By Harvard University, Department of Organismic and Evolutionary Biology
January 26, 2021

Pederpes Reconstruction

Life reconstruction of the early tetrapod Pederpes showing the forelimb bones underneath the skin. Credit: Image copyright 2021, Julia Molnar

When tetrapods(four-limbed vertebrates) started to move from water to land roughly 390 million years ago it set in motion the rise of lizards, birds, mammals, and all land animals that exist today, consisting of human beings and some marine vertebrates such as whales and dolphins.

The earliest tetrapods stemmed from their fish ancestors in the Devonian period and are more than twice as old as the oldest dinosaur fossils. They looked like a cross between a giant salamander and a crocodile and had to do with 1-2 meters long, had gills, webbed feet and tail fins, and were still greatly connected to water. Their brief arms and legs had up to 8 digits on each hand and foot and they were probably ambush predators, lurking in shallow water awaiting prey to come near.

Scientists know how the fins of fish changed into the limbs of tetrapods, but debates remain about where and how the earliest tetrapods used their limbs. And, while lots of hypotheses have been proposed, really couple of studies have actually rigorously evaluated them using the fossil record.

In a paper released on January 22, 2021, in Science Advances a global group of scientists examined three-dimensional digital models of the bones, joints, and muscles of the fins and limbs of two extinct early tetrapods and a carefully related fossil fish to reveal how function of the forelimb changed as fins progressed into limbs. The research study led by Julia Molnar, Assistant Professor at New York Institute of Technology College of Osteopathic Medication and Stephanie Pierce, Thomas D. Cabot Partner Professor of Organismic and Evolutionary Biology at Harvard University, found three distinct functional stages in the shift from fins to limbs, which these early tetrapods had a very unique pattern of muscle leverage that didn’t look like a fish fin or contemporary tetrapod limbs.


Virtual skeleton of the early tetrapod Pederpes from micro-CT scanned fossil and musculoskeletal reconstruction of its forelimb. Credit: Copyright 2021, Julia Molnar

To rebuild 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) very first required to determine what muscles were present in the fossil animals. A tough task as muscles are not maintained in fossils, and the muscles of modern-day fish fins are entirely different from those of tetrapod limbs. The team spent numerous years trying to respond to the question, how precisely did the couple of basic muscles of a fin become dozens of muscles that carry out all sorts of functions in a tetrapod limb?

” Identifying what muscles existed in a 360- million-year-old fossil took many years of work just to get to the point where we could begin to construct very complicated musculoskeletal designs,” said Pierce. “We needed to understand how many muscles were present in the fossil animals and where they attached to on the bones so we could test how they operated.”

They constructed three-dimensional musculoskeletal models of the pectoral fin in Eusthenopteron(a fish carefully 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 completion of the Late Devonian period) and Pederpes (348-347 million years old living throughout the early Carboniferous duration). For comparison, they also constructed comparable 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 method had been utilized just recently to study locomotion in the ancestors of people and also dinosaurs like T. rex, but never in something as old as an early tetrapod.

Controling the models in the software, the group had the ability to determine two functional qualities: the joint’s maximum series of motion and the muscles’ capability to move the fin or limb joints. The two measurements would expose compromises in the locomotor system and allow the scientists to check hypotheses of function in extinct animals.

The team found the forelimbs of all terrestrial tetrapods gone through 3 unique functional stages: a “benthic fish” phase that resembled contemporary lungfish, an “early tetrapod” phase unlike any extinct animal, and a “crown tetrapod” phase with attributes of both lizards and salamanders.

” The fin from Eusthenopteron had a pattern that was reminiscent of the lungfish, which is among the closest living relatives of tetrapods,” said Pierce. “However the early tetrapod limbs showed more resemblances to each besides either fish or modern tetrapods.”

“I believed Pederpes, and perhaps Acanthostega, would fall pretty well within the variety of contemporary tetrapods. They formed their own unique cluster that didn’t look like a contemporary tetrapod limb or a fish fin.

The results revealed that early tetrapod limbs were more adjusted for propulsion rather than weight bearing. In the water, animals utilize their limbs for propulsion to move themselves forward or backward permitting the water to support their body weight. Moving on land, however, needs the animal act versus gravity and push downward with their limbs to support their body mass.

This doesn’t mean that early tetrapods were incapable of proceeding land, however rather they didn’t move like a modern-day living tetrapod. Their means of locomotion was most likely distinct to these animals that were still very much connected to the water, however were also venturing onto land, where there were many opportunities for vertebrate animals however little competition or worry from predators.

” These outcomes are interesting as they separately support a study I released last year utilizing entirely various fossils and methods,” said Pierce. “That study, which concentrated on the arm bone, indicated that early tetrapods had some capacity for land motion but that they may not have been very good at it.”

The scientists are better to rebuilding the evolution of terrestrial locomotion, but more work is required. They prepare to next model the hind limb to examine how all four limbs collaborated. It has actually been recommended that early tetrapods were utilizing their forelimbs for propulsion, but modern-day tetrapods get most of their propulsive power from the hind limb.

” We prepare to try to find any evidence of a shift from forelimb driven mobility toward hind limb driving locomotion, like contemporary tetrapods,” said Molnar. Looking at the forelimb and hind limb together could expose more about the shift from water to land and how tetrapods ultimately concerned dominate the terrestrial world.

Reference: “Development of forelimb musculoskeletal function across the fish-to-tetrapod transition” by J. L. Molnar, J. R. Hutchinson, R. Diogo, J. A. Clack and S. E. Pierce, 22 January 2021, Science Advances
DOI: 10.1126/ sciadv.abd7457

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