Saurosuchus

Saurosuchus (meaning "lizard crocodile") is a genus of large loricatan pseudosuchian archosaur that lived in South America during the Late Triassic period. It was a heavy, ground-dwelling, quadrupedal carnivore, being the major predator in the Ischigualasto Formation.[1]

Saurosuchus
Temporal range: Late Triassic, 231.4 Ma
↓
Mounted skeleton of Saurosuchus galilei in Mori Arts Center Gallery, Japan
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Paracrocodylomorpha
Clade: Loricata
Family: †Prestosuchidae
Genus: †Saurosuchus
Reig, 1959
Species:
†S. galilei
Binomial name
†Saurosuchus galilei
Reig, 1959

Discovery and naming
Posterior view of the mount in the Ischigualasto Provincial Park

The holotype, PVL 206, was discovered by Galileo J. Scaglia and Leocadio Soria in 1957, lying in a greenish sandstone on the Cancha de Bochas Member of the Ischigualasto Formation in the Ischigualasto-Villa Unión Basin in northwestern Argentina. It consists of a nearly complete, but deformed skull. Saurosuchus was formally described and named later in 1959 by Osvaldo. A. Reig. The generic name, Saurosuchus, is derived from the Greek σαῦρος (sauros, meaning lizard) and σοῦχος (souchus, meaning crocodile). The specific name, galilei, is in honour to Galileo J. Scaglia, who unearthed and prepared the holotype.[1]

Referred specimens

Saurosuchus is known from numerous specimens coming from the Ischigualasto Formation. Apart from the holotype, it was identified another individual: specimen PVL 2198; consisting of a partial maxilla, left illium, right and left ischium, eleven dorsal vertebrae, osteoderms, ribs and teeth.[1] Sill referred additional specimens, PVL 2557, 2472 and 2267. The specimen PVL 2557 consists of two dorsal vertebrae, right and left sacrals, nine caudal vertebrae, right illium, ischium and partial pubis, right femur, tibia, fibula, tarsus and pes, ribs and chevrons. PVL 2472 compromises one cervical vertebra, tibia and astragalus. Lastly, PVL 2267 is composed by a fragmented illium, femur, tibia, fibula, a tarsus and a partial pes.[2] In 2010, during the redescription of the skull of Saurosuchus, Alcober referred and described the immature specimen PVSJ 32; consisting of a complete skull, complete cervical and dorsal vertebral series, four anterior caudal vertebrae, ribs and two dorsal osteoderm rows.[3] The postcranial remains of this specimen, were properly described by Trotteyn et al. 2011.[4]

Here, more specimens of Saurosuchus were referred: PVSJ 369, 675 and 615. In addition to this, the previous specimens PVL 2472 and PVL 2267 were excluded, both of them being not referable to Saurosuchus.[4]

Saurosuchus was also reported from the Chinle Formation of Arizona in 2002 on the basis of isolated teeth and small skull fragments.[5] The diagnostic value of these bones has been questioned in later studies, which considered them to be from an indeterminate species of rauisuchian.[6]

Description
Life restoration of S. galilei

Initially, Saurosuchus was estimated at 5.5 m (18 ft) long[1] with a weight over 250 kg (550 lb).[7] However, a complete skeleton is not known, and size estimates range from around 6 to 9 m (20 to 30 ft) in total body length.[3] There are two rows to either side of the midline, with each leaf-shaped osteoderm joining tightly with the ones in front of and behind it.[2][4] It has a deep, laterally compressed skull. The teeth are large, recurved, and serrated. The skull is wide at its back and narrows in front of the eyes. The skull roof and maxilla are somewhat pitted.[3]

Pitting is also seen in aquatic phytosaurs and crocodilians, but the ridges and grooves are deeper and much more extensive across the skulls of these forms. The frontal bones, located at the top of the skull, are enlarged to form thick ridges over the eyes. As in more evolved rauisuchians, a small rod projects down from the lacrimal bone in front of the eye, but it does not attach firmly to the jugal bone below it. Ridges along the upper surface of the supraoccipital bone at the back of the skull are attachment points for strong neck ligaments. The cervical vertebrae are shortened and robust, forming a strong neck.[3][4]

Classification

Saurosuchus was considered a member of the Rauisuchia, although now it is considered part of the more basal Loricata, a clade comprising both Rauisuchia and the true crocodylians. Below is the cladogram of the Loricata conducted by Nesbitt 2011:[8]

Paracrocodylomorpha 
 

Poposauroidea

 
 Loricata 
 

Prestosuchus

 
 
 

Saurosuchus

 
 
 

Batrachotomus

 
 
 

Fasolasuchus

 
 
 

Rauisuchidae

 
 

Crocodylomorpha

 
 
 
 
 
 
 

Paleoecology
Saurosuchus hunting a group of Hyperodapedon

Saurosuchus was unearthed in the Cancha de Bochas Member from the Ischigualasto Formation, being the major predator on its environment.[3] The Ischigualasto Formation was dominated by fluvial and floodplain environments with strongly seasonal rainfalls. Interlayered volcanic ash layers above the base and below the top of the formation provide chronostratigraphic control and have yielded ages of 231.4 ± 0.3 Ma and 225.9 ± 0.9 Ma respectively.[9]

Contemporaneous fauna

Animals that lived alongside included numerous non-dinosaurian tetrapods, as well as basal dinosauromorphs. Notable paleofauna that were contemporaneous with Saurosuchus in the Cancha de Bochas Member include: Hyperodapedon, Exaeretodon, Herrerasaurus, Sillosuchus, Eoraptor, Trialestes, Aetosauroides and Ischigualastia. Herrerasaurus with Saurosuchus, are some of the most common predators in the formation, with numerous specimens and remains.[10][11]

See also

References
  1. Reig, O. A. (1959). "Primeros datos descriptivos sobre nuevos arcosaurios del Triásico de Ischigualasto (San Juan, Argentina)". Revista de la Asociación Geológica Argentina. 13 (4): 257–27.
  2. Sill, W. D. (1974). "The anatomy of Saurosuchus galilei and the relationships of the rauisuchid thecodonts". Bulletin of the Museum of Comparative Zoology. 146: 317–362.
  3. Alcober, O. (2000). "Redescription of the skull of Saurosuchus galilei (Archosauria: Rauisuchidae)". Journal of Vertebrate Paleontology. 20 (2): 302–316. doi:10.1671/0272-4634(2000)020[0302:ROTSOS]2.0.CO;2.
  4. Trotteyn, M.J.; Desojo, J.; Alcober, O. (2011). "Nuevo material postcraneano de Saurosuchus galilei (Archosauria: Crurotarsi) del Triásico Superior del centro-oeste de Argentina". Ameghiniana. 48 (1): 13–27. doi:10.5710/amgh.v48i1(265). S2CID 129032513.
  5. Heckert, A.B.; Lucas, S.G.; Krzyzanowski, S.E. (2002). "The rauisuchian archosaur Saurosuchus from the Upper Triassic Chinle Group, Southwestern U.S.A., and its biochronological significance" (PDF). In Heckert, A.B.; Lucas, S.G. (eds.). Upper Triassic Stratigraphy and Paleontology. 21. New Mexico Museum of Natural History and Science Bulletin. pp. 241–247.
  6. Irmis, R.B. (2005). "The vertebrate fauna of the Upper Triassic Chinle Formation in Northern Arizona". In Nesbitt, S.J.; Parker, W.G.; Irmis, R.B. (eds.). Guidebook to the Triassic Formations of the Colorado Plateau in northern Arizona: Geology, Paleontology, and History. 9. Mesa Southwestern Museum.
  7. Reig, O. A. (1961). "Acerca de la posición sistemitica de la familia Rauisuchidae y del género Saurosuchus (Reptilia-Thecodontia)". Publicaciones del Museo Municipal de Ciencias Naturales y Tradicionales de Mar del Plata. 1: 73–114.
  8. Nesbitt, S.J. (2011). "The early evolution of archosaurs: relationships and the origin of major clades" (PDF). Bulletin of the American Museum of Natural History. 352: 1–292. doi:10.1206/352.1. hdl:2246/6112. S2CID 83493714.
  9. Wallace, R. V. S. (2018). "A new close mammal relative and the origin and evolution of the mammalian central nervous system" (PDF). Cite journal requires |journal= (help)
  10. Martínez, R. N.; Apaldetti, C.; Alcober, O. A.; Colombi, C. E.; Sereno, P. C.; Fernández, E.; Malnis, P. S.; Correa, G. A.; Abelin, D. (2013). "Vertebrate succession in the Ischigualasto Formation". Journal of Vertebrate Paleontology. 32: 10–30. Bibcode:2013JVPal..32S..10M. doi:10.1080/02724634.2013.818546. S2CID 37918101.
  11. Weishampel, D. B.; Dodson, P.; Osmolska, H. (2007). "Dinosaur Distribution". The Dinosauria, Second Edition. University of California Press. p. 528.
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