''Hesperosuchus'' is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

crocodylomorph Crocodylomorpha is a group of pseudosuchian archosaurs that includes the crocodilians and their extinct relatives. They were the only members of Pseudosuchia to survive the end-Triassic extinction. During Mesozoic and early Cenozoic times, cro ...

reptile that contains a single species, ''Hesperosuchus agilis''. Remains of this

pseudosuchian Pseudosuchia is one of two major divisions of Archosauria, including living crocodilians and all archosaurs more closely related to crocodilians than to birds. Pseudosuchians are also informally known as "crocodilian-line archosaurs". Prior to ...

have been found in

Late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch ...

(

Carnian The Carnian (less commonly, Karnian) is the lowermost stage of the Upper Triassic Series (or earliest age of the Late Triassic Epoch). It lasted from 237 to 227 million years ago (Ma). The Carnian is preceded by the Ladinian and is followed by t ...

) strata from

Arizona Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣonak ) is a state in the Southwestern United States. It is the 6th largest and the 14th most populous of the 50 states. Its capital and largest city is Phoenix. Arizona is part of the Fou ...

and

New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe , LargestCity = Albuquerque , LargestMetro = Tiguex , OfficialLang = None , Languages = English, Spanish ( New Mexican), Navajo, Ker ...

.Colbert, E. H. 1952. A pseudosuchian reptile from Arizona. Bulletin of the American Museum of Natural History 99:561–592. Because of similarities in skull and neck anatomy and the presence of hollow bones ''Hesperosuchus'' was formerly thought to be an ancestor of later

carnosauria Carnosauria is an extinct large group of predatory dinosaurs that lived during the Jurassic and Cretaceous periods. Starting from the 1990s, scientists have discovered some very large carnosaurs in the carcharodontosaurid family, such as ''Gig ...

n dinosaurs, but based on more recent findings and research it is now known to be more closely related to

crocodilia Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period ( Cenomanian stage) and are the closest living ...

ns rather than dinosaurs.Brinkman, D. 1981. The origin of the crocodiloid tarsi and the interrelationships of thecodontian archosaurs. Breviora 464:1–23.Benton, M. J. and J. M. Clark. 1988. Archosaur phylogeny and the relationships of the Crocodylia. pp. 295–338. In M. J. Benton (ed.). The Phylogeny and Classification of the Tetrapods Vol. 1.Amphibians, Reptiles, Birds. Clarendon Press, Oxford.

Description

Stance and limbs

At only about 4 to 5 feet in length, ''Hesperosuchus'' is a relatively small and lightly built pseudosuchian. It is very closely comparable to the different genera, '' Ornithosuchus woodwardi'' and '' Saltoposuchus longipes'',Baczko, M. B. von & Ezcurra, M. D. 2013. Ornithosuchidae: a group of Triassic archosaurs with a unique ankle joint. Geological Society, London, Special Publications 379(1), 187–202. which are both pseudosuchians as well. The hind limbs of ''Hesperosuchus'' are large and strong, while the forelimbs were smaller and much more slender. This observation lead to the hypothesis that ''Hesperosuchus'' was a

bipedal Bipedalism is a form of terrestrial locomotion where an organism moves by means of its two rear limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from Latin ''bis'' 'double' ...

animal. Comparing the hind limbs of ''Saltoposuchus'' with those of ''Hesperosuchus'', they are evenly as large and strong. The length of the extended hind limb in both genera are approximately equal to the length of the presacral vertebrae. ''Saltoposuchus'' was described by van Huene, and portrayed as a facultative

quadruped Quadrupedalism is a form of locomotion where four limbs are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four limbs is said to be a quadruped (from Latin ''quattuor' ...

. It is believed that ''Hesperosuchus'' also practiced both bipedialism and quadrupedalism. Though it is believed to be more often on two feet as the long slender hands looked as if it was adapted for grasping, which may have been useful for food gathering, digging or defense. Five digits were found on both the hind limbs as well as the forelimbs. In order to counterbalance the weight of its body, ''Hesperosuchus'' is inferred to have had a relatively long tail. Since the caudal vertebrae aren't completely restored, it is inferred based on similar archosaurians, that the tail contained somewhere around 45 caudal vertebrae. The strong hind limbs and overall light weight made ''Hesperosuchus'' very quick and able to move rapidly. This advantage of speed allowed for it to catch small prey and escape from larger predators.

Skull

The skull of ''Hesperosuchus'' was only partially preserved and is missing many segments. The

mandible In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...

and the skull of the specimen found was very poorly preserved, but there is just enough bone present to provide for an indication of what the basic structure of the jaw and skull may have looked like. It was found that the skull very closely resembled that of ''Ornithosuchus''. In the fronto-parietal regions of the skull, along with a flat cranial roof, marked depressions were found in the frontal and post orbital bones, in front of and lateral to the

supratemporal fenestrae The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, the ...

. Fragments of the left

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has b ...

and

maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The t ...

were found to have sockets for nine teeth, with four being in the premaxilla. The first premaxillary teeth start out small in size and progressively get larger with the fourth tooth being clearly enlarged. This was compared to the skull of ''Ornithosuchus'', which is defined with characteristics of two enlarged teeth in this similar area; the first two maxillary teeth. The teeth of ''Hesperosuchus'' are serrated in both the posterior and anterior edges which supports the fact that ''Hesperosuchus'' was a meat eating animal. With the two fragments of the jaws found, only 14 teeth in total were reported with five in the posterior fragment and nine in the anterior one. The basioccipital region is defined as typically

archosaur Archosauria () is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaurs are broadly classified as reptiles, in the cladistic sense of the term which includes birds. Extinct archosaurs include non-avian d ...

ian, with a rounded condyle, a rather elongated surface above it for the

medulla oblongata The medulla oblongata or simply medulla is a long stem-like structure which makes up the lower part of the brainstem. It is anterior and partially inferior to the cerebellum. It is a cone-shaped neuronal mass responsible for autonomic (involun ...

, and an extended ventral plate. These basioccipital characteristics are seen in extinct archosaurians such as primitive theropod dinosaurs as well as seen in crocodylians. It can be seen that the skull would have been relatively large which was compared to carnosaurian dinosaurs, which too had fairly large skulls. It can be said that both these groups were active and carnivorous as large skulls allow for wide gaping jaws to catch and attack prey. Such large skulls need to be light as a large antorbital opening is present and clearly shown in ''Hesperosuchus''.

Discovery

''Hesperosuchus'' was discovered in upper Triassic rocks of Northern Arizona by Llewellyn I. Price, William B. Hayden, and Barnum Brown in the fall of 1929 and the summer of 1930. The specimen was then taken to a museum for Otto Falkenbach to carefully and precisely put together. Many different illustrations of the bones were done by Sydney Prentice from the Carnegie Museum of Pittsburgh. In addition, models and figures were also made by John LeGrand Lois Darling from the Museum Illustrators Corps. ''Hesperis'' in

ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic peri ...

means “Evening Star”; the reason this was chosen is unknown. ''Suchos'' (σοῦχος) on the other hand is ancient Greek for the Egyptian crocodile god, as ''Hesperosuchus'' is related to crocodilians. ''Agilis'' means agile for the hypothesis that ''Hesperosuchus'' was a very agile animal, based on its hindlimb structures. The exact location where the specimen was found is an area 6 miles southeast of Cameron, Arizona, close to the old Tanner Crossing of the

Little Colorado River The Little Colorado River () is a tributary of the Colorado River in the U.S. state of Arizona, providing the principal drainage from the Painted Desert region. Together with its major tributary, the Puerco River, it drains an area of about in ...

.Lucas, S.G., 1993, The Chinle Group: revised stratigraphy and chronology of Upper Triassic nonmarine strata in the western United States: Museum of Northern Arizona, Bulletin 59, p. 27-50. This area, in particular, is a very prevalent for finding many Triassic

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...

s.Blakey, R. C. and R. Gubitosa. 1983, Late Triassic paleogeography and depositional history of the Chinle Formation, southern Utah and northern Arizona: ''in'' Reynolds, M.W., and Dolly, E.D., eds., Mesozoic paleogeography of west-central U.S.: Society of Economic Paleontologists and Mineralogists, Rocky Mountain Section, Denver, p. 57–76.HAUGHTON, S. H.1924. The fauna and stratigraphy of the Stormberg series. Ibid., vol. 12, pp. 323- 497. This area is about 160 miles on top of the Moen kopi formation, in the portion of the

Chinle formation The Chinle Formation is an Upper Triassic continental geological formation of fluvial, lacustrine, and palustrine to eolian deposits spread across the U.S. states of Nevada, Utah, northern Arizona, western New Mexico, and western Colorado. In Ne ...

Irmis, R.B., 2005, The vertebrate fauna of the Upper Triassic Chinle Formation in northern Arizona: Mesa Southwest Museum, Bulletin 9, p. 63- 88. where the Little Colorado River flows through a canyon. Along with ''Hesperosuchus'', many other specimens were found in the same general area. There were many

ganoid scale A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as we ...

s believed to have belonged to Triassic freshwater

holostean Holostei is a group of ray-finned bony fish. It is divided into two major clades, the Halecomorphi, represented by a single living species, the bowfin ('' Amia calva''), as well as the Ginglymodi, the sole living representatives being the gars ...

fish, several

phytosaur Phytosaurs (Φυτόσαυροι in greek) are an extinct group of large, mostly semiaquatic Late Triassic archosauriform reptiles. Phytosaurs belong to the order Phytosauria. Phytosauria and Phytosauridae are often considered to be equivalent ...

teeth, and many small

stereospondyl The Stereospondyli are a group of extinct temnospondyl amphibians that existed primarily during the Mesozoic period. They are known from all seven continents and were common components of many Triassic ecosystems, likely filling a similar ecologi ...

vertebrae. In addition, a large number of small teeth were found, some which definitely belonged to ''Hesperosuchus'', and some belonging to

amphibian Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...

related animals. It is hypothesized that these other teeth may have belonged to animals that Hesperosuchus may have preyed upon. ''Hesperosuchus'' was a contemporary of ''

Coelophysis ''Coelophysis'' ( traditionally; or , as heard more commonly in recent decades) is an extinct genus of coelophysid theropod dinosaur that lived approximately 228 to 201.3 million years ago during the latter part of the Triassic Period from t ...

'', a primitive predatory

theropod Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally c ...

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

. ''Coelophysis'' was long thought to have been a cannibal, based on the presence of putative juvenile ''Coelophysis'' bones in the gut regions of a few adults. However, in some of these cases, it was later found that the "juvenile ''Coelophysis''" bones were actually those of a ''Hesperosuchus'' (or something very similar) instead.

Paleoecology

''Hesperosuchus'' was a

terrestrial Terrestrial refers to things related to land or the planet Earth. Terrestrial may also refer to: * Terrestrial animal, an animal that lives on land opposed to living in water, or sometimes an animal that lives on or near the ground, as opposed to ...

animal, where its speed and ability to run fast is the most advantageous as a fitness trait. Northern Arizona's landscape during the Triassic period was surrounded by numerous bodies of water like lakes and streams. This supports that ''Hesperosuchus'' likely lived close to water although being a full on land-dwelling animal. The ganoid scales found in the general area where ''Hesperosuchus'' was found belong to freshwater fish of the Triassic period, belonging to the genus '' Semionolus'' or '' Lepidolus'', which lived in shallow lakes and streams. The phytosaur teeth and small stereospondyl vertebrae found near ''Hesperosuchus'' support the presence of lakes or streams crossing a flood plain. Also the many small teeth found, which some, belong to amphibians of the Triassic period supports the occupying of near water habitats.

References