Molnar et al. 2021: Forelimb function across the fish–tetrapod transition

Molnar et al. 2021
bring us a deep and complex look into the hypothetical muscles (based on muscle scars) of Eusthenopteron (Fig. 1), Acanthostega (Fig. 1) and Pederpes. The authors compare these distinctly different taxa to “show that early tetrapods share a suite of characters including restricted mobility in hurmerus long-axis rotation, increased muscular leverage of humeral retraction, but not depression/adduction, and increased mobility in elbow flexion-extension.” The authors infer the earliest ‘steps’ in tetrapod forelimb evolution were related to limb-substrate interactions. Weight support appeared later.

Figure x. The fin to finger transition in the LRT with the addition of Elpistostege.

Figure 1. The fin to finger transition in the LRT with the addition of Elpistostege.

Unfortunately, 
without a valid phylogenetic context, what these authors deliver is not quite germane to the topic of their headline. The actual fin-to-finger transition occurred between Panderichthys (Figs. 1, 2) and the extremely similar Trypanognathus (Figs. 1, 2). The former had fins. The latter had fingers and toes. Otherwise they were very much alike.

Molnar et al. looked at the wrong taxa. Neither Panderichthys nor Trypanognathus are mentioned in the Molnar et al. text.

What can we conclude given
the similarities and differences of Panderichthys and Trypanognathus?

  1. Small fins and limbs at the transition were incapable of weight bearing
  2. Elbows and knees were incapable of bending, pushing, pulling
  3. Torso much longer than tail, lots of flexible ribs
  4. Low, wide, flexible torso at the transition provided serpentine locomotion
  5. Little risk of tipping over due to low center of gravity
Figure 6. Dorsal and ventral views of Panderichthys and several basal tetrapods demonstrating the low, flat skulls and bodies with small limbs and relatively straight ribs.

Figure 2. Dorsal and ventral views of Panderichthys and several basal tetrapods demonstrating the low, flat skulls and bodies with small limbs and relatively straight ribs.

Molnar et al. conclude:
“Together, these results suggest that competing selective pressures for aquatic and terrestrial environments produced a unique, ancestral “early tetrapod” forelimb locomotor mode unlike that of any extant animal.”

Not really. Consider the moray eel chasing crabs on land without fins or fingers. Click the pic to view video on YouTube. David Attenborough is the narrator.

Now put predator and prey in a Devonian swamp setting,
with lots of growing and rotting vegetation and no rocky place to find safety. Note (in figure 2), the rather slow phylogenetic growth of the limbs relative to the torso in this sequence. Other lineages did their own thing in their own time. Ossinodus, for instance (Fig. 1), had a shorter torso and longer limbs, and was a phylogenetic ancestor to Ichthyostega and Acanthostega.


References
Molnar JL, Hutchinson JR, Diogo R, Clack JA and Pierce SE. 2021. Evolution of forelimb musculoskeletal function across the fish-to-tetrapod transition. Science Advances 2021; 7: eabd7457 22 January 2021

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