13 Mammalian Explosion
From Amniote to Synapsid to Mammals
Of all the great transitions between major structural grades within vertebrates, the tran-
sition from basal amniotes to basal mammals is represented by the most complete and
continuous fossil record, extending from the Middle Pennsylvanian to the Late Triassic
and spanning some 75 to 100 million years.
—James Hopson, “Synapsid Evolution and the
Radiation of Non-eutherian Mammals”Of all the transitional series that we have examined between major groups of verte-
brates, one of the best documented is the transition from primitive amniotes to mammals via
the synapsids, formerly known as the “mammal-like reptiles.” As we explained previously
(fig. 11.1), however, the synapsids that evolve into mammals are not reptiles and never had
anything to do with the lineage that leads to reptiles. Both the earliest true reptiles (Westlo-
thiana from the Early Carboniferous—fig. 11.4) and the earliest synapsids (Protoclepsydrops
from the Early Carboniferous and Archaeothyris form the Middle Carboniferous) are equally
ancient, demonstrating that their lineages diverged at the beginning of the Carboniferous.
Older pre-cladistic interpretations had synapsids evolving from a paraphyletic wastebasket
of primitive amniotes known as the “anaspid reptiles.” This idea is now completely discred-
ited, and anyone who still uses the obsolete and misleading term “mammal-like reptile”
clearly doesn’t know much about the current understanding of vertebrate evolution.
Focusing on the lineage of synapsids, we can put together an almost continuous series of
well-preserved fossils (figs. 13.1, 13.2, 13.3, and 13.4) that span the Carboniferous and Perm-
ian and straggle into the Triassic before most lineages died out (possibly in competition from
the newly emergent dinosaurs), and the remaining lineages gave rise to true mammals. Each
taxon along the way shows a mosaic of mammalian characters, with some advanced features
appearing early in the series, while others appeared quite late. The most primitive group is
the paraphyletic wastebasket known as “pelycosaurs” (fig. 13.2A and B), which includes not
only the oldest and most primitive taxa (such as Protoclepsydrops and Archaeothyris) but also
the spectacular “finbacks” Dimetrodon and Edaphosaurus, which were among the largest land
animals of the Early Permian (fig. 13.2B). Although the earliest forms are almost indistin-
guishable from the earliest true reptiles in most features, creatures such as Protoclepsydrops
and Archaeothyris still show a number of unique synapsid specializations, including a hole
in the side of the skull (temporal opening) beneath the postorbital and squamosal bones,
the beginnings of true canine-like teeth, and a number of other subtle features in the skull
and palate. Even more advanced “pelycosaurs” like Dimetrodon are still primitive in most
features but clearly have the lower temporal opening in the skull and the large canines in the
front of the mouth.