and 3 sacral vertebrae. Their scapulae had large acromion processes.[2] Their forelimbs were much shorter than their hindlimbs, with humeri less than two-thirds the length of their femurs.[3] Their pelvic girdles consisted of a long pubis with a strong symphysis in the middle, a plate-like ischium, a highly recurved ilium, and a deep, imperforate acetabulum.[3] They had relatively long, straight femurs, crurotarsal ankles, and calcaneal tubers that gave them large heels.[3]

Their skulls were relatively small relative to their bodies, on average about 37 centimeters long, 18 centimeters wide, and 15 centimeters high. Their braincases were very firmly fused with the skull roof and palate. They had slender,forked premaxillae that turned up and expanded in the front, creating a shovel-like structure.[2] Desmatosuchus are unique among Aetosaurs in that they are the only known aetosaurs that lacked teeth on their premaxillae.[2] Their premaxillae fit loosely together with their maxillae, indicating flexibility at that joint.[2] Their maxilla contained 10 to 12 teeth.[2] Desmatosuchus also had very thin vomers, which bounded the medial side of the internal nares.[2] These internal nares were relatively large, roughly half the length of the entire palate.[2] Their lower jaw typically carried 5 or 6 teeth, and had a toothless beak on the end.[2] The dentary was about half the length of the lower jaw, with the front portion being toothless and covered by a horny sheath.[2] Behind the dentary was a moderately large mandibular fenestra.

Desmatosuchus were heavily armored. The carapace was made up of two rows of median scutes surrounded by two more rows of lateral scutes. The lateral scutes have well-developed spines that point out laterally and dorso-posteriorly.[4] There are typically five rows of spines, and they increase in size anteriorly. The front spine is much larger, around 28 centimeters long, and is recurved. The fourth spine varies in length in each specimen, but remains shorter than the fifth in all of them.[2] Desmatosuchus are the only aetosaurs known to have possessed spines like these.[4]

Discovery and Classification[edit]

D. haplocerus from the Late Triassic of Texas

The first Desmatosuchus discovery occurred in the late 1800's when E.D. Cope classified armor from the Dockum Group in Texas as the new species Episcoposaurus haplocerus.[5] Case later classified a partial skeleton found in the Tecovas Formation as Desmatosuchus spurensis.[6] Since the the localities of Cope and Case were only a few kilometers apart, the two taxa were synonymized into Desmatosuchus haplocerus, the initial type species of the genus.[5]

A revision of Desmatosuchus by Parker (2008) found the lectotype of Episcoposaurus haplocerus to be referable to Desmatosuchus but indeterminate at the species level. Therefore, E. haplocerus was considered to be a nomen dubium and D. spurensis was named the type species of the genus. Two species were accepted as valid: D. spurensis and D. smalli.[7] Desmatosuchus chamaensis is recognized as a distinct genus, but there is some dispute about whether the name Heliocanthus or Rioarribasuchus applies.[5]

The following cladogram simplified after an analysis presented by Julia B. Desojo, Martin D. Ezcurra and Edio E. Kischlat (2012).[8]

Desmatosuchus skeleton from the Museum of Natural History
 Aetosauria 

Aetosauroides scagliai


 Stagonolepididae 

Aetosaurus ferratus



Coahomasuchus kahleorum




Neoaetosauroides engaeus




Calyptosuchus wellesi



Stagonolepis robertsoni




Aetobarbakinoides brasiliensis



 Typothoracisinae 


Typothorax



Redondasuchus



 Paratypothoracisini 

Tecovasuchus chatterjeei




Rioarribasuchus chamaensis



Paratypothorax andressorum





 Desmatosuchinae 

Sierritasuchus macalpini




Longosuchus meadei




Lucasuchus hunti




Acaenasuchus geoffreyi


 Desmatosuchus 

Desmatosuchus haplocerus



Desmatosuchus smalli













Paleobiology[edit]

D. spurensis skull

Desmatosuchus bones and armor pieces are abundant in the Dockum formation, Chinle formation, and Post quarry, indicating that Desmatosuchus were widespread and abundant during the Late Triassic.[2] It is possible that Desmatosuchus traveled in packs or family units. This is evidenced by several findings of multiple Desmatosuchus skeletons in relatively small areas.[2]

As an aetosaur, Desmatosuchus were herbivorous thecodonts. Desmatosuchus had blunt, bulbous, slightly recurved teeth. Furthermore, they are believed to have had homodont dentition.[2] This, combined with its shovel like snout, indicate that Desmatosuchus fed by digging up soft vegetation.[4] This method of feeding is further evidenced by its toothless premaxilla and dentary tip, which were covered in horny sheaths. These sheaths protected the bones and could be used for cutting or holding objects.[9] It is believed that Desmatosuchus dug for food in the soft mud near bodies of water due to the abundance of lakes and rivers in the Dockum area and the the fact that Desmatosuchus scutes are often found among parts of other reptiles that are known to have fed along waterways. [2] It is unknown whether or not Desmatosuchus replaced their teeth and, if so, how. The low number of Desmatosuchus teeth that have been discovered indicates that they were only held in place by soft tissue connections.[2] The jaw articulation point is below the tooth line, holding its upper and lower tooth rows parallel while biting in a way that is reminiscent of ornithischian dinosaurs.[9]

Postosuchus and Desmatosuchus

The armor and spikes of Desmatosuchus were its only ways to defend itself from predators. The lateral spike rows showed variation in size among individuals, especially the second most anterior spike. This spike was always shorter than the one in front of it, but to what extent varied drastically. This variation may indicate sexual dimorphism.[4] It has also been hypothesized as a form of sexual display.[2] Aside from this armor, Desmatosuchus was defenseless from attacks from carnivores. Several Desmatosuchus bones have been found amongst Postosuchus skeletons, indicating predation by Postosuchus.[2] The pack nature of Desmatosuchus did little to ward off predators, as Postosuchus along with several other Late Triassic carnivores also traveled in groups.[2]

Most thecodonts of the Late Triassic lacked certain pelvic features that aided locomotion, such as a deep acetabulum or a crest over the acetabulum. This, in spite of their upright posture, rendered them only slightly more mobile than sprawling reptiles. [10] Desmatosuchus possessed both of these features, along with its long femur and elongate pubis, making it more mobile than most thecodonts of its time.[10] This mobility, along with the Desmatosuchus' size, abundance, and specialized beak made it the chief herbivore in the Dockum area.[2]

In popular culture[edit]

Desmatosuchus was featured in When Dinosaurs Roamed America, driving away a Coelophysis and a predatory Rutiodon. Desmatosuchus was also featured in an episode of Animal Armageddon, where it was hunted by Staurikosaurus, one of the first true dinosaurs, into eventual extinction. In both, Desmatosuchus was used to contrast the anatomy of the earlier archosaur reptiles with that of the first true dinosaurs.

References[edit]

  1. ^ Parker, W.G. (2005). A new species of the Late Triassic aetosaur Desmatosuchus (Archosauria:Pseudosuchia). Compte Rendus Palevol 4(4): 327-340.
  2. ^ a b c d e f g h i j k l m n o p q r s t Small, B. J. (1985). The Triassic thecodontian reptile Desmatosuchus: osteology and relationships.
  3. ^ a b c Charig, A. J., 1972. The evolution of the archosaur pelvis and hindlimb: an explanation in functional terras. In K. A. Joysey and T. S. Kemp (eds.). Studies in Vertebrate Evolution, 121-155. Oliver and Boyd, Edinburgh
  4. ^ a b c d Palmer, D., ed. (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. p. 96. ISBN 1-84028-152-9.
  5. ^ a b c Parker, W.G. (2007). Reassessment of the aetosaur “Desmatosuchus” chamaensis with a reanalysis of the phylogeny of the Aetosauria (Archosauria: Pseudosuchia). Journal of Systematic Palaeontology 5(1): 41-68.
  6. ^ J.T. Gregory, Typothorax and Desmatosuchus, Postilla 16 (1953) 1–27.
  7. ^ Parker, W.G. (2008). Description of new material of the aetosaur Desmatosuchus spurensis (Archosauria: Suchia) from the Chinle Formation of Arizona and a revision of the genus Desmatosuchus. PaleoBios 28(1): 1-40.
  8. ^ Julia B. Desojo, Martin D. Ezcurra and Edio E. Kischlat (2012). "A new aetosaur genus (Archosauria: Pseudosuchia) from the early Late Triassic of southern Brazil". Zootaxa 3166: 1–33. ISSN 1175-5334. 
  9. ^ a b Walker, A. D., 1961. Triassic reptiles from the Elgin area: Stagonolepis, Dasygnathus and their allies. Phil. Trans. R. So c Lond. B. 244:103-204.
  10. ^ a b Bakker, R. T., 1981. Dinosaur physiology and the origin of the mammals. Evolution 25, 636-658.