One of the main differences between fish and tetrapods,
other than the transition from fins to feet, is the origin of the neck. famously in the amphibian-like fish, Tiktaalik (Fig. 2). The proximal outgroup taxon in the large reptile tree (LRT, 1016 taxa), Panderichthys (Fig. 1), does not have a neck. The skull and opercular bones are jammed up against the cleithrum (pectoral girdle) permitting no wiggle room. That wiggle room ultimately comes from the disappearance of those opercular bones.
It is noteworthy
that more primitive taxa than Tiktaalik, in the Paratetrapoda, like Pholidogaster and Colosteus (Fig. 3) also lack a neck. The pectoral girdle extends beneath the posterior jaws, as in Osteolepis.
The first tetrapod clade,
(Fig. 9) with flat-headed Greererpeton at its base, had a neck, though not much of one. In related taxa like Gerrothorax (Fig. 4), the skull and torso were so wide that a neck would have been useless for lateral movements, but essential to help the skull rise during feeding (famously, like a toilet bowl lid!). More derived taxa in this clade, like Metoposaurus, had a little more neck represented by more space between the skull and pectoral girdle.
The second tetrapod clade,
(Fig. 9) with Ossinodus and Acanthostega (Fig. 5) at its base, likewise did not have much of a neck. Perhaps there was less of a neck than in more basal Tiktaalik. This is a small clade with just these two members, so far.
The third tetrapod clade,
(Fig. 9) with Pederpes and Crassigyrinus (Fig. 6) at its base likewise had very little wiggle room between the skull and cleithrum. Crassigyrinus had a short neck between its cheeks, so likely was immobile. In this clade derived members, Sclerocephalus and Eryops, document the third appearance of the neck in tetrapods. Even so, it was a very short relatively immobile neck.
The fourth tetrapod clade
(Fig. 9) with Ichthyostega (Fig. 7) as its base, might have had some wiggle room between the skull and tall cleithrum. Not sure whether the small skull or large skull is correct. Certainly its phylogenetic successor, the reptilomorph Proterogyrinus (Fig. 8), had a substantial neck as did most of its descendants (but see below for notable exceptions).
and in the clade Seymouriamorpha, like Seymouria, and in the LRT leads to both Reptilia and Lepospondyli, had an increasingly mobile neck.
The number of cervicals
remains low (under 4) in basal lepospondyls, and sometimes that number decreases to one. An exception, Eocaecilia, had 5 elongate cervicals. Basal amniotes, like Gephyrostegus, had six flexible cervicals.
Notable reversals, back to lacking a neck, include:
- Rana, the frog.
- Cacops the basal lepospondyl
- Mixosaurus, the ichthyosaur and
- Eubaelana, the right whale, with short fused cervicals