Sphenosuchia

Sphenosuchia Temporal range: Late Triassic - Late Jurassic, 228–152  Ma PreꞒ Ꞓ O S D C P T J K Pg N
Life restoration of Hesperosuchus agilis
Skeletal diagram of Pseudhesperosuchus
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Archosauria
Clade:	Pseudosuchia
Clade:	Crocodylomorpha
Informal group:	† Sphenosuchia von Huene, 1942
Genera
See below
Synonyms
Pedeticosauria Walker, 1968

Sphenosuchia is a grouping of basal crocodylomorphs that first appeared in the Triassic and persisted into the Jurassic. Most were small, gracile animals with an erect limb posture. Although sometimes recovered as a monophyletic group, they are now generally thought to represent various early offshoots of the lineage leading to Crocodyliformes, with some sphenosuchians more closely related to crocodyliforms than to other "sphenosuchians", rendering the group paraphyletic.

Stratigraphic range

The earliest known members of the group (i.e. Hesperosuchus ) are early Norian in age, found in the Blue Mesa Member of the Chinle Formation. Only one sphenosuchian is currently known from the Middle Jurassic, Junggarsuchus , from the Junggar Basin (Shishugou Formation) of China during either the Bathonian or the Callovian (~165 Ma) age, ^[1] and the Hallopodidae are known from the Late Jurassic of North America. ^[2]

Phylogeny

The monophyly of the group is debated, although several synapomorphies characterize the clade, including extremely slender limbs, a compact carpus and an elongate coracoid process.

In 2002, Clark and Sues found a possible sphenosuchian clade of Dibothrosuchus , Sphenosuchus , and possibly Hesperosuchus and Saltoposuchus , with several other genera in unresolved positions ( Kayentasuchus , Litargosuchus , Pseudhesperosuchus , and Terrestrisuchus ). ^[3] More recently, however, Clark et al. (2004) argued for the paraphyly of the group, contending that morphological characters were secondarily lost in more highly derived crocodylomorphs. ^[1] Further analysis and study is required before the group's monophyly is resolved with certainty — a perfect phylogenetic analysis is, at present, impossible due to a paucity of fossil remains demonstrating phylogenetically informative characters.

Below is a cladogram modified from Nesbitt (2011). ^[4] Sphenosuchians are marked by the green bracket.

Crocodylomorpha CM 73372 Hesperosuchus Dromicosuchus Sphenosuchus Dibothrosuchus Terrestrisuchus Litargosuchus Kayentasuchus Crocodyliformes Orthosuchus Protosuchus Alligator

Sphenosuchians

Genera

Genus	Status	Age	Location	Unit	Notes	Images
Dibothrosuchus	Valid	Early Jurassic	China	Lower Lufeng Series
Dromicosuchus	Valid	Late Triassic	USA	Newark Supergroup
Dyoplax [5]	Valid	Late Triassic (Carnian)	Germany	Schilfsandstein Formation	A possible sphenosuchian; [5] alternatively, it could be an erpetosuchid. [6]
Hesperosuchus	Valid	Late Triassic (Carnian)	USA	Chinle Formation
Junggarsuchus	Valid	Middle Jurassic	China	Shishugou Formation
Kayentasuchus	Valid	Early Jurassic (Sinemurian - Pliensbachian)	USA	Kayenta Formation
Litargosuchus	Valid	Early Jurassic	South Africa	Elliot Formation
Parrishia	Nomen dubium	Late Triassic	USA	Dockum Group	An indeterminate sphenosuchian known only from undiagnostic vertebrae [7]
Phyllodontosuchus [8]	Valid	Early Jurassic	China	Lower Lufeng Series
Pseudhesperosuchus	Valid	Late Triassic (Norian)	Argentina	Los Colorados Formation
Redondavenator	Valid	Late Triassic	USA	Redonda Formation
Saltoposuchus	Valid	Late Triassic (Norian)	Germany   Switzerland UK	Löwenstein Formation Trossingen Formation Lossiemouth Sandstone
Sphenosuchus	Valid	Early Jurassic	South Africa	Elliot Formation
Terrestrisuchus	Valid.	Late Triassic	UK
Trialestes	Valid	Late Triassic	Argentina	Ischigualasto Formation

References

1. Clark, J.M., et al. (2004).A Middle Jurassic 'sphenosuchian' from China and the origin of the crocodylian skull Nature430:1021-1024.
2. Leardi, Juan Martin; Pol, Diego; Clark, James Matthew (2017-01-19). "Detailed anatomy of the braincase of Macelognathus vagans Marsh, 1884 (Archosauria, Crocodylomorpha) using high resolution tomography and new insights on basal crocodylomorph phylogeny". PeerJ. 5 e2801. doi: 10.7717/peerj.2801 . ISSN   2167-8359. PMC   5251941 . PMID   28133565.
3. Clark, James M.; Sues, Hans-Dieter (2002). "Two new basal crocodylomorph archosaurs from the Lower Jurassic and the monophyly of the Sphenosuchia". Zoological Journal of the Linnean Society. 136: 77–95. doi: 10.1046/j.1096-3642.2002.00026.x .
4. Nesbitt, S.J. (2011). "The early evolution of archosaurs: relationships and the origin of major clades". Bulletin of the American Museum of Natural History. 352: 1–292. doi: 10.1206/352.1 . hdl:2246/6112. S2CID   83493714.
5. Lucas, S. G.; Wild, R.; Hunt, A. P. (1998). "Dyoplax O. Fraas, a Triassic sphenosuchian from Germany". Stuttgarter Beiträge zur Naturkunde, B. 263: 1–13.
6. Michael W. Maisch; Andreas T. Matzke; Thomas Rathgeber (2013). "Re-evaluation of the enigmatic archosaur Dyoplax arenaceus O. Fraas, 1867 from the Schilfsandstein (Stuttgart Formation, lower Carnian, Upper Triassic) of Stuttgart, Germany". Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen. 267 (3): 353–362. doi:10.1127/0077-7749/2013/0317.
7. Clark, J. M.; Sues, H.-D.; Berman, D. S. (2001). "A new specimen of Hesperosuchus agilis from the Upper Triassic of New Mexico and the interrelationships of basal crocodylomorph archosaurs". Journal of Vertebrate Paleontology. 20 (4): 683–704. doi:10.1671/0272-4634(2000)020[0683:ANSOHA]2.0.CO;2. S2CID   7455338.
8. Harris, Jerald D.; Lucas, Spencer G.; Estep, J. W.; Jianjun Li (2000). "A new and unusual sphenosuchian (Archosauria: Crocodylomorpha) from the Lower Jurassic Lufeng Formation, People's Republic of China". Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen. 215 (1): 47–68. doi:10.1127/njgpa/215/2000/47.