Fish fossils unearthed in China have injected new life into the evolutionary hypothesis of fins and limbs

A batch of fossils unearthed in Chinese rocks dating back about 436 million years ago first revealed the mysterious armored fish, a member of an extinct jawless fish with pairs of fins
.

The findings, led by an international team led by Professor ZHU Min of the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences and Professor Philip Donoghue of the University of Bristol, show the original state
of the pair fins before separating into pectoral and ventral fins (the forerunners of arms and legs).

The findings were published in the Sept.
28 issue of the
journal Nature.

So far, the only surviving fossil of an armored fish (galeaspid) is the head, but these new fossils include the entire body
.
They are found in rocks in Hunan and Chongqing and are named Tujiaaspis
after the local Tujia family.

There are many theories about the evolutionary origins of vertebrate fins and limbs (evolutionary precursors to arms and legs), and most of them are based on comparative embryology
.
There is a rich record of early vertebrate fossils, but they either have separate pairs of fins or none of them
.
There is little evidence that fins evolved gradually
.

First author GAI Zhikun, Professor of IVPP, said: "Since the first discovery of armored fish more than half a century ago, their anatomy has been a mystery
.
Thousands of fossils have been found in China and Vietnam, but until now, almost all of them are just heads, with no knowledge
of the rest of their bodies.
”

The newly discovered fossils are spectacular, preserving the entire body for the first time and revealing that the animals have pairs of fins that extend from the back of the head all the way to the tip of
the tail.
This was a big surprise, as scientists have long believed that armored whales have no pairs of fins at all
.

Corresponding author Professor Donoghue said: "The evolution of a pair of fins has been given new life
through the differentiation of successive cephalocaudate fin precursors to pectoral fins (arms) and ventral fins (legs) during the evolutionary process.
"

This "fin-fold" hypothesis has been popular, but until now there was a lack of evidence
to support it.
Tujiaaspis' discovery re-proposes the fin folding hypothesis and harmonizes
it with contemporary data on the genetic control of extant vertebrate embryonic development fins.

Tujiaaspis shows the "primordial conditions" for the evolution of pairs of fins, Professor Zhu said, and later jawless fish show the first evidence
of this fin folding separation into pectoral and ventral fins.
The professor also noted that traces of slender fin folds can be seen in embryos of live jawfish, which can be manipulated to produce fin folds
.

Dr Humberto Ferron of the University of Bristol, who is a co-author of the study, used computational engineering methods to simulate the behavior of animal models with or without pairs of fins
.
"The paired fins act as Tujiaaspis hydrofoils, passively generating lift for the fish without any muscle input
from the fins themselves," he said.
Tujiaaspis' lateral fin pleats allow it to swim more efficiently
.
”

Dr Joseph Keating, from the University of Bristol, said: "Our new analysis suggests that the ancestors of jaw vertebrates may have had pairs of fin folds, which were separated into pectoral fins and pelvic areas
.
"

He noted that the primitive fins evolved muscle tissue and bone support, which allowed our fish ancestors to better control the direction of swimming and increase propulsion
.

"Surprisingly, we see that the evolutionarily innovative Tujiaaspis may be the basis for
various animal movements such as birds, whales, bats and humans.
"

Galeaspid anatomy and the origin of vertebrate paired appendages