For the first time, a collection of fossils from 436-million-year-old rock in China has shown that the enigmatic galeaspids, a freshwater fish without a jaw, had paired fins.
The discovery, made by an international team led by Professor Philip Donoghue from the University of Bristol‘s School of Earth Sciences and Min Zhu of the Institute of Vertebrate Palaeontology and Palaeoanthropology in Beijing, reveals the early state of paired fins before they split into pectoral and pelvic fin, the ancestors of arms and legs.
The only remaining galeaspid fossils up until this point have been heads, but these new fossils, named Tujiaaspis after the local Tujia people, come from rocks in Hunan Province and Chongqing and show their entire bodies.
Many theories about the origins of vertebrate fins and limbs, which are the evolutionary forerunners of arms and legs, are based on comparative embryology. Despite the abundance of fossils, early vertebrates either had fin or didn’t. There wasn’t much proof of their slow evolution.
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 anatomy of galeaspids has been something of a mystery since they were first discovered more than half a century ago.
The newly discovered fossils are amazing because they preserve the entire body for the first time and show that the animals had paired fins that extended continuously from the back of the head to the very tip of the tail. It was thought before that galeaspids didn’t have any pairs of fins, so this is a big surprise.
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, according to corresponding author Professor Donoghue.
The discovery of Tujiaaspis revives the fin-fold hypothesis and reconciles it with current data on the genetic controls on the embryonic development of fins in living vertebrates. The “fin-fold” hypothesis has been widely accepted but has been devoid of any supporting evidence up until now.
Tujiaaspis shows the primitive condition for paired fin first evolved,” continued corresponding author Min Zhu of VPP, Beijing. Later groups, like the jawless osteostracans, show the first evidence for the separation of muscular pectoral fins, retaining long pelvic fins that are reduced to the short muscular fins in jawed vertebrates, such as in groups like placoderms and sharks. However, we can see remnants of an elongated fin.
Dr. Humberto Ferron of Bristol used computational engineering techniques to model the behavior of Tujiaaspis models with and without the paired fins. The lateral fin folds of Tujiaaspis helped it swim better, according to the co-author: “The paired fins of Tujiaaspis act as hydrofoils, passively generating lift for the fish without any muscular input from the fins themselves.”
Co-author Dr. Joseph Keating of Bristol developed a model to predict how paired fins evolved. He said: “The evolution of paired fins has been the subject of intense debate because fossil jawless vertebrates exhibit a bewildering variety of fin types.”
According to “our new analysis, the ancestor of jawed vertebrates probably had paired fin-folds that later split into pectoral and pelvic regions. Our fishy ancestors eventually added musculature and skeletal support to these primitive fins, which improved swimming control and added propulsion. It is incredible to think that animals as different as birds, whales, bats, and humans all rely on evolutionary innovations seen in Tujiaaspis to move.
