Sphenosuchus Acutus
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''Sphenosuchus'' ("wedge crocodile") is an
extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of
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. Extinct crocodylomorphs were considerably mor ...
from the
Early Jurassic The Early Jurassic Epoch (geology), Epoch (in chronostratigraphy corresponding to the Lower Jurassic series (stratigraphy), Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic ...
Elliot Formation The Elliot Formation is a geological formation and forms part of the Stormberg Group, the uppermost geological group that comprises the greater Karoo Supergroup. Outcrops of the Elliot Formation have been found in the northern Eastern Cape, so ...
of
South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...
, discovered and described early in the 20th century. The skull is preserved very well but other than elements of the forelimb and isolated parts of the hind limb, the ''Sphenosuchus'' material is incomplete. It was probably
quadrupedal Quadrupedalism is a form of Animal locomotion, locomotion in which animals have four legs that are used to weight-bearing, bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four l ...
, but may have been a
facultative biped A facultative biped is an animal that is capable of walking or running on two legs (bipedal), as a response to exceptional or discretionary (facultative) circumstances, while normally walking or running on four limbs or more. In contrast, obligat ...
. ''Sphenosuchus'' was first thoroughly described in 1972 by the British palaeontologist
Alick Walker Alick Donald Walker (26 October 1925 – 4 December 1999) was a British palaeontologist, after whom the ''Alwalkeria'' genus of dinosaur is named. He was born in Skirpenbeck, near York and attended Pocklington School from 1936 to 1943. He began ...
, in a paper in the journal ''Nature''. Walker suggested, based on detailed (but still preliminary at that time) studies of the skull of ''Sphenosuchus'' and modern
birds Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
, that crocodylomorphs and
birds Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
might share an immediate
common ancestor Common descent is a concept in evolutionary biology applicable when one species is the ancestor of two or more species later in time. According to modern evolutionary biology, all living beings could be descendants of a unique ancestor commonl ...
. Walker recanted his hypothesis in 1985, but restated and elaborated on it (in essence 'de-recanting') in a monograph published in 1990, which provided the most comprehensive description and discussion of ''Sphenosuchus'' yet published. Broom believed, in 1927, with the evidence present in the specimens, that ''Sphenosuchus'' is a
pseudosuchian Pseudosuchia, from Ancient Greek ψεύδος (''pseúdos)'', meaning "false", and σούχος (''soúkhos''), meaning "crocodile" is one of two major divisions of Archosauria, including living crocodilians and all archosaurs more closely relat ...
which is well along the line that leads to the true
crocodiles Crocodiles (family Crocodylidae) or true crocodiles are large, semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term "crocodile" is sometimes used more loosely to include all extant member ...
. In a paper published by
Friedrich von Huene Baron Friedrich Richard von Hoyningen-Huene (22 March 1875 – 4 April 1969) was a German nobleman paleontologist who described a large number of dinosaurs, more than anyone else in 20th-century Europe. He studied a range of Permo-Carbonife ...
, he emphasizes ''Sphenosuchus'' as a very important stage of
crocodile Crocodiles (family (biology), family Crocodylidae) or true crocodiles are large, semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term "crocodile" is sometimes used more loosely to include ...
evolution.


Species

The
type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...
, ''Sphenosuchus acutus'' is the only described species in this genus.


Description

Out of the early crocodylomorphs, ''Sphenosuchus'' was one of the largest. The skull was in the length and the body an estimated total length of . This genus is considered to have been
carnivorous A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose nutrition and energy requirements are met by consumption of animal tissues (mainly mu ...
and
cursorial A cursorial organism is one that is adapted specifically to run. An animal can be considered cursorial if it has the ability to run fast (e.g. cheetah) or if it can keep a constant speed for a long distance (high endurance). "Cursorial" is often ...
. The found specimen consists of a nearly perfect but slightly crushed skull, a shoulder girdle, and a few limb bones so most of the known features come from the skull. The primary head of the quadrate meets the prootic and
squamosal The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestra ...
. The quadrate and
pterygoid Pterygoid, from the Greek for 'winglike', may refer to: * Pterygoid bone, a bone of the palate of many vertebrates * Pterygoid processes of the sphenoid bone ** Lateral pterygoid plate ** Medial pterygoid plate * Lateral pterygoid muscle * Medial ...
are not fused to the
braincase In human anatomy, the neurocranium, also known as the braincase, brainpan, brain-pan, or brainbox, is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calv ...
and the basipterygoid articulation is free. The
dentary In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone ...
is not gracile and has anterior swelling. The parietals are fused together. The post temporal fenestrae are known for the ''Sphenosuchus'' with the sizes being very variable among early
archosaurs Archosauria () or archosaurs () is a clade of diapsid sauropsid tetrapods, with birds and crocodilians being the only extant taxon, extant representatives. Although broadly classified as reptiles, which traditionally exclude birds, the cladistics ...
but rather small in size for ''Sphenosuchus'' Has a
scapula The scapula (: scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on either side ...
blade in a triangular shape. No
clavicles The clavicle, collarbone, or keybone is a slender, S-shaped long bone approximately long that serves as a strut between the shoulder blade and the sternum (breastbone). There are two clavicles, one on each side of the body. The clavicle is the ...
present.
Metatarsal The metatarsal bones or metatarsus (: metatarsi) are a group of five long bones in the midfoot, located between the tarsal bones (which form the heel and the ankle) and the phalanges ( toes). Lacking individual names, the metatarsal bones are ...
I is reduced, II and IV are symmetrical and III is the longest. The
nasals In phonetics, a nasal, also called a nasal occlusive or nasal stop in contrast with an oral stop or nasalized consonant, is an occlusive consonant produced with a lowered velum, allowing air to escape freely through the nose. The vast majorit ...
are long and narrow and pass from near the plane passing through the front of the
orbits In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an satellite, artificia ...
to the extreme anterior points of the
snout A snout is the protruding portion of an animal's face, consisting of its nose, mouth, and jaw. In many animals, the structure is called a muzzle, Rostrum (anatomy), rostrum, beak or proboscis. The wet furless surface around the nostrils of the n ...
. The frontals are large relatively and help to form a small part of the upper orbital margin and a large portion of the upper
temporal fossa The temporal fossa is a fossa (shallow depression) on the side of the skull bounded by the temporal lines above, and the zygomatic arch below. Its floor is formed by the outer surfaces of four bones of the skull. The fossa is filled by the te ...
. The
jugal The jugal is a skull bone found in most reptiles, amphibians and birds. In mammals, the jugal is often called the malar or zygomatic. It is connected to the quadratojugal and maxilla, as well as other bones, which may vary by species. Anatomy ...
is a large bone forming the whole of the lower orbital and almost all of the lower temporal bar. The parietal is a good sized element forming nearly the entire intertemporal region along with the majority of the occiput. There is no parietal foramen present. The dentary is slender and moderate in length, it appears to have roughly fourteen
teeth A tooth (: teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tear ...
with a third of them being larger than the others. Overall the skull is similar to those of crocodiles however it is slightly deeper. From one of the forelimbs it appears that ''Sphenosuchus'' has a
carpus In human anatomy, the wrist is variously defined as (1) the carpus or carpal bones, the complex of eight bones forming the proximal skeletal segment of the hand; "The wrist contains eight bones, roughly aligned in two rows, known as the carpal ...
with the
radius In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is th ...
and
ulna The ulna or ulnar bone (: ulnae or ulnas) is a long bone in the forearm stretching from the elbow to the wrist. It is on the same side of the forearm as the little finger, running parallel to the Radius (bone), radius, the forearm's other long ...
elongated and
metacarpal In human anatomy, the metacarpal bones or metacarpus, also known as the "palm bones", are the appendicular bones that form the intermediate part of the hand between the phalanges (fingers) and the carpal bones ( wrist bones), which articulate ...
like in structure.


Specimens

A new specimen of ''Hesperosuchus agilis'' was found from the
Upper Triassic The Late Triassic is the third and final epoch of the Triassic Period in the geologic time scale, spanning the time between Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch and followed by the Early Jurassic Epoch. T ...
of
New Mexico New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...
. According to Colbert ''Hesperosuchus'' is in a close relationship with ''Sphenosuchus'' and this assessment has been widely accepted since. There are several physical similarities between the two. The
maxilla In vertebrates, the maxilla (: maxillae ) 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 two maxil ...
of the specimen lacks a lateral
lamina Lamina may refer to: People * Saa Emerson Lamina, Sierra Leonean politician * Tamba Lamina, Sierra Leonean politician and diplomat Science and technology * Planar lamina, a two-dimensional planar closed surface with mass and density, in mathem ...
that would conceal the medial wall of the fossa in lateral view similar to ''Sphenosuchus''. The quadrate is similar as well in that it has a distinct lateral ridge along the
anterior Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
margin. Another new specimen of an early
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. Extinct crocodylomorphs were considerably mor ...
was found in
Arizona Arizona is a U.S. state, state in the Southwestern United States, Southwestern region of the United States, sharing the Four Corners region of the western United States with Colorado, New Mexico, and Utah. It also borders Nevada to the nort ...
at the
Petrified Forest National Park Petrified Forest National Park is a national park of the United States in Navajo and Apache counties in northeastern Arizona. Named for its large deposits of petrified wood, the park covers about , encompassing semi-desert shrub steppe as wel ...
from the
Upper Triassic The Late Triassic is the third and final epoch of the Triassic Period in the geologic time scale, spanning the time between Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch and followed by the Early Jurassic Epoch. T ...
. A major
synapomorphy In phylogenetics, an apomorphy (or derived trait) is a novel Phenotypic trait, character or character state that has evolution, evolved from its ancestral form (or Plesiomorphy and symplesiomorphy, plesiomorphy). A synapomorphy is an apomorphy sh ...
shared by the specimen and ''Sphenosuchus'' is the swelling of the anterior end of the
dentary In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone ...
. Parrish and his team constructed 27
parsimonious In philosophy, Occam's razor (also spelled Ockham's razor or Ocham's razor; ) is the problem-solving principle that recommends searching for explanations constructed with the smallest possible set of elements. It is also known as the principle o ...
trees and in every one the new specimen was placed as the
sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...
to ''Sphenosuchus''. The specimen was rather incomplete therefore if more of the fossil is to be found this could be subject to revision. Also there is a slight issue with the synapomorphy that relates the two so closely. The swelling of the denture could be in part an ontogenetic feature. However the lack of this feature in the other specimens up for debate to be related to the specimen suggests that is in fact
phylogenetically In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical data ...
significant.


Classification

In Alick Walker's first paper looking at the origin of
birds Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
and
crocodiles Crocodiles (family Crocodylidae) or true crocodiles are large, semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term "crocodile" is sometimes used more loosely to include all extant member ...
it is suggested that they could in fact have a close common ancestor with ''Sphenosuchus''. The evidence being similarities in several features including: quadrate articulation, inner ear regions, pneumatic spaces connected with the middle ear, the palatal structure, occiput and odd basipterygoid process, and the upper temporal bar positioning in the skull. According to these findings Walker thought that the conclusion that birds and crocodiles could be more closely related than previously thought could not be ignored. In 1990 Walker revised his previous work with a new, more comprehensive paper regarding ''Sphenosuchus'' and the relationship with modern crocodiles and birds. Several homologies were discussed. Main cavities in the crocodilian
skull The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...
are also present in ''Sphenosuchus'', sometimes in a less clearly defined state. Certain
pneumatic Pneumatics (from Greek 'wind, breath') is the use of gas or pressurized air in mechanical systems. Pneumatic systems used in Industrial sector, industry are commonly powered by compressed air or compressed inert gases. A centrally located a ...
spaces in the ''Sphenosuchus'' skulls are not found in modern crocodiles but are of closer resemblance to cavities in the bird skull. The otic capsule of ''Sphenosuchus'' closely resembles those of birds and crocodiles. Modern crocodiles' heads are in the same morphological position as in ''Sphenosuchus'' although reduced in comparison. Due to these homologies Walker concludes that the most important steps of crocodylomorph
evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...
, especially the skull, took place in Sphenosuchia and therefore they should be included in the same taxon as Protosuchia. In 1995 Johann Welman published a paper "Euparkeria and the origin of birds". where he argues that while ''Sphenosuchus'' had been thought to potentially be related to birds he finds the brain case structures of ''Sphenosuchus'' to be too specialized to be the sister group of birds. Another feature he found to be different was the eustachian system. Crocodiles and ''Sphenosuchus'' have a median eustachian system while birds do not. The internal
carotid arteries In anatomy, the left and right common carotid arteries (carotids) () are arteries that supply the head and neck with oxygenated blood; they divide in the neck to form the external and internal carotid arteries. Structure The common carotid ...
in birds run on the lateral sides of the basiparaspehnoid while in ''Sphenosuchus'' arteries are more or less parallel in ventral view. Also in ''Sphenosuchus'' the
facial nerve The facial nerve, also known as the seventh cranial nerve, cranial nerve VII, or simply CN VII, is a cranial nerve that emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of ta ...
exits anteriorly to the otic capsule which is much more derived than birds; birds facial nerve passes between the two parts of the otic capsules. Welman finds that the evidence he presented makes it clear that crocodilomorphs are too specialized in a number of key characteristics to be closely related to birds.Welman, J. "Euparkeria and the origin of birds." South African Journal of Science 91.10 (1995): 533-537.


References

{{Taxonbar, from=Q3725154 Early Jurassic crocodylomorphs Terrestrial crocodylomorphs Jurassic reptiles of Africa Jurassic South Africa Fossils of South Africa Fossil taxa described in 1924 Prehistoric pseudosuchian genera