Darwin's solitary illustration of evolutionary branching has left a lasting impression in the minds of readers. From an ancestral form, speciation occurs and the diversity of descendants increases. This can be visualised as a cone of morphological variation, extending from the source. However, the Cambrian Explosion provides empirical evidence against this concept, as a large number of organisms appear abruptly. (For more, see here) Yet it has been tempting for Darwinists to interpret the Cambrian species in terms of a number of cones of increasing diversity that all have their origins deeper in the Precambrian. This was something Stephen Jay Gould attempted to counter by proposing an "inverted cone" model. But there is a need for a third model to be on the table for consideration. For some years, it has been recognised that two groups of animals have very similar disparities in the Cambrian and the Recent. These are the arthropods and the priapulid worms.
"Both are reported to have comparable morphological disparity in the Cambrian and the Recent. This is important for our understanding of the manner in which metazoans radiated, because it implies that Cambrian animals had already explored a variety of 'design' options similar to that realized by their present-day counterparts. [. . .] This challenges the traditional 'cone of increasing diversity' (or, more precisely, it shifts this evolutionary model back in time) in favour of an approximately cylindrical model of bodyplan diversity from the Cambrian to the Recent. It also implies that the magnitude of Cambrian morphological diversity requires some form of explanation: whether in terms of Pre-Cambrian evolution at a small size, the gradual Pre-Cambrian differentiation of internal bodyplans decoupled from the appearance of external (and fossilizable) characters in the Cambrian, or some other mechanism." (p.2057)
Priapulus caudatus (source here)
A recent examination of the available evidence about priapulids and their allies has been undertaken by Matthew Wills and colleagues. Previous analyses appeared in 1998 and 2001, but since then there have been a good number of important fossil finds and two published appraisals of the phylogeny of the group. The authors considered that the time was opportune for a reassessment of the cylindrical model of diversity.
"Does this considerable expansion of our knowledge influence the conclusions of the original paper? We therefore ask to what extent the findings of the initial study are sensitive to factors such as improvements in taxon sampling, the discovery of new fossils through research time and differences in the characters coded." (p.2057)
We shall pass over the "Materials & Methods" and address the results and their implications. The core conclusions of the earlier assessment of disparity have been confirmed.
"It is not surprising that the discovery and description of new fossil taxa (along with better and more detailed studies of both fossil and extant species) has influenced some generalities and many details of the morphospace analyses. However, the general conclusions drawn from the first such study appear robust to these additions and changes." (p.2070)
"Our results were consistent with previous findings: Cambrian priapulids, archaeopriapulids and palaeoscolecids are only marginally less disparate than their extant counterparts." (p.2071)
After pointing out that some of the newer fossil finds are quite close in appearance to some modern forms, the authors point out the significance for grappling with the Cambrian Explosion. Those who seek to smooth out the phenomenon to make it more compatible with Darwinian gradualism are being led by their theoretical premises and are failing to engage with evidence.
"What implications do these new findings have for our understanding of the Cambrian explosion? Most significantly, indices of morphological disparity (unlike morphological phylogenies) are robust to modest differences in taxon and character sampling, lending our results a new rigour. Hence, the near-extant levels of disparity embodied by the Cambrian worms indicate that their appearance in the fossil record must have been preceded by large amounts of morphological change during their divergence from a presumed common ancestor. Although debates on the time available for this divergence are set to continue, the enormous magnitude of the explosion it represents is clear." (p.2072)
The press release about the paper provided this commentary on the significance of the findings:
"Dr Wills first pioneered a study on existing and extinct priapulids in 1998. Fourteen years on, the team looked at a new and expanded data set of anatomical features to see how knowledge of these worms has been affected by new fossil finds. He explained: "The fossils from the Cambrian period can cause a real headache for evolutionary biologists. Instinct tells us to expect simple organisms evolving over time to become increasingly more complex. However during the Cambrian period there was an apparent explosion of different major groups of animals, all appearing simultaneously in the fossil record. We looked at priapulid worms, which were among the first ever predators. What's remarkable is that they had already evolved into a diverse array of forms -- comparable to the morphological variety of their living cousins -- when we first encounter them in the Cambrian fossil record. It's precisely this apparent explosion of anatomical diversity that vexed Darwin and famously attracted the attention of Harvard biologist Stephen Jay Gould.""
The focus on morphological disparity is a welcome aspect of the research, as it allows discussion of overall trends and allows overlaps between fossil and Recent species. The authors present this as one of the strengths of their approach:
"Unlike the results of cladistic analyses (which are extremely sensitive to the details of taxon and character sampling), the results of disparity analyses are much more robust. The intensity of local clustering and discontinuity with which our morphospaces are occupied increases with research time, thus providing greater insights into the structure of the morphospace. Cambrian and post-Cambrian time bins now display positive clustering of their constituent species. Their nonoverlapping distributions also yield a globally heterogeneous pattern when the data for all species are pooled." (p.2072)
From a design perspective, this research is exemplary in drawing out patterns in the data. For too long, we have been led (via the educational system) to view the fossil record through the coloured glasses of Darwinism. This has led to the shoe-horning of data to fit the theory, with very little attention given to testing the theory against the data. With attention to bauplan (or bodyplan) thinking linked to measures of morphospace, and recognising that organisms rich in biological information have phenotypic plasticity that operates independently of natural selection acting on mutations (see here), we have a workable research paradigm. We can enthusiastically endorse the words of co-author Dr Marcello Ruta:
"Detailed scrutiny of other groups of organisms is needed, in order to decipher the rate at which structural, functional and ecological changes occur and how acquisition of new traits impact on group diversification. Ultimately, combined results from these investigations will offer a solid framework for understanding the very roots of Life's grandeur and the astounding variety of species alive today." (source here)
The disparity of priapulid, archaeopriapulid and palaeoscolecid worms in the light of new data
M. A. Wills, S. Gerber, M. Ruta, M. Hughes
Journal of Evolutionary Biology, October 2012, 25(10), 2056-2076 | DOI: 10.1111/j.1420-9101.2012.02586.x
Abstract: Priapulids and their extinct relatives, the archaeopriapulids and palaeoscolecids, are vermiform, carnivorous ecdysozoans with an armoured, extensible proboscis. These worms were an important component of marine communities during the Palaeozoic, but were especially abundant and diverse in the Cambrian. Today, they comprise just seven genera in four families. Priapulids were among the first groups used to test hypotheses concerning the morphological disparity of Cambrian fossils relative to the extant fauna. A previous study sampled at the generic level, concluding that Cambrian genera embodied marginally less morphological diversity than their extant counterparts. Here, we sample predominantly at the species level and include numerous fossils and some extant forms described in the last fifteen years. Empirical morphospaces for priapulids, archaeopriapulids and palaeoscolecids are relatively insensitive to changes in the taxon or character sample: their overall form has altered little, despite the markedly improved sampling. Cambrian and post-Cambrian genera occupy adjacent rather than broadly overlapping regions of these spaces, and Cambrian species still show lower morphological disparity than their post-Cambrian counterparts. Crucially, the significance of this difference has increased with improved taxon sampling over research time. In contrast with empirical morphospaces, the phylogeny of priapulids, archaeopriapulids and palaeoscolecids derived from morphological characters is extremely sensitive to details of taxon sampling and the manner in which characters are weighted. However, the extant Priapulidae and Halicryptidae invariably resolve as sister families, with this entire clade subsequently being sister group to the Maccabeidae. In our most inclusive trees, the extant Tubiluchidae are separated from these other living taxa by a number of small, intervening fossil clades.
Marine Worms Reveal the Deepest Evolutionary Patterns, ScienceDaily (9 October 2012)
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