Shark evolution mystery grows older and deeper

Humans and sharks last shared a common ancestor about 440 million years ago – earlier than previously thought, a new analysis of an ancient German shark reveals.

In a paper published in the journal Proceedings of the Royal Society B, scientists led by Michael Coates from the Department of Organismal Biology and Anatomy at the University of Chicago, US, describe Gladbachus adentatus, a 385-million-year-old shark originally found in Germany and now held at Cambridge University in the UK.

The specimen – comprising three sections, compressed, in a slab of resin – is the only example of the species ever found. In its rarity, it typifies the difficulties palaeontologists experience in trying to trace the evolution early sharks and their relationship to later cartilaginous fish and their more numerous bony cousins.

The early evolution of sharks and other cartilaginous species (known collectively as chondrichthyes), the researchers note, “has long been obscured by an impoverished fossil record”.

Although Gladbachus is known only from a single specimen it is significant because its resin casing has preserved much of its exterior, or endoskeleton. Coates and his colleagues carried out tissue sampling and computed tomography on the fossil to uncover details of its gills, jaws, cranium, cartilage and teeth.

The analysis revealed that the shark is a transitional form between a class of extinct fish known as acanthodians – which first show up in the fossil record some 50 million years earlier and show features of both bony and cartilaginous species – and true chondrichthyans.

The scientists report that their analysis simultaneously clarifies and complicates the story of the evolution of sharks and cartilaginous fish. 

The work, they report, leads to a “more balanced” interpretation of the early radiation of jawed-fish (a group called gnathostomes, today incorporating 99% of all species), but makes the stem lineages that led to cartilaginous fish “increasingly populated”. 

The data to date reveals that there is no single line of adaptation leading to modern chondrichthyans. Rather, the researchers write, there is evidence “implying repeated and convergent evolution of chondrichthyan-like specialisations”.

Any such conclusion must be extremely tentative, they caution, because the poor fossil record from the early Silurian period, 440 million years ago, to the middle Devonian 65 million years later when Gladbachus was swimming around means that species diversity “is fundamentally under-sampled.”

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