When Synapsids and Diapsids split

At some point
on every reptile cladogram the Synapsida emerges and somewhere else the Diapsida emerges.

In contrast to all prior cladograms,
on the large reptile tree, the traditional Diapsida is diphylletic, with lepidosaurs no longer related to archosaurs except by way of the basalmost Viséan reptiles (at the archosauromorph/ lepidisauromorph split). The reduced Diapsida (sans lepidosaurs) arises from the Prodiapsida, which splits from the Synapsida at the common base of both clades, near Protorothyris (Fig. 1), a basal archosauromorph. What happened at that split is today’s topic.

One of the basalmost synapsids
is Varanosaurus. One of the basal prodiapsids is Heleosaurus (Fig. 1). Both have a synapsid temporal morphology. Among traditional paleontologists, both are considered traditional synapsids.

Now let’s take a look
at some of the characters that split these sister taxa that otherwise share so many traits and put forth some hypotheses as to what they may mean in the grand scope of reptile evolution.

Figure 1. Taxa at the split between Synapsida and Diapsida (Prodiapsida): Varanosaurus and Heleosaurus to scale along with their common ancestor, Protorothyris.

Figure 1. Taxa at the split between Synapsida and Diapsida (Prodiapsida): Varanosaurus and Heleosaurus to scale along with their common ancestor, Protorothyris.

In many respects,
Varanosaurus was just a bigger Heleosaurus. And both were much larger than their predecessor, Protorothyris. So size was a major factor in the Early Permian. Basal synapsids were larger than prodiapsids and both were larger than their Carboniferous predecessors.

Distinct from Varanosaurus,
Heleosaurus had 19 rather minor traits in the large reptile tree. As a rule they’re not very interesting or informative (but see the next topic header):

  1. Remained < 60 cm long
  2. Slightly wider skull relative to height at orbit
  3. The nasal shape retains ‘narrows anteriorly’ description (not arrowhead)
  4. Orbit stays in anterior half of the skull
  5. Supratemporal/squamosal overhang
  6. Shorter jugal quadratojugal process
  7. Quadrate rotates to vertical
  8. Lateral temporal fenestra larger, circumtemporal bones more gracile
  9. Occiput remains close to quadrates
  10. Basipterygoid lateral processes prominent
  11. Mandible tip straight
  12. Mandible fenestra remains absent
  13. Olecranon process not present (Heleosaurus clade only)
  14. Clavicles medially not broad
  15. Radius + ulna > 3x longer than wide
  16. Retained pubis angled ventrally
  17. Acetabulum opens ventrally (Heleosaurus clade only)
  18. Tibia < 2x ilium length
  19. Dorsal osteoderms present (restricted to Heleosaurus

In summary,
these Heleosaurus traits break down to four major and a few minor distinctions from Varanosaurus:

  1. Smaller size, larger orbit, shorter rostrum, relatively less bone in the skull – all attributable to neotony (retention of embryo/juvenile traits)
  2. Relatively longer hind limbs and more slender tail (shorter chevrons and transverse processes (ribs). Together these two make prodiapsids speedy, not lumbering. Ideal for avoiding larger enemies and attacking insect prey.
  3. Relatively larger orbit: possible nocturnal hunter.
  4. Longer, more gracile ribs: fast locomotion requires more efficient and rapid respiration provided by expanding ribs
  5. Minor traits: Fewer teeth, ‘solid’ palate, larger choanae: all part of the insectivore, rapid respiration bauplan.

In my opinion
the smaller size of Heleosaurus helped it retain an insect diet, rather than moving into carnivory, piscivory or herbivory, as proposed for the pelycosaurs. Heleosaurus was probably faster and more agile than its larger and smaller relatives, better adapted to hunt insects and avoid predators.

Later taxa
‘improved’ on these traits as the clade Diapsida appeared, followed quickly by a division into terrestrial younginiforms and aquatic younginiforms.

These lizardy archosauromorph diapsids competed with
outwardly similar lepidosauromorphs lepidosaur pseudo-diapsids, like Tjubina. The lepidosaur branch retained insectivory, for the most part. The archosauromorph branch did not, for the most part, with the exception that several extant mammals and birds today are insectivores.

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