When elbows and knees start bending in basal tetrapods

You might remember,
earlier we looked at the non-traditional origin of fingers and toes in the Tetrapoda in the basal dvinosaur/plagiosaur, Trypanognathus (Fig. 1). 

Today
we’ll look at the origin of elbows and knees able to bend (distinct from lobefins and kin).

And then a quick peek
at bendable limbs large enough to sustain/lift the weight of the skull and body off the substrate, an ability chronicled in Middle Devonian tracks.

Backstory
Lobefin fish, like Eusthenopteron through Panderichthys, have one proximal limb bone, two more distally and many tiny bones further out, ending in lepidotrichia (fin filaments). In these taxa the radius is much longer than the ulna. The tibia is much longer than the fibula.

Dvinosaurs (basalmost tetrapods, and by definition, reptiles and humans; Fig. 2) like Trypanognathus (Figs. 1, 2), have the same limb bone arrangements (1 bones, 2 bones, then several bones), but the ulna length catches up to the radius and the fibula resembles the tibia. Notably this occurs on limbs too small to support weight. Only tiny fingers and toes are present. These replace the lepidotrichia. There is little indication that a strongly bendable elbow or knee is present.

In colosteids,
like Colosteus (Fig. 3) and Pholidogaster, the limbs have modern proportions, with bendable elbows and knees, but they remain far too small to support the weight of the head and torso. Another traditionally considered colosteid, Greererpeton (Fig. 1) nests at the base of the next derived clade in the large reptile tree (LRT, 1444 taxa).

Thereafter
limbs get bigger, as documented in Ossinodus (Fig. 1), able to support weight lifted above the substrate. At this stage and with this innovation basal tetrapods split into three clades: Temnospondyli, Lepospondyli and Reptilomorpha in the LRT. Even so, lateral undulation of the backbone is the main driver for stride length.

Temnospondyli
Limbs are larger in temnospondyls (Fig. 1). Bodies are rounder. Tails are longer. The limbs would have been advanced more by lateral undulation than by extension and flexion.

Acanthostega represents a rare reversal,
among temnospondyls. Phylogenetically it is a smaller, apparently neotonous taxon with extra fingers and toes, a reversal to a longer radius than ulna, smaller limbs, but also a smaller, narrower body and a robust tail. Both girdles were quite large and both the humerus and femur had large processes not seen in more primitive dvinosaurs.

Lepospondyli
Trimerorhachis (Fig. 5) is either a basal taxon retaining a wide torso and relatively small limbs or it is yet another reversal because its sister taxon in the LRT is Dendrerepton (Fig. 4) a small taxon with robust limbs and a small body. This clade gives rise to frogs, like Rana, which hyper-emphasizes the limbs and reduces the torso, along with Cacops, which shortens the torso and emphasizes the girdles.

Reptilomorpha
This clade includes reptiles, like Silvanerpeton, their proximal ancestors and microsaurs, like Tuditanus, in the LRT. Basal taxa were increasingly terrestrial with robust limbs on smaller bodies. Much later several clades within both Reptilia and Microsauria (e.g. Diplocaulus) returned to a more aquatic niche, typically reducing the limbs. In some reptiles (e.g. Ichthyosaurus, Orcinus) limbs evolved back into fins/flippers and tails evolved fish-like flukes.