There is little doubt now that the paired terrestrial tetrapod limbs, possessed by all terrestrial vertebrates, and those such as whales and turtles that he reverted to a marine existence, evolved out of the paired pectoral and pelvic fins of fish that first crawled onto land. The remaining question was how did these paired limbs evolve in the early fish? Because of gaps in the fossil record, it had looked as though fins suddenly appeared fully formed.
That question has now been answered by an international team, led by Min Zhu of the Institute of Vertebrate Palaeontology and Palaeoanthropology, Beijing and Professor Philip Donoghue from the University of Bristol’s School of Earth Sciences, whose discovery shows the primitive condition of paired fins before they separated into pectoral and pelvic fins, the forerunner to arms and legs.
The evidence was found in fossils in the rocks of Hunan Province and Chongqing and named Tujiaaspis after the indigenous Tujia people who live in this region. Unlike previous fossils records which are mostly just the heads, these fossils contain the whole body.
Earlier fossils either had fins or they didn't, with no fossil record of transition to paired fins. That gap has now been filled by Tujiaaspis. There had been a hypothesis that fins probably began as folds but is was just that - a hypothesis awaiting confirmation or falsification. That confirmation has now been provided.
As the press release from the University of Bristol explains:
What started out as simple skin folds that provided the early jawless fish with a better swimming ability by improving hydrodynamic flow over their bodies and improved buoyancy, rather like the wings of a glider, were to evolve into bilateral, paired, pectoral and pelvic fins, complete with a skeleton and muscles for active , powered swimming and improved control, rather than passive gliding, so laying down the basic pattern that was to evolved into the limbs of terrestrial tetrapods such as tiktaalik. And yet another set of specimens is added to the growing collection of Creationism's famous 'non-existent' transition fossils, so giving the frauds two more, albeit smaller, gaps in which to shoehorn their shrinking little god.A trove of fossils in China, unearthed in rock dating back some 436 million years, have revealed for the first time that the mysterious galeaspids, a jawless freshwater fish, possessed paired fins.The holotype specimen of the fossil Tujiaaspis vividus from 436 million year old rocks of Hunan Province and Chongqing, China.
Zhikun Gai
Computed fluid dynamic simulations of the flow of water over a digital model of Tujiaaspis vividus, viewed from above.
Zhikun Gai
The discovery, by an international team, led by Min Zhu of the Institute of Vertebrate Palaeontology and Palaeoanthropology, Beijing and Professor Philip Donoghue from the University of Bristol’s School of Earth Sciences, shows the primitive condition of paired fins before they separated into pectoral and pelvic fins, the forerunner to arms and legs.
Until now, the only surviving fossils of galeaspids were heads, but these new fossils originating in the rocks of Hunan Province and Chongqing and named Tujiaaspis after the indigenous Tujia people who live in this region, contain their whole bodies.
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 that 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.
Zhikun Gai, first author
Laboratory of Vertebrate Evolution and Human Origins
Institute of Vertebrate Paleontology and Paleoanthropology
Chinese Academy of Sciences (CAS), Beijing, China.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 to 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.
Professor Philip C. J. Donoghue.
Bristol Palaeobiology Group
School of Earth Sciences
University of Bristol, Bristol, UKTujiaaspis 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?
Min Zhu of VPP, corresponding author
Laboratory of Vertebrate Evolution and Human Origins
Institute of Vertebrate Paleontology and Paleoanthropology
Chinese Academy of Sciences (CAS), Beijing, ChinaThe 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.
Dr Humberto G. Ferrón, co-author
Bristol Palaeobiology Group
School of Earth Sciences
University of Bristol, Bristol, UK.Theories abound on the evolutionary beginnings of vertebrate fins and limbs – the evolutionary precursors of arms and legs - mostly based on comparative embryology. There is a rich fossil record, but early vertebrates either had fins or they didn’t. There was little evidence for their gradual evolution.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 that 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 that the evolutionary innovations seen in Tujiaaspis underpin locomotion in animals as diverse as birds, whales, bats and humans.
Dr Joseph N. Keating, co-author
Bristol Palaeobiology Group
School of Earth Sciences
University of Bristol, Bristol, UK.
Bristol’s Dr Humberto Ferron used computational engineering approaches to simulate the behaviour of models of Tujiaaspis with and without the paired fins.
Co-author Dr Joseph Keating at Bristol modelled the evolution of paired fins.
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