Evolution: transitional fish (Evolution)

by David Turell , Wednesday, February 07, 2024, 16:45 (80 days ago) @ David Turell

380 million years ago:

https://www.sciencealert.com/bizarre-prehistoric-predator-fish-breathed-air-had-fangs-a...

"More than 380 million years ago, a sleek, air-breathing predatory fish patrolled the rivers of central Australia.

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"Known from at least 17 fossil specimens, Harajicadectes is the first reasonably complete bony fish found from Devonian rocks in central Australia. It has also proven to be a most unusual animal.

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"Harajicadectes was a fish in the Tetrapodomorpha group. This group had strongly built paired fins and usually only a single pair of external nostrils.

"Tetrapodomorph fish from the Devonian period (359–419 million years ago) have long been of great interest to science. They include the forerunners of modern tetrapods – animals with backbones and limbs such as amphibians, reptiles, birds and mammals.

"For example, recent fossil discoveries show fingers and toes arose in this group.

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"Up to 40 centimetres long, Harajicadectes is the biggest fish found in the Harajica rocks. Likely the top predator of those ancient rivers, its big mouth was lined with closely-packed sharp teeth alongside larger, widely spaced triangular fangs.

"It seems to have combined anatomical traits from different tetrapodomorph lineages via convergent evolution (when different creatures evolve similar features independently). An example of this are the patterns of bones in its skull and scales. Exactly where it sits among its closest relatives is difficult to resolve.

"The most striking and perhaps most important features are the two huge openings on the top of the skull called spiracles. These typically only appear as minute slits in most early bony fishes.

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"Other Devonian animals that sported such spiracles were the famous elpistostegalians – freshwater tetrapodomorphs from the Northern Hemisphere such as Elpistostege and Tiktaalik.

"These animals were extremely close to the ancestry of limbed vertebrates. So, enlarged spiracles seem to have arisen independently in at least four separate lineages of Devonian fishes.

"The only living fishes with similar structures are bichirs, African ray-finned fishes that live in shallow floodplains and estuaries. It was recently confirmed they draw surface air through their spiracles to aid survival in oxygen-poor waters.

"That these structures appeared roughly simultaneously in four Devonian lineages provides a fossil "signal" for scientists attempting to reconstruct atmospheric conditions in the distant past."

Comment: Something had to be pre-Tiktaalik. This fills another small gap in the fossil record.