Fossil fish illustrate the evolutionary origin of fins and limbs

Sep. 28 () –

Fossils in rocks from China, dated to 436 million years ago, have revealed for the first time that the mysterious galleaspids, members of an extinct clade of jawless fishes, They had paired fins.

The discovery, published in the magazine ‘Nature’shows the primitive state of the paired fins before they separated into pectoral and pelvic fins, precursors of the arms and legs.

Until now, the only preserved galeaspian fossils were heads, but these new fossils comprise entire bodies, according to the study, carried out by an international team led by Professor ZHU Min, from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences, and Professor Philip Donoghue, from the University of Bristol (United Kingdom). They have been found in rocks in Hunan and Chongqing province. and they have been baptized with the name of Tujiaaspis in honor of the indigenous Tujia people who live in the region.

Theories about the evolutionary beginnings of vertebrate fins and limbs – the evolutionary precursors of arms and legs – abound, and are based largely on comparative embryology. There is a rich fossil record of early vertebrates, but they either had separate paired fins or they didn’t.. There is little evidence for the gradual evolution of fins.

According to the first author, GAI Zhikun, a professor at IVPP, “the anatomy of galleaspids has been a mystery since they were 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 was known about the rest of the body) until now”.

The new fossils are spectacular as they preserve a full body for the first time and reveal that these animals had paired fins that extended from the back of the head to the tip of the tail. This is a big surprise, as scientists previously thought galleaspids lacked paired fins at all.

“Tujiaaspis breathes new life into a centuries-old hypothesis about the evolution of paired fins, through the differentiation of pectoral (arms) and pelvic (legs) fins. throughout evolution from a fin precursor continuous from head to tail“, emphasizes the corresponding author, Professor Donoghue.

This “fin fold” hypothesis has been very popular, but until now had no evidence to support it. Tujiaaspis discovery resurrects fin fold hypothesis and reconciles it with current data on the genetic control of embryonic fin development in living vertebrates.

Tujiaaspis shows the “primitive condition” of paired fin evolution, according to Professor ZHU, who said later jawless fish showed the first evidence of the separation of this fin fold into pectoral and pelvic fins. Professor ZHU also noted that traces of elongated fin folds could be seen in embryos of living jawed fish, that could be manipulated to produce them.

Study co-author Dr Humberto Ferrón, from Bristol, used computational engineering methods to simulate the behavior of models of Tujiaaspis with and without paired fins. The paired fins of the Tujiaaspis act as hydrofoils, passively generating lift for the fish without the fins themselves providing anything. The lateral folds on the fins of Tujiaaspis allowed it to swim more efficiently.”

“Our new analyzes suggest that the ancestor of jawed vertebrates probably possessed paired fin folds, which were separated into pectoral and pelvic regions,” adds co-author Dr Joseph Keating, from the University of Bristol, who notes that primitive fins developed musculature and skeletal support that allowed our fish ancestor to better direct its swimming and add propulsion.

“It is surprising to think that the evolutionary innovations observed in Tujiaaspis support locomotion in animals as diverse as birds, whales, bats and humans,” he concludes.