A Chronology of Basal Reptilia

The dual origin of the Reptilia (following Cephalerpeton) was blogged earlier. Here we’ll take a look at the chronology of basal reptiles during the Carboniferous and Permian.

A chronology of the basal Reptilia.

Figure 1. Click to enlarge. A chronology of the basal Reptilia.

The Most Primitive Known Reptile Was Not the Oldest
Cephalerpeton nested as the most basal reptile, the one closest to the nearest outgroup taxon, Gephyrostegus. Unfortunately the fossil record of both succeed the earliest known reptiles, Westlothiana and Casineria, by some 40 million years. That means the first appearance of Cephalerpeton had to precede its first (and only) appearance in the fossil record by a similar time span. Thus Cephalerpeton was a long-lived taxon. Supporting this hypothesis, the appearance of the proximal sister to Gephyrostegus, Silvanerpeton, first appeared in the fossil record alongside Casineria.

The first appearance of Cephalerpeton during the Kasimovian, some 305 million years ago, also followed the first appearances of several other reptilian taxa, including Hylonomus, Paleothyris and Solenodonsaurus. The first appearance of Cephalerpeton also coincided with the first appearances of Haptodus and Archaeothyris. Such timing demonstrates long ghost lineages in which one can expect to find more Cephalerpeton specimens back to the basal Visean, 345 million years ago.

Protorothyris and Limnoscelis Ghost Lineages
Protorothyris, an outgroup taxon to the Synapsida and Protodiapsida, first appeared in the fossil record about 290 million years ago. That succeeded the first appearances of phylogenetic descendants by some 15 million years. So, Protorothyris was also a long-lived taxon with an earlier origin.

Limnoscelis, a basal diadectomorph, succeeded its phylogenetic successor, Solenodonsaurus.

Turtle Ancestry
The earliest know turtles, Odontochelys and Proganochelys, first appeared in the Late Triassic, 225 million years ago. Their phylogenetic predecessor, Stephanospondylus, appeared 290 million years ago. That gives turtles 65 million years (nearly the entire Permian and Triassic) to develop their unique morphologies from their closest known sister taxon at the base of the Permian.

Therapsid Ancestry
Basal therapsids, like Nikkasaurus and Biarmosuchus, first appeared some 250-255 million years ago. Their closest outgroup sisters, Ophiacodon and Archaeothyris, lived some 50 million years earlier.

Heleosaurus and Milleretta Ghost Lineage
Heleosaurus appears in the fossil record approximately 270 mya, but its phylogenetic successors appeared 305 mya, 35 million years earlier. Thus Heleosaurus was a long-lived taxon.

Milleretta appears in the fossil record approximately 255 mya, but its phylogenetic successors, including Bolosaurus, appeared 300 mya, 45 million years earlier. Thus Milleretta was also a long-lived taxon.

Most of the rest of the taxa are chronologically ordered with regard to their phylogenetic order. Sphenodon, the living Tuatara, is a living example of a long-lived taxon.

Morphological Stasis and Rapid Radiation
The chronological tree (Fig. 1) illustrates the twin topics of morphological stasis and rapid radiation. The Tournaisian (Early Carboniferous) was a time of rapid change in our reptilian predecessors. Most of the rest of the Early Carboniferous was a time of stasis with a rapid radiation in the Pennsylvanian, producing all of the major reptilian clades. The Permian is where we find most of the basal reptile fossils. Here we find some basal taxa (presumably earliest Permian) surviving to the end of the Permian, a case of stasis. The Triassic, as I need not remind anyone, was a time of rapid radiation following the Permian extinction event.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

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