''Mesosuchus'' ("middle crocodile") is an
extinct genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...
of
basal
Basal or basilar is a term meaning ''base'', ''bottom'', or ''minimum''.
Science
* Basal (anatomy), an anatomical term of location for features associated with the base of an organism or structure
* Basal (medicine), a minimal level that is nec ...
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. Rhynchosaur ...
from early
Middle Triassic
In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided in chronostratigraphy. The Middle Triassic spans the time between Ma ...
(early
Anisian
In the geologic timescale, the Anisian is the lower stage or earliest age of the Middle Triassic series or epoch and lasted from million years ago until million years ago. The Anisian Age succeeds the Olenekian Age (part of the Lower Trias ...
stage) deposits of Eastern
Cape
A cape is a clothing accessory or a sleeveless outer garment which drapes the wearer's back, arms, and chest, and connects at the neck.
History
Capes were common in medieval Europe, especially when combined with a hood in the chaperon. T ...
,
South Africa
South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring count ...
. It is known from the
holotype
A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of seve ...
SAM
Sam, SAM or variants may refer to:
Places
* Sam, Benin
* Sam, Boulkiemdé, Burkina Faso
* Sam, Bourzanga, Burkina Faso
* Sam, Kongoussi, Burkina Faso
* Sam, Iran
* Sam, Teton County, Idaho, United States, a populated place
People and fictional ...
5882, a partial
skeleton
A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside ...
, 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
David Meredith Seares Watson in 1912.
Fossil discovery
SAM 5882, the
holotype
A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of seve ...
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 nor a syntype). O ...
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.
[ ]
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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
In phylogenetics, an autapomorphy is a distinctive feature, known as a derived trait, that is unique to a given taxon. That is, it is found only in one taxon, but not found in any others or outgroup taxa, not even those most closely related to t ...
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 group first appeared about three hundred million years ...
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 (, pl. 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 togethe ...
. 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
''Howesia'' is an extinct genus of basal rhynchosaur from early Middle Triassic (early Anisian stage) deposits of Eastern Cape, South Africa. It is known from the holotype SAM 5884, a partial skeleton with palate and partial lower jaws and from ...
'', 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 adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthp ...
.
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'', 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.
Discovery & etymology
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 Mr
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 new species which he named ''Mesosuchus browni''. As the
etymology
Etymology () The New Oxford Dictionary of English (1998) – p. 633 "Etymology /ˌɛtɪˈmɒlədʒi/ the study of the class in words and the way their meanings have changed throughout time". is the study of the history of the form of words ...
of the name suggests, Watson believed that ''Mesosuchus'' was an ancestral crocodile with close affinities to other presumed primitive crocodilians such as ''
Proterosuchus'', ''
Erythrosuchus'', and ''
Ornithosuchus''. However, in 1913,
Robert Broom
Robert Broom FRS FRSE (30 November 1866 6 April 1951) was a British- South African doctor and palaeontologist. He qualified as a medical practitioner in 1895 and received his DSc in 1905 from the University of Glasgow.
From 1903 to 1910, ...
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
''Euparkeria'' (; meaning "Parker's good animal", named in honor of W.K. Parker) is an extinct genus of archosauriform from the Middle Triassic of South Africa. It was a small reptile that lived between 245-230 million years ago, and was close to ...
''. Looking back, we see the confusion Watson had when giving the name ''Mesosuchus'' as the data for his opinion were derived from the skeletons of ''Euparkeria'', in particular the slender lower jaw with thecodont implantation, the crocodilian-like ilium, and the construction of the tarsus and pes.
References
{{Taxonbar, from=Q1186485
Rhynchosaurs
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