Origin of arms and legs traced to armored fish that lived 436 million years ago

By Mark Waghorn via SWNS

The origin of arms and legs has been traced back to a bizarre armored fish that swam the oceans 436 million years ago.

It had primitive paired fins before they separated into pectoral and pelvic versions for locomotion - forerunners to human limbs.

The creature belonged to an enigmatic group of jawless freshwater fish called galeaspids.

Tujiaaspis vivid had a large head shield that provided protection against predators and bolstered foraging ability.

The remarkably preserved remains were among a trove of fossils unearthed in two newly discovered prehistoric graveyards in southern China.

Tujiaaspis is a 'missing link' in the evolution of tetrapods - vertebrates that include amphibians, reptiles, birds and mammals.

Pectoral fins developed into arms and legs - in the case of humans. Most genetic machinery that builds a walking limb is already present in the swimming fin of a fish.

First author Dr. Zhikun Gai, of the University of Bristol, said: "The anatomy of galeaspids has been something of a mystery since they were first discovered more than half a century ago.

"Tens of thousands of fossils are known from China and Vietnam, but almost all of them are just heads. Nothing has been known about the rest of their bodies - until now.

"The new fossils are spectacular, preserving the whole body for the first time and revealing these animals possessed paired fins that extended continuously, all the way from the back of the head to the very tip of the tail.

"This is a great surprise since galeaspids have been thought to lack paired fins altogether."

They were encased in rocks dug up in Hunan Province and Chongqing. Tujiaaspis is named after the indigenous Tujia people who live there.

Corresponding author Professor Philip Donoghue, also from Bristol, said: "Tujiaaspis breathes new life into a century-old hypothesis for the evolution of paired fins, through differentiation of pectoral (arms) and pelvic (legs) fins over evolutionary time from a continuous head-to-tail fin precursor.

"This 'fin-fold' hypothesis has been very popular but it has lacked any supporting evidence until now.

"The discovery of Tujiaaspis resurrects the fin-fold hypothesis and reconciles it with contemporary data on the genetic controls on the embryonic development of fins in living vertebrates."

It lived around 500 million years before the first fish began crawling out of the sea onto land.

Corresponding author Dr. Min Zhu, of the Chinese Academy of Sciences, Beijing, said : "Tujiaaspis shows the primitive condition for paired fins first evolved.

"Later groups, like the jawless osteostracans show the first evidence for the separation of muscular pectoral fins, retaining long pelvic fins that reduced to the short muscular fins in jawed vertebrates, such as in groups like placoderms and sharks.

"Nevertheless, we can see vestiges of elongate fin-folds in the embryos of living jawed fishes, which can be experimentally manipulated to reproduce them. The key question is why did fins first evolve in this way?"

Co-author Dr. Humberto Ferron, also from Bristol, used computational engineering to simulate the behaviour of Tujiaaspis - with and without the paired fins.

He explained: "The paired fins of Tujiaaspis act as hydrofoils, passively generating lift for the fish without any muscular input from the fins themselves. The lateral fin-folds of Tujiaaspis allowed it to swim more efficiently."

Galeaspids lived in a shallow sea that covered Southern China, Tibet and Vietnam during the Silurian and Devonian ages.

Co-author Dr. Joseph Keating, also from Bristol who modelled the evolution of paired fins, said: "Fossil jawless vertebrates display a dizzying array of fin types, which has provoked extensive debate about the evolution of paired fins.

"Our new analyses suggest the ancestor of jawed vertebrates likely possessed paired fin-folds, which became separated into pectoral and pelvic regions.

"Eventually, these primitive fins evolved musculature and skeletal support, which allowed our fishy ancestors to better steer their swimming and add propulsion.

"It is amazing to think the evolutionary innovations seen in Tujiaaspis underpin locomotion in animals as diverse as birds, whales, bats and humans."

Tujiaaspis was among the collection of astonishingly well-preserved fish fossils described in a series of papers in the journal Nature.

They include new species and the oldest known teeth from a jawed vertebrate. They belonged to a shark relative named Qianodus duplicis.

They date back to around 439 million years ago - extending the minimum age for the origin of vertebrate jaws and dentitions back by approximately 14 million years.