Fossils of ancient fish unearthed in two separate digs in China are helping piece together how the first jaws and paired limbs evolved – informing understanding of human evolution.
A treasure trove of 436-million-year-old fossils has revealed for the first time that an extinct clade of jawless fish called galeaspids had paired fins. It shows how early fins began separating into pectoral and pelvic fins, the forerunner of arms and legs.
The first vertebrates which left the water around 375 million years ago – the ancestors of all land vertebrates including humans – did so on primitive walking limbs which evolved from paired fins.
Until now, the only galeaspid fossils found were of the armoured fish’s head. The new fossils, described in a new study published in Nature, comprise whole bodies. The specimens were named Tujiaaspis after the indigenous Tujia people who live in the Hunan Province and Chongqing where the fossils were found.
Early vertebrate fossils either show they had separated fins, or they didn’t. There is little evidence of the gradual evolution of paired fins.
“The anatomy of galeaspids has been something of a mystery since they were first discovered more than half a century ago,” says first author Zhikun Gai, a professor at the Institute of Vertebrate Palaeontology and Palaeoanthropology (IVPP) of the Chinese Academy of Sciences. “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.”
“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,” adds corresponding author Professor Philip Donoghue from the University of Bristol in the UK.
Called the “fin-fold” hypothesis, this theory, though lacking in supporting evidence until now, has been favoured by palaeontologists. It also reconciles contemporary genetic data which studies the effect of controlling the embryonic development of fins in living vertebrates.
Using computer simulations, the team determined that the paired fins of Tujiaaspis gave it an edge while swimming.
“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,” says co-author Dr Humberto Ferron, also from Bristol.
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Another fossil discovery from China, only 3 million years older than Tujiaaspis, is believed to be the oldest undisputed jawed fish. The palaeontologists who discovered the 439-million-year-old specimen believe that it suggests we need to rethink the timeline of vertebrate evolution.
The fish belongs to an ancient group of “sharks” collectively known as acanthodians and predates the previous oldest jawed fish by 15 million years.
Named Fanjingshania, this ancient fish is covered in external, bony “armour” and sports multiple pairs of fins. It is described in a separate Nature paper. What is shocking about the specimen is that it presents evidence of a diversification of major vertebrate groups well before the so-called “Age of Fishes” which began around 420 million years ago.
“This is the oldest jawed fish with known anatomy,” says senior author Professor Min Zhu from the Chinese Academy of Sciences IVPP. “The new data allowed us to place Fanjingshania in the phylogenetic tree of early vertebrates and gain much needed information about the evolutionary steps leading to the origin of important vertebrate adaptations such as jaws, sensory systems, and paired appendages.”
Analysis of Fanjingshania and algorithmic analysis of the phylogenetic tree have major implications for our understanding of when jawed fish originated.
“The new discovery raises questions about existing models of vertebrate evolution by significantly condensing the timeframe for the emergence of jawed fish from their closest jawless ancestors. This will have profound impact on how we assess evolutionary rates in early vertebrates and the relationship between morphological and molecular change in these groups,” says co-author Dr Ivan J. Sansom from the University of Birmingham in the UK.