The fossil record of fish has provided numerous enigmas for those who like to construct evolutionary trees. A major branching point is at the inferred transition from jawless fish to jawed vertebrates (gnathostomes). The next notable inferred branching point relates to the separation of cartilaginous and bony fishes. But somewhere in the picture, a place must be found for the placoderms and the acanthodians, both archaic jawed fish. (For a previous blog on placoderms, go here)
The acanthodians are thought to be close to the branching point, but more than this is unclear. The problem has been one of very limited data - the fossil record of acanthodians is sparse. The best known specimen is Acantholes, a Permian acanthodian (just before the group became extinct). One specimen is comparatively well preserved and this has been used as evidence for bony fish affinities.
Climatius (top left; Lower Devonian), Diplacanthus (top right; Middle Devonian)and Acanthodes (center; Lower Permian) (copyright Dennis C. Murphy, Source: Devonian Times)
New research is based on an Early Devonian fossil with the name Ptomacanthus anglicus. First described about 30 years ago, this fossil comes from the Wayne Herbert Quarry in Herefordshire. The animal's braincase is preserved, and this has proved to be very significant. The research, by palaeontologist Martin Brazeau, has revealed characters similar to sharks and placoderms. This undermines the previous association with the bony fishes. The question for us is: does this new knowledge clarify the "tree of life" story, or does it undermine its foundations?
Cladistic analyses should always make clear the assumptions made by the analyst. (For a discussion of some of the principles of cladistics, go here). Brazeau summarises the previous work in this area with these words;
"Most of the recent hypotheses of acanthodian relationships expressed in cladistic terms have focused on their sister-group relations with chondrichthyans or osteichthyans. All of these studies have presupposed acanthodian monophyly and stereotyped acanthodian endoskeletal morphology on Acanthodes, the latest-occurring, and a highly apomorphic, genus."
The braincases of Acanthodes and Ptomacanthus show significant differences. Characters Acanthodes shared with bony fish led to cladograms where the acanthodians are aligned with the osteichthyans. The newly reported braincase of Ptomacanthus places the acanthodians tentatively with the chondrichthyans.
"A cladistic analysis of 45 ingroup and two outgroup taxa was performed on the basis of 134 characters. Ptomacanthus is placed as a basal stem chondrichthyan, but this result should be viewed with caution. [. . .] Many of the supporting characters are not known or applicable in recognized crown-group chondrichthyans."
These findings have led to big questions being raised about acanthodian monophyly. Do we have two groups here, or even more? This is not a simple case of moving the acanthodians from near the base of the bony fish branch to a similar place in the cartilaginous fish branch. There are too many characters that are distinctive to the early acanthodians, so this means organising the data using the perspective of an evolutionary tree is a challenge that is even more of a challenge than it was. Brazeau puts a brave face on this when he concludes with these words:
"Current conceptions of gnathostome phylogeny depict a rather simplistic arrangement of nominally monophyletic and, apparently, morphologically disparate groups. The emerging picture of acanthodian (and perhaps placoderm) paraphyly does not overturn a general consensus about gnathostome interrelationships. Instead, it populates the long, naked internal branches, revealing a much richer picture of character evolution in early gnathostomes."
Whatever this "richer picture" is, we can say with confidence that it is not Darwinian. The fossil record demonstrates that diversity did not emerge gradually, but appears abruptly. Then, the drivers for innovation apparently declined. Those familiar with other animal groups will not be too surprised at these comments: fish are not providing a pattern of diversification that is at all exceptional. A cause for concern, however, is the Darwinian insistence that their theoretical framework brings integration and coherence to biology. The inconvenient data tells us something rather different: the gradualist integration is imposed upon the data and is not required by it. The observed patterns of initial diversity followed by relative stasis fit well into a design paradigm, as has often been pointed out.
Also of concern is the way Darwinians claim ownership of the research findings. They keep presenting each new discovery as another successful step towards reaching their goal:
"Prehistoric Fish Provides New Piece In Evolution's Jigsaw Puzzle." (Science Daily)
"This figures in nicely with the emerging idea that acanthodians don't form a group of fishes that are all closely related to each other. Some of these fossils are primitive sharks while others are primitive bony fishes." (BBC News)
"It's an important piece in the puzzle for trying to understand one of the biggest events not only in our own evolutionary history, but also the vast majority of living animals with backbones." (National Geographic)
Why is this a concern? It is because press releases and media reports are packaging the research into tidy boxes for the masses. No one is thinking outside the box. No one is even questioning whether Darwinism is the appropriate paradigm for analysing the data. Whatever is found, the response is the same: we are putting the jigsaw puzzle together and it will eventually give us the picture we anticipate of the Tree of Life. What we need are methodologies that are open to the possibility that the picture may not be a tree: when the puzzle pieces are allowed to fit together properly, we may be looking at a forest.
The braincase and jaws of a Devonian 'acanthodian' and modern gnathostome origins
Martin D. Brazeau
Nature 457, 305-308 (15 January 2009) | doi:10.1038/nature07436
Modern gnathostomes (jawed vertebrates) emerged in the early Palaeozoic era, but this event remains unclear owing to a scant early fossil record. The exclusively Palaeozoic 'acanthodians' are possibly the earliest gnathostome group and exhibit a mosaic of shark- and bony fish-like characters that has long given them prominence in discussions of early gnathostome evolution1. Their relationships with modern gnathostomes have remained mysterious, partly because their un-mineralized endoskeletons rarely fossilized. Here I present the first-known braincase of an Early Devonian (approximately 418-412 Myr bp) acanthodian, Ptomacanthus anglicus, and re-evaluate the interrelationships of basal gnathostomes. Acanthodian braincases have previously been represented by a single genus, Acanthodes, which occurs more than 100 million years later in the fossil record. The braincase of Ptomacanthus differs radically from the osteichthyan-like braincase of Acanthodes in exhibiting several plesiomorphic features shared with placoderms and some early chondrichthyans. Most striking is its extremely short sphenoid region and its jaw suspension, which displays features intermediate between some Palaeozoic chondrichthyans and osteichthyans. Phylogenetic analysis resolves Ptomacanthus as either the most basal chondrichthyan or as the sister group of all living gnathostomes. These new data alter earlier conceptions of basal gnathostome phylogeny and thus help to provide a more detailed picture of the acquisition of early gnathostome characters.
Origin of Jawed Vertebrates: Prehistoric Fish Provides New Piece in Evolution's Jigsaw Puzzle, ScienceDaily (Jan. 19, 2009)
Syed, T. Fossil illuminates jaw evolution, BBC News (19 January 2009)
Dell'Amore, C. Oldest Shark Braincase Shakes Up Vertebrate Evolution, National Geographic News (January 14, 2009)
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