Synapsid Reptiles and Mammals

    Synapsid reptiles and the mammals that evolved from them form one monophyletic clade.  There are two groups of synapsid reptiles, the pelycosaurs and the therapsids, the latter of which includes an advanced group known as the cynodonts.

PELYCOSAURS

     From anapsid reptiles evolved two groups of higher reptiles: the synapsids (which would lead to mammals) and the diapsids (which would lead to crocodiles, pterosaurs, dinosaurs, and birds).  Although the diapsids would produce many successful lineages which would reach enormous size during the Mesozoic, the earliest diapsid reptiles were small reptiles that probably fed on insects.  In contrast, the earlier synapsids were the dominant carnivores and herbivores of their time, unlike the mouse- to opossum-sized mammalsof the Mesozoic.   The first known pelycosaurs were large reptiles with large skulls and large canine teeth.  These pelycosaurs were adapted to eating large prey and were the first carnivorous amniotes (Kemp, 1982; Carroll, 1988). Synapsids possessed both mammalian and reptilian traits.

(Kemp, 1982; Carroll, 1988). 

     Although pelycosaurs were the earliest “mammal-like reptiles”, there are only a few characteristics which have been modified from the ancestral condition of the anapsid reptiles.  The most prominent are the synapsid opening and the presence of canines.

     The first pelycosaur,  Archaeothyris, would have resembled a large iguana with short limbs.  There is an earlier species Protoclepsydrops known only from a few bone fragments.  Like early anapsid reptiles, Archaeothyris was a sprawling reptile and it was closely related to the first anapsid reptiles, the captorhinomorphs.  Captorhinomorphs had a few derived characteristics compared to synapsids such as a reduced tabular bone; supratemporal and postorbital bones which don’t contact each other, a single coronoid bone in lower jaw as opposed to two in synapsids, and lacked the large medial centrale bone in the foot which synapsids possessed.   These characteristics of the earliest known reptiles which are more advanced than the condition of pelycosaurs indicate that the pelycosaurs evolved from a slightly more primitive but as yet unknown group of anapsid reptiles.  There is an early reptile, Promeriscus from the Upper Carboniferous, in which the supratemporal and postorbital bones contact each other as in pelycosaurs.  Although Promeriscus lacks the temporal fenestra that gives the synapsids their name, there is an open suture (a line of weakness) in this same region.  Many amphibians have this characteristic as well and it is thought that this open suture was a remnant of a movable hinge from the skull of crossopterygians fish (Kemp, 1982; Carroll, 1988).  .

      Archaeothyris measured about 50 cm and possessed the single temporal fenestra (with the postorbital and squamosal bones above and the jugal bone below) which would characterize the synapsids.  It had a long snout and the region of the skull behind the eye socket was short.  The canine teeth were slightly larger than the others.  Archaeothyris was thoroughly reptilian with a brain about the average size of a reptile’s brain.

 All known groups of pelycosaurs appeared before the end of the Carboniferous and they, not the anapsid or diapsid reptiles, are the dominant terrestrial vertebrates of the Permian.  All three suborders of pelycosaurs evolved from forms similar to Archaeothyris.

1)     Ophiacodontia

The ophiacodonts were the least modified reptiles and include the earliest pelycosaurs and some fish-eating species of the Permian.  The vertebral column retained the primitive condition of intercentra between the neighboring pleurocentra.  The ribs possessed 2 heads, there were 2 sacral ribs,  the ilium was not expanded, the skull was low, and snout was long and narrow.  Eothyris had enlarged canines.  Ophiacodon could reach over 4 meters in length.  Because wrist and foot bones of Ophiacodon don’t ossify well, it might have been aquatic (Kemp, 1982; Carroll, 1988). Ianthasaurus was a small, primitive pelycosaur which developed a sail, measured only 60 cm, and probably fed on insects (Czerkas, 1990).

    The sphenacodonts were the advanced pelycosaurs from which the therapsids evolved.  There were a number of modifications in sphenacodonts that would be important for later therapsids and mammals.  Their skull was deeper, their canines were enlarged, and there was increase in jaw musculature.  The angular bone of lower jaw was modified,  the occipital region solidified and could receive stresses from jaw musculature, the intercentra of vertebrae were reduced, the limbs were elongated, the ilium was expanded, the supratemporal bone of the skull was reduced, and the frontal bone of the skull forms part of orbit (eye socket).

     All sphenacodonts had tall neural spines on their vertebrae and this reached extreme lengths in species such as Dimetrodon.  The sail made of these neural spines may have functioned in both thermoregulation and display.  Sphenacodonts still retained the  pineal opening of the skull and the ribs still had 2 heads.  Haptodus of early Permian still retains most of ophiacondont characteristics (Kemp, 1982; Carroll, 1988).