The Vertebrate Invasion of the Land II: Synapsid amniotes

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Amniota: the most recent common ancestor of mammals and birds and all of its descendants. Amniotes represent a major departure in the evolution of full terrestriality in vertebrates, and are the first vertebrates fully to break the dependance on bodies of water.

  • Synapomorphies:
    Who are the amniotes?

    Note well: In older literature non-mammalian synapsids are often referred to as "mammal-like reptiles." In the age of phylogenetic systematics, this just doesn't fly. Members of Reptilia are, by definition, not synapsids and vice versa. If you use this archaic term on an exam, I will have to suppress the urge to flunk you. Just thought you should know.


    The interval from the Pennsylvanian to early Triassic saw synapsids dominating the world's land faunas. A great diversity of synapsids lived, and inaugurated three major trends in synapsid evol.

    Synapsid diversity:

  • Caseidae: Including Casea and Cotylorhynchus (above) Pennsylvanian age large herbivores with a very broad flat torsos and improbably small heads. Nevertheless, the synapsid synapomorphy is plainly visible.

  • Varanopseidae: Small-bodied, primitive looking synapsid predators (see skull. Namre derives from the general similarity of body profile to modern varanoid (i.e. "monitor") lizards. Unremarkable but for the recent discovery of evidence of parental care of young.

    Dimetrodon: An early large predator and largest representative of Sphenacodontidae (don't worry). We have speculated on the function of the sail. The rest of the animal was a large (up to alligator-sized) predator with skull and teeth adapted for slicing flesh from prey. In this animal we see two of the synapsid trends well under way:

    Therapsida: (Permian - Recent) During the Permian, we start to pick up members of a synapsid group that is more highly derived along the trends that we discussed before, Therapsida. This group has many morphological synapomorphies, but for us, a general trend is enough to note:

    During the middle and late Permian, therapsids replaced the more basal synapsids as the dominant land fauna, both as predators and as herbivores. They encompass much more diversity than we can address, so here are some high-points.:

  • Dicynodontia: (Permian - Triassic) Four facts to note.

    We have considered the biological implications of the case of the snuggling dicynodonts in a previous lecture. They were preyed upon by a range of therapsid predators, including:

  • Gorgonopsia: (Permian).

  • Cynodontia (Permia n- Recent): The Synapsids that had carried the major trends the farthest. Consider the basal cynodont Procynosuchus.

    Procynosuchus marks the starting point of two new trends that mark cynodont evolution.:

    By the Middle Jurassic, any synapsid you saw would be difficult to distinguish from a mammal.

    Mammalia (Cretaceous - Recent) appeared in the Cretaceous.