| SYSTEMATIC BIOLOGY |
|
| CLASS PROAMNIOTA+[i] |
Proamniota (pro-am-ne-O-ta) is coined from two Greek roots meaning before the membrane [before -pro (προ); and membrane -amen (υμήν)]. In this case, I coined the name to mean before the amnion (αμνίον), the specialized membrane in the cleidoic egg that allows the exchange of gasses (O2 and CO2) without losing water.
Benton (2005) grouped the Lepospondyli and Reptilomorpha together as classes in an unnamed class that I call Proamniota (see the footnote on this). He defined the clade with the following characters: both premaxillae make up less than 2/3 of the skull width, the vomers are elongate and strip-like, and the tarsus has an L-like process at the proximal end. Evidence is that they were aquatic, semi-aquatic, or even mainly terrestrial, but they all had larval forms, that presumably were aquatic. The clade is paraphyletic; however, because it includes also the sister group to all of the amniotes (mammals, birds, reptiles). Thus is the problem with stem groups. In a strict cladistic hierarchy the Proamniotes would rank as high as all of the amniotes. I choose to treat them as a separate stem group and acknowledge that the sister to the amniotes lies within it.
| HIERARCHICAL TAXONOMY OF THE CLASS PROAMNIOTA. The following descriptions come from Benton (2005). The structure of the following system is also based, in part, on the phylogenetic treatment by Mikko's Phylogeny Web (from the Finnish Museum of Natural History). Taxa in red are extinct. |
|
Diplocaulus (Left), a paedomorphic Lepospondyl, and Seymouria (Right), a Reptilomorph that was very similar to the earliest amniotes. Drawing of Diplocaulus from: http://macroevolution.narod.ru/vertebrates/nectridia2.gif Drawing of Seymouria from: http://www.bertsgeschiedenissite.nl/geschiedenis%20aarde/reptilomorpha.htm CLASS PROAMNIOTA[i]+ (LEPOSPONDYLI + REPTILOMORPHA) This is a taxonomy of two Subclasses, each with three orders. Reptilomorpha is the sister group to the amniotes. The synapomorphies that define this group include the loss of the supratemporal, and the vertebrae are cylindrical pleirocentra. ORDER MICROSAURIA Small and mainly terrestrial animals, almost lizard-like. Some show secondary adaptations to water and others to burrowing in soil or leaf litter. Carboniferous to lower Permian Microbrachis, Tuditanus ORDER
NECTRIDEA Mainly
aquatic and newt-like. Some
developed large lateral wing-like extensions of their skulls that enlarged as
they aged. Upper
Carboniferous (Pennsylvanian) to Permian Sauropleura, Diplocaulus, Diploceraspis ORDER
AISTOPODA Very
similar to snakes, the aistopods had many vertebrae and no limbs or limb girdle
elements. Also, they seem to have
been able to open their jaws very wide through extra joints in the skull. Carboniferous
and lower Permian Aornerpeton SUBCLASS REPTILIOMORPHA (SISTER TO THE AMNIOTES)+ The synapomorphies for this subclass include premaxillae less than 1/2 of the skull width, and the vomers taper forward (Benton, 2005). They dominated terrestrial habitats from the Carboniferous to the Permian. This group also included the first known tetrapod herbivore. ORDER
ANTHRACOSAURIA Fish
eating animals with elongate skulls, which seem to have been able to flex when
opening the jaws. It had large
vertebrae and a flat tail that could have been useful for swimming.
Some had legs that were well developed and suited to rapid terrestrial
movement, but later forms had reverted to a more aquatic existence.
Lower
Carboniferous to Upper Carboniferous Proterogyrinus, Pholiderpeton ORDER
SEYMOURIAMORPHA The
seymouriomorphs were a diverse and powerful group of animals.
The terrestrial forms held their bodies high off the ground.
Aquatic forms had broad skulls and fed on fish. Upper Carboniferous
(Pennsylvanian) to Upper Permian Seymouria ORDER
DIADECTOMORPHA These
were very close to the origin of the amniotes.
Members of this group were among the first herbivorous tetrapods.
One, Diadectes, was adapted to
herbivory by having anterior nipping teeth and posterior crushing teeth.
These animals were heavily built with very strong legs and limb girdles.
Upper
Carboniferous (Pennsylvanian) to Lower Permian Diadectes [i]
|
