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SYNAPSIDA | THERAPSIDA |--Biarmosuchia `--+--Dinocephalia | |--Anteosauria | `--Tapinocephalia `--Neotherapsida |--Anomodontia | |--Venyukovioidea | `--+--Dromasauria | `--Dicynodontia `--Theriodontia |--Gorgonopsia `--+--Therocephalia `--CYNODONTIA
One of the three major evolutionary lines (clades) deriving from the early Therapsida (the biarmasuchia - phthinosuchian - Eotitanosuchia base), were the anomodonts, mostly toothless herbivores. These developed at first (during the Wordian and Capitanian epochs) along several paths of adaptive radiation, such as the Venyukovioidea, the Dromasauria, and the Eodicynodont, Endothiodont. and the higher dicynodonts. Of those groups only the higher Dicynodontia were to be successful, and in fact these stocky, toothless and beaked animals remained the dominant terrestrial herbivores right up until the Carnian epoch (Late Triassic period).
The venyukovioid, represented by a handful of genera, seem to be intermediate in position between the ancestral therapsid condition and the dicynodonts. In these therapsids, as exemplified by the genus Venyukovia from the Capitanian (middle Permian) of Russia, there was a reduction in the dentition, and the skull exhibited changes in proportions that were clearly related to the highly evolved skull in the dicynodonts
Very different at first glance are the Dromasaurs. These are small therapsids, with a slight build, slender legs and long tail. The skull is short, with very large orbits (eye sockets). It has been suggested that these animals were actually juveniles, but no adult forms have been found, and it is more likely that they simply resepresent part of the early anomodont radiation.
Note by Christian Kammerer: Dromasaurs are undoubtedly an artificial group, and they are actually quite anomodontian if you consider venyukoviamorphs typical basal anomodonts. Actually, the most basal "dromasaurs" may form a clade (Galechiridae) consisting of Galechirus, Galepus, and Patranomodon, but Galeops is probably even closer to the dicynodonts than several "venyukoviamorphs". Several wonderful new basal anomdonts (for example Suminia, Anomocephalus) have been described recently and greatly elucidate basal anomodont relationships. Modesto et al. (1999).
Neotherapsida: Anomodontia + Theriodontia.
Range: from the Middle Permian.
Phylogeny: Eutherapsida: Dinocephalia + *: Anomodontia + Theriodontia.
Characters: $ ventrally expanded squamosal [RS01]; temporal fenestra further enlarged; $ squamosal covers most of quadrate and quadratojugal in posterior view [RS01]; $ quadrate not sutured to skull; $ epipterygoid broadly contacts parietal [RS01]; coronoid shortened below tooth row; $ atlantal epipophyses present [RS01] (no, I don't know exactly what this means); $ large obturator process present on suture between pubis & ischium [RS01].
References: Rubidge & Sidor (2001) [RS01] ATW020219.
Anomodontia: Dicynodonts and relations. Anomocephalus, Patranomodon, Venjukovia.
Range: Middle Permian to Late Triassic worldwide.
Phylogeny: Neotherapsida: Theriodontia + *: Venyukovioidea + (Dromasauria + Dicynodontia).
Characters: Dominated Late Permian terrestrial vertebrate fauna at all size ranges. Only herbivorous therapsid group to survive PermoTriassic extinction; no teeth except maxillary fangs; beak presumably present; jaw articulation allowed anterior-posterior movement of jaw for slicing; long posterior process of premaxilla; nares not terminal; coronoid bone lost; masseter muscle probably absent or poorly developed, so that jaw was closed by very large adductors originating on sagittal crest and on strongly developed, dorsally bowed, zygomatic arch (squamosal + jugal); secondary palate (convergent with mammals), suggesting possible homeothermy and, perhaps, hair; temporal openings extend posterior to occipital condyle, creating central sagittal crest from parietals; post-orbital skull generally light and formed of arches; typical therapsid semi-sprawling posture with short heavy femur and humerus; reduction or loss of internal trochanter of femur.
Image: Dicynodon trautscholdi modified from More Dicynodon trautscholdi, courtesy of mathematical.com, and from a similar figure in Carroll (1988).
Note how the adductors operate almost horizontally to both close the jaw and pull it back in a slicing and clipping motion. This is characteristic of all anomodonts. ATW020526.
Venyukovioidea: Venjukovia, Otsheria, Ulemica, Suminia. Likely paraphyletic basal anomodonts, including dicynodonts.
Range: Middle Permian (to Late Triassic assuming paraphyly).
Phylogeny: Anomodontia: (Dromasauria + Dicynodontia) + *.
Characters: skulls small & short [C88]; premaxilla forms broad shelf anterior to nares [C88]; zygomatic arch high & thin, exposing adductor muscles broadly between arch & lower jaw [C88]; postorbital contacts squamosal [C88]; temporal fenestrae extended posteriorly beyond occipital condyle, but not as far dorsally as in dinocepahlians [C88]; palate with moveable basicranial articulation [C88]; pterygoid has distinct transverse flange, but without denticles [C88]; lower jaw with coronoid absent [C88]; dentaries may be fused at symphysis [C88].
References: Carroll (1988) [C88]. ATW020525.
Dromasauria: again, likely paraphyletic. Galechirus, Galeops, Galepus
Range: Middle Permian of South Africa.
Phylogeny: Anomodontia:: Dicynodontia + *.
Characters: small & slightly built; skull short; orbits very large; postorbital bar narrow [C88]; postorbital does not contact squamosal [C88]; high narrow zygomatic arch [C88]; temporal fenestrae open dorsally [C88]; squamosal narrow [C88]; partial secondary palate formed by premaxillae [C88]; outer surface of mandible grooved (insertion of external mandibular adductor?) [C88]; canines reduced [C88]; premaxillary teeth reduced in number [C88]; slender legs; long tail.
Links: NAPC Abstracts, Ka - Ku (Kurkin abstract); ANNOTATIONS 52 (Meyer abstract).
References: Carroll (1988) [C88]. ATW020913.
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