Evolution News - How Fish Fins Became Tetrapod Limbs

Perdepes (reconstruction) showing the forelimb skeleton.

Researchers reconstruct changes in forelimb function as vertebrates moved onto land | Department of Organismic and Evolutionary Biology

Another bad day for Creationists!

Now an international team of research biologists led by Julia Molnar, Assistant Professor at New York Institute of Technology College of Osteopathic Medicine and Stephanie Pierce, Thomas D. Cabot Associate Professor of Organismic and Evolutionary Biology at Harvard University have published details of how fish fins evolved into the tetrapod limb. And they use the dreaded word 'transition'!

To work out how this transition occurred using evidence from the fossil record, the team first spent a long time working out which muscles were present and how and where they were attached to the skeleton. They then fed this information into computer software originally developed to study human locomotion. The Harvard University department of Organismic & Evolutionary Biology news article explains:

To reconstruct how limbs of the earliest known tetrapods functioned, Molnar, Pierce and co-authors John Hutchinson (Royal Veterinary College), Rui Diogo (Howard University), and Jennifer Clack (University of Cambridge) first needed to figure out what muscles were present in the fossil animals. A challenging task as muscles are not preserved in fossils, and the muscles of modern fish fins are completely different from those of tetrapod limbs. The team spent several years trying to answer the question, how exactly did the few simple muscles of a fin become dozens of muscles that perform all sorts of functions in a tetrapod limb?

"Determining what muscles were present in a 360-million-year-old fossil took many years of work just to get to the point where we could begin to build very complicated musculoskeletal models," said Pierce. "We needed to know how many muscles were present in the fossil animals and where they attached to on the bones so we could test how they functioned".

Pederpes Forelimb Reconstruction Julia MolnarThey built three-dimensional musculoskeletal models of the pectoral fin in Eusthenopteron (a fish closely related to tetrapods that lived during the Late Devonian period about 385 million years ago) and the forelimbs of two early tetrapods, Acanthostega (365 million years old living towards the end of the Late Devonian period) and Pederpes (348-347 million years old living during the early Carboniferous period). For comparison, they also built similar models of the pectoral fins of living fishes (coelacanth, lungfish) and forelimbs of living tetrapods (salamander, lizard).

To determine how the fins and limbs worked, the researchers used computational software originally developed to study human locomotion. This technique had been used recently to study locomotion in the ancestors of humans and also dinosaurs like T. rex, but never in something as old as an early tetrapod.

Manipulating the models in the software, the team were able to measure two functional traits: the joint's maximum range of motion and the muscles' ability to move the fin or limb joints. The two measurements would reveal trade-offs in the locomotor system and allow the researchers to test hypotheses of function in extinct animals.

The team found the forelimbs of all terrestrial tetrapods passed through three distinct functional stages: a "benthic fish" stage that resembled modern lungfish, an "early tetrapod" stage unlike any extinct animal, and a "crown tetrapod" stage with characteristics of both lizards and salamanders.

"The fin from Eusthenopteron had a pattern that was reminiscent of the lungfish, which is one of the closest living relatives of tetrapods," said Pierce. "But the early tetrapod limbs showed more similarities to each other than either fish or modern tetrapods."

"That was perhaps the most surprising," said Molnar. "I thought Pederpes, and possibly Acanthostega, would fall pretty well within the range of modern tetrapods. But they formed their own distinct cluster that didn't look like a modern tetrapod limb or a fish fin. They were not smack dab in the middle but had their own collection of characteristics that probably reflected their unique environment and behaviors."

The results showed that early tetrapod limbs were more adapted for propulsion rather than weight bearing. In the water, animals use their limbs for propulsion to move themselves forward or backward allowing the water to support their body weight. Moving on land, however, requires the animal act against gravity and push downward with their limbs to support their body mass.

This doesn't mean that early tetrapods were incapable of moving on land, but rather they didn't move like a modern-day living tetrapod. Their means of locomotion was probably unique to these animals that were still very much tied to the water, but were also venturing onto land, where there were many opportunities for vertebrate animals but little competition or fear from predators.

Relationships of study taxa and example musculoskeletal model showing definitions of forelimb movements.

(A) Cladogram showing relationships between extinct (†) and extant taxa and the node defining the fin-to-limb and hypothesized water-to-land transitions. Three fossil taxa with well-preserved appendages were sampled: the finned tetrapodomorph Eusthenopteron foordi, the Devonian tetrapod Acanthostega gunnari, and the Carboniferous tetrapod Pederpes finneyae. The extant taxa included in the study were chosen as representative examples of the two closest sister groups of tetrapods (Actinistia and Dipnoi) and the two major clades of extant tetrapods (Amniota and Lissamphibia).
(B) Musculoskeletal model of the pectoral appendage of Pederpes in dorsolateral view, showing bony elements (shoulder girdle, humerus, radius, and ulna), cylinder representing the body profile, and reconstructed muscle paths (red lines).
(C to H) Pederpes model in dorsolateral (C, D, G, and H) and anterior (E and F) views showing definitions of forelimb movements at the glenohumeral (shoulder) and humeroradioulnar (elbow) joints. Colored lines in (G) indicate axes for long-axis rotation (blue), elevation/depression (red), and protraction/retraction (green). Colored lines in (H) indicate axes for long-axis rotation (blue), flexion/extension (red), and radial/ulnar deviation (green). mm. D, deltoid; EACU, extensor antebrachii et carpi ulnaris; ECR, extensor carpi radialis; LD, latissimus dorsi; P.a, pectoralis anterior; Pch, procoracohumeralis; Scs, subcoracoscapularis; S, supinator; T, triceps. See figs. S1 and S2 and movies S1 to S3 for musculoskeletal models of all taxa sampled. See Materials and Methods for model construction.

Source

The team's results were published open access in Science Advances a few days ago:

Abstract

One of the most intriguing questions in vertebrate evolution is how tetrapods gained the ability to walk on land. Although many hypotheses have been proposed, few have been rigorously tested using the fossil record. Here, we build three-dimensional musculoskeletal models of the pectoral appendage in Eusthenopteron, Acanthostega, and Pederpes and quantitatively examine changes in forelimb function across the fin-to-limb transition. Through comparison with extant fishes and tetrapods, we show that early tetrapods share a suite of characters including restricted mobility in humerus long-axis rotation, increased muscular leverage for humeral retraction, but not depression/adduction, and increased mobility in elbow flexion-extension. We infer that the earliest steps in tetrapod forelimb evolution were related to limb-substrate interactions, whereas specializations for weight support appeared later. 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.

Molnar, J. L.; Hutchinson, J. R.; Diogo, R.; Clack, J. A.; Pierce, S. E.
Evolution of forelimb musculoskeletal function across the fish-to-tetrapod transition
Science Advances 22 Jan 2021: Vol. 7, no. 4, eabd7457 DOI: 10.1126/sciadv.abd7457

Copyright: © 2021 The authors. Published by The American Association for the Advancement of Science
Open access
Reprinted under a Creative Commons Attribution 4.0 International License 4.0 (CC BY).

Of particular interest here is how the environment strongly influenced the stages in this transition with locomotion coming first, so that the opportunities available on land could be exploited - arthropods and primitive plants had already colonised the land - then weight-bearing becoming more important as the evolving amphibians lost the weight-supporting water, so weight-bearing and more efficient locomotion gave an advantage on land. The earliest land vertebrates had no threats from predators so there was less benefit from being able to run. Like modern mud-skippers and lungfish, the main locomotory limbs were the forelimbs.

Of course this paper was not written to debunk creationist claims; it just does that incidentally by revealing the facts. It will be interesting to see if Creationist frauds like Ken Ham misrepresent and lie about this science like his AiG site did with a paper on the reduction of active genes for egg production in the monotremes recently.

---

Tweet Link