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Comprehensive Description


The earliest synapsids were probably ectothermic (their body temperature depended on the environment), as shown by the presence of a large "sail" on the back of edaphosaurids and some sphenacodontids. This sail was composed of greatly elongated neural spines that was grooved anteriorly and posteriorly to accommodate blood vessels, and skin probably connected all the neural spines together. This sail may have allowed these early synapsids to raise their body temperature faster than similarly-sized tetrapods lacking a sail. It was also thought that the sail was used to radiate excess heat into the environment, but recent studies suggest otherwise (Haack, 1986). The presence of the sail suggests that early synapsids lacked hair. Hair usually does not fossilize, but its presence in most monotremes, marsupials, and placentals suggests that it was present in their last common ancestor. Endothermy is also difficult to determine in extinct taxa, but it probably appeared slightly before the first mammal was born. The presence of epidermal horny scales cannot be determined in most fossil taxa, but bony ventral scales were primitively present, as shown by their presence in most reasonably complete fossils of early synapsids.


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All known early synapsids had a lateral temporal fenestra (a hole in the lateral surface of the skull behind the orbit) that is still present in a modified form in mammals. Primitively, the lateral temporal fenestra was bordered by only three bones (Fig. 2).

Figure 2. Temporal fenestration primitively found in synapsids.
Abbreviations: J, jugal; Po, postorbital; Sq, squamosal. Scale equals 1 cm.

The occiput was broad and it faced posterodorsally. In other early amniotes, the endochondral bones of the braincase formed a narrower surface, and it was more or less vertical.

The pubis had a long, ridged anterodorsal edge.


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Evolution and Systematics


Discussion of Phylogenetic Relationships

View Synapsida Tree

Phylogeny of synapsids, after Reisz (1986) and Laurin (1993). Terminology after Kemp (1982), Reisz (1986), and Reisz et al. (1992).

Early synapsid phylogeny is relatively uncontroversial. Recent studies by Reisz (1980, 1986), Brinkman and Eberth (1983), Gauthier et al. (1988), and Laurin (1993) have resolved the relationships between the largest groups of early synapsids. More recent studies by Reisz et al. (1992) and Berman et al. (1995) have confirmed these findings.

Reisz (1980) produced the first phylogeny of synapsids using phylogenetic systematics. He argued that most carnivorous synapsids (ophiacodontids, varanopseids, sphenacodontines, and therapsids formed a clade that excluded the herbivorous edaphosaurids and caseids.

Brinkman and Eberth (1983) argued that varanopseids were closely related to caseids and that edaphosaurids were closely related to sphenacodontids:

Reisz (1986) agreed with Brinkman and Eberth (1983) that edaphosaurids were closely related to sphenacodontids, but he argued that varanopseids were more closely related to ophiacodontids than to caseids:

This new position of varanopseids has not been directly questioned by subsequent studies. The phylogeny of Gauthier et al. (1988) resembled that of Brinkman and Eberth (1983), but they did not test the monophyly of the varanopseid-caseid clade.

Gauthier et al. (1989), Hopson (1991), and Reisz et al. (1992) suggested that Haptodus, previously believed to be a sphencodontid, did not belong to this group:

Laurin (1993) studied the relationships of several problematic taxa previously called haptodontines. He suggested that haptodontines were a paraphyletic group:

See the page on Evolution of early synapsids for synapomorphies justifying the latest phylogenies and for anatomical drawings.


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