''Mesosuchus'' ("middle 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 basal

rhynchosaur 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 ...

from early

Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epoch (geology), epochs of the Triassic period (geology), period or the middle of three series (stratigraphy), series in which the Triassic system (stratigraphy), system is di ...

(early

Anisian In the geologic timescale, the Anisian is the lower stage (stratigraphy), stage or earliest geologic age, age of the Middle Triassic series (stratigraphy), series or geologic epoch, epoch and lasted from million years ago until million years ag ...

stage) deposits of Eastern

Cape A cape is a clothing accessory or a sleeveless outer garment of any length that hangs loosely and connects either at the neck or shoulders. They usually cover the back, shoulders, and arms. They come in a variety of styles and have been used th ...

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 ...

. It is known from the

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

SAM 5882, a partial

skeleton A skeleton is the structural frame that supports the body of most animals. There are several types of skeletons, including the exoskeleton, which is a rigid outer shell that holds up an organism's shape; the endoskeleton, a rigid internal fra ...

, and from the paratypes SAM 6046, SAM 6536, SAM 7416 and SAM 7701 from the Aliwal North ''Euparkeria'' site. ''Mesosuchus'' is quite small, spanning around 30 cm in length. ''Mesosuchus'' was discovered and named by D. M. S. Watson in 1912.

Discovery and naming

Bones of ''Mesosuchus'' were first found by David Meredith Seares Watson in 1912 after examining a block of sandstone kept in a private collection of Alfred Brown. This block was found in the middle deposits of the Burgersdorp Formation, in the ''Cynognathus'' Assemblage Zone near the town of Aliwal North in the Cape Province of South Africa. The block of sandstone contained many intermingled partial skeletons of several small reptiles, and after careful sorting, Watson considered the unidentified skeletons to belong to a single species, which he named ''Mesosuchus browni''. As the

etymology Etymology ( ) is the study of the origin and evolution of words—including their constituent units of sound and meaning—across time. In the 21st century a subfield within linguistics, etymology has become a more rigorously scientific study. ...

of the name suggests, Watson believed that ''Mesosuchus'' was an ancestral crocodile with close affinities to other presumed primitive crocodilians such as '' Proterosuchus'', ''

Erythrosuchus ''Erythrosuchus'' (from , 'red' and , 'crocodile') is an extinct genus of archosauriform reptiles from the early Triassic of South Africa. Remains have been found from the Cynognathus Assemblage Zone of the Beaufort Group in the Karoo of South ...

'', and '' Ornithosuchus''. However, in 1913,

Robert Broom Robert Broom Fellow of the Royal Society, FRS FRSE (30 November 1866 6 April 1951) was a British- South African medical doctor and palaeontologist. He qualified as a medical practitioner in 1895 and received his DSc in 1905 from the University ...

looked more closely at the partial skeletons and immediately determined it to be in fact skeletons of two distinct, though related, species. Broom designated an articulated skeleton with a single external naris and a pair of supposed acrodont premaxillary teeth as the type of ''Mesosuchus'', and the remainder of the specimens were assigned to a new genus and species, '' Euparkeria capensis''.

Fossil specimens

SAM 5882, the

for ''Mesosuchus'', consists of a partial rostrum, palate, braincase, lower jaws, sections of articulated presacral vertebral column, nine articulated caudal vertebrae, portions of scapula and pelvic girdle, and partial forelimb and hindlimbs. SAM 6046, one of the

paratype In zoology and botany, a paratype is a specimen of an organism that helps define what the scientific name of a species and other taxon actually represents, but it is not the holotype (and in botany is also neither an isotype (biology), isotype ...

s of ''Mesosuchus'', consists of an incomplete right maxilla, an articulated series of the last ten presacrals, both sacrals, and first six caudals, partial forelimbs, left and right pelvic girdles, right hind limb, as well as element of left tarsus. SAM 6536, another paratype, consists of a virtually complete skull with lower jaws, articulated cervical vertebrae and ribs, dorsal vertebrae and ribs, complete left scapulocoracoid and partial right scapula, interclavicle, clavicles, distal end of left humerus, and gastralia. This article contains quotations from this source, which is available under th
Creative Commons Attribution 4.0 International (CC BY 4.0)
license. SAM 7416, another paratype, consists of an articulated vertebral column composed of the last dozen presacrals, both sacrals and at least the first 15 caudal vertebrae, fragments of right forelimb, pelvic girdle, complete right femur, right crus and partial left crus, and right and left tarsi and pedes.

Description

Diagnosis

All diagnostic traits of ''Mesosuchus'' come from the diagnosis of ''Mesosuchus browni'', as it is the only known species. Most of the autapomorphies used to identify ''Mesosuchus browni'' pertain to the skull due to the relatively large amount of skull material known. It was a small rhynchosaurian

diapsid Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The earliest traditionally identified diapsids, the araeosc ...

with multiple rows of maxillary and dentary teeth, with each row consisting of only a small number of teeth. The two premaxillary teeth are approximately twice the size of the maxillary teeth. The maxillary teeth are inset medially and project below the internal naris. There is also an occlusion between the vomerine teeth and dentary teeth with saddle-shaped vomers that overhang the dorsal side of the premaxillary

symphysis A symphysis (, : symphyses) is a fibrocartilaginous fusion between two bones. It is a type of cartilaginous joint, specifically a secondary cartilaginous joint. # A symphysis is an amphiarthrosis, a slightly movable joint. # A growing together o ...

. Looking at the spine, the length of the axis neural spine is greater than the length of axis centrum. It also has a craniocaudally narrow neural spine of the third cervical and a prominent midventral groove on the first two caudal centra.

Skull

The skull of ''Mesosuchus'' has a broadly triangular shape with a wide temporal region that tapers sharply along the orbits, expands abruptly at the prefrontals, and then tapers to the blunt rostrum. There is a large, median external naris located at the front of the rostrum that faces dorsally and cranially. The orbits face laterally and slightly cranially. One feature on the skull that can distinguish the ''Mesosuchus'' from all other rhynchosaurs, with possible exception of '' Howesia'', is the presence of a beak-like rostrum, formed primarily by huge premaxillary teeth rather than by tapering, edentulous premaxillae. The palate has a pronounced, sagittally elongated vault that is formed primarily by the vomers. The vomers contact the premaxillae at a level with the maxillary tooth margin towards the tip of the rostrum. However, it does curve strongly dorsally towards the palatines, and so we see the choanae very far above the tooth margin. To withdraw the choana further from the oral cavity, the vomerine and palatine borders of the choana are recessed dorsally. On the maxilla, there is a distinct notch that contributes most of the border of a dorsal fenestra. It appeared to be the cranialmost of a series of maxillary foramina that extended across the lateral face of the maxilla. This most likely hinted to cutaneous blood vessels and nerves in that area. This notch is hidden by an overlap of the premaxilla by the maxilla. The postorbital is composed of the anteroventral, posterodorsal, and dorsomedial processes in equal length. The anteroventral process does not come into contact with the ectopterygoid but it does indeed reach farther down the medial side of the jugal. The posterodorsal process forms a great portion of the smooth cranial rim of the upper temporal fenestra and contacts the parietal below the postfrontal. Information of the lower jaw had to be put together from the pieces of the different SAM fossils as there is not one that had a complete lower jaw. When looking at the different parts however, there are indications of at least six elements present in the lower jaw of the ''Mesosuchus'': dentary, splenial, surangular, angular, prearticular, and articular.

Dentition

Evident on SAM 5882, 6046, and 6536 was a clear line of separation between the tooth and surrounding bone indicating that the teeth are implanted in the jaw, which went against the previous idea that the marginal teeth of ''Mesosuchus'' were ankylosed to the jaws. Although there are indications of deep implantation, it is still uncertain whether the mode of attachment could be described as thecodont. Looking at the maxillary teeth, a circular cross-section is maintained throughout the teeth and the majority are worn out heavily to short, blunt pegs. This suggests that ''Mesosuchus'' is in fact an

herbivore A herbivore is an animal anatomically and physiologically evolved to feed on plants, especially upon vascular tissues such as foliage, fruits or seeds, as the main component of its diet. These more broadly also encompass animals that eat ...

. In 1963, M.E. Malan observed an interesting pattern in the positioning of the maxillary and dentary teeth. The middle section, where the medial expansion of the maxilla is wideset, had a zig-zag arrangement of teeth while the first and last four teeth are aligned in a row that is parallel to the maxilla. In the dentary, a simpler arrangement with only a slight zig-zag arrangement. Comparing this feature to ''

Captorhinus aguti ''Captorhinus'' (from , 'to gulp down' and , 'nose') is an extinct genus of Captorhinidae, captorhinid reptiles that lived during the Permian period. Its remains are known from North America (Oklahoma, Texas) and possibly South America. Descr ...

'', which also possessed a zig-zag pattern of teeth, they hypothesized that ''Mesosuchus'' could have had multiple teeth on at least the maxilla. This hypothesis was supported when examining the erosional features on SAM 6536.

Phylogeny

''Mesosuchus'' in a

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

based on Ezcurra ''et al.'' (2016):

References

{{Taxonbar, from=Q1186485 Rhynchosauria Prehistoric reptile genera Anisian genera Middle Triassic reptiles of Africa Triassic South Africa Fossils of South Africa Fossil taxa described in 1912 Taxa named by D. M. S. Watson