Post details: Critical transitions in fish evolution lack fossil documentation


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Critical transitions in fish evolution lack fossil documentation

Recent discoveries of fish fossils from Devonian sedimentary rocks have raised expectations for a major rethink of fish phylogeny. The recent report of a Late Silurian sarcopterygian fish has made it certain that earlier evolutionary accounts are in need of a thorough overhaul.

artistic reconstruction
Artist's restoration of Guiyu oneiros (Image credit: Brian Choo, source here)

The new fossil, Guiyu oneiros, is exceptionally well preserved in part and counterpart. It is an "articulated and three-dimensionally preserved bony fish lacking only the caudal fin." Although fish fragments have been found in Silurian rocks before, this discovery transcends them all, because it is so complete. Speculations about fragments suddenly have to address the realities of this new fossil. Previously, the important transitions in jawed vertebrates (gnathostomes) were considered a Devonian phenomenon. The (unresolved) discussions concerned placoderms and acanthodians, and their relationships with the chondrichthyans (cartilaginous fish) and osteichthyans (bony fish, subdivided into the ray-finned fishes - the Actinopterygii - and the lobe-finned fishes - the Sarcopterygii). The Silurian fish fossil fundamentally changes this debate.

"The straightforward message is that the origin of modern gnathostomes is not a Devonian phenomenon, after all. The basal divergence between osteichthyans and chondrichthyans occurred somewhat earlier.
[. . .]
"The very fact that Guiyu can be identified as a sarcopterygian provides further and arguably clinching evidence that a whole series of major branching events within the gnathostome crown group must have taken place well before the end of the Silurian."

The generic name given to the fossil draws attention to the mosaic of gnathostome characters it possesses. Gui is Chinese Pinyin for "ghost" or "secret" and yu is Chinese Pinyin for "fish". The research team interprets the mosaic morphology in terms of the animal being an intermediate, demonstrating "the incremental acquisition of osteichthyan apomorphies". However, mosaic morphologies are not exceptional (this was noted with the acanthodians), and the proof of "incremental acquisition" of characters is whether the fossil record documents this. Does it provide us with a tree, a bush, or something else? Like so many other groups of animals, the data appears to point to a bush.

Guiyu is said to have a mixture of "primitive and advanced features". Needless to say, the judgment of what is primitive and what is advanced rests largely with the researcher. This is the door for evolutionary presuppositions to colour the resulting analysis. The research team provide a cladistic analysis of these characters and present a cladogram in their Figure 5. It shows fossilised stem sarcopterygians co-existing in time with fossilised crown sarcopterygians. The crucial divergences are undocumented by fossil material. Guiyu is interpreted as a early stem sarcopterygian.

"The discovery of Guiyu offers an exceptional example of a primitive fish close to the split of crown osteichthyans. However, our understanding of the stem section of the Osteichthyes phylogenetic tree still remains vague owing to the rarity of relevant fossils."

To be fair to the authors, they do caution their readers to hold their analysis tentatively. "Our phylogenetic scenario, although it should be approached with some caution considering the lower Bremer indices at several nodes [. . .]". An example of the way presuppositions have guided the analysis comes in the placement of Ligulalepis at the base of the Sarcopterygii. They consider that "its neurocranium reveals the primitive osteichthyan condition". Guiyu and other species placed as stem sarcopterygians are said to be "more derived with regard to its dermal and endoskeletal intracranial joints". Yet, Guiyu and these other stem sarcopterygians, as well as most of the crown sarcopterygian analysed, all predate Ligulalepis as far as fossil evidence is concerned. More on the issues raised by cladistic analysis is here).

The new fossil necessarily brings big changes to the ways people should think about fish evolution. Although commentators are queuing up to say that the new fossil casts light on fish phylogeny and "illuminates an early stage in the evolutionary history of the bony fishes", the reality is that all the key transitions are pushed back into the Silurian - and the link with physical data is broken. The schemes that will emerge (unless significant new fossils are found) will all be based on cladistic analyses which presuppose evolutionary transformation. This may suit some, but this is not an acceptable way to pursue a scientific approach to origins. Coates is right to say:
"By pushing a whole series of branching points in gnathostome evolution out of the Devonian and into the Silurian, the discovery of Guiyu also signals that a significant part of early vertebrate evolution is unknown."

The oldest articulated osteichthyan reveals mosaic gnathostome characters
Min Zhu, Wenjin Zhao, Liantao Jia, Jing Lu, Tuo Qiao & Qingming Qu
Nature 458, 469-474 (26 March 2009) | doi:10.1038/nature07855 (abstract)

Abstract: The evolutionary history of osteichthyans (bony fishes plus tetrapods) extends back to the Ludlow epoch of the Silurian period. However, these Silurian forms have been documented exclusively by fragmentary fossils. Here we report the discovery of an exceptionally preserved primitive fish from the Ludlow of Yunnan, China, that represents the oldest near-complete gnathostome (jawed vertebrate). The postcranial skeleton of this fish includes a primitive pectoral girdle and median fin spine as in non-osteichthyan gnathostomes, but a derived macromeric squamation as in crown osteichthyans, and substantiates the unexpected mix of postcranial features in basal sarcopterygians, previously restored from the disarticulated remains of Psarolepis. As the oldest articulated sarcopterygian, the new taxon offers insights into the origin and early divergence of osteichthyans, and indicates that the minimum date for the actinopterygian-sarcopterygian split was no later than 419 million years ago.

See also:

Coates, M.I. Beyond the Age of Fishes, Nature 458, 413-414 (26 March 2009) | doi:10.1038/458413a (text)


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