Monday, 13 July 2015

Coelacanths are not living fossils

One of the standard special creationist arguments against evolution is the so-called 'living fossil', organisms that allegedly have not changed at all over millions of years. The coelacanth is frequently cited by special creationists as an example of a fish that has 'forgotten to evolve'. For example, the Institute of Creation Research claims that:
One of the most spectacular living fossils is the coelacanth, a lobe-finned fish. Once known only from fossilized remains, this fish was considered by many to be a key transitional form (“missing link”) between fish and amphibians. Its fossils are found in Devonian strata, which are assigned a stunningly vast age of 400 million years. However, a live coelacanth hauled up in a fishing net off Madagascar in 1938 showed the same well-designed form as the fossils. It uses its unique fins to orient itself vertically in the deepest seas of the Indian Ocean, not for “walking” onto land from shallow waters. Where is any evidence of natural selection having made even one significant change in this fish over its supposed 400-million-year existence? A similar question could be asked of a host of living fossils.
There are many problems with this argument, ranging from the fact that such claims are often made on examination of photographs of fossil and living creature by non-specialists (detailed examination of both is needed to comment authoritatively on the degree of morphological change) to the creationist belief that evolution should take place along orthogenetic lines, as evidenced by their assumption that these fossils should have 'evolved' into something else, a comment betraying gross ignorance of evolutionary theory.

With respect to the coelacanth, the simple fact is that modern coelacanths differ dramatically in size and shape from extinct coelacanths. Once again, a YEC argument is made which can be debunked by simply looking up the facts. When we take the time to refer to the primary literature, we find out that the coelacanth has changed over time:
No fossil is available either for extant coelacanth species or for the genus Latimeria itself. This suggests that palaeontologists – even those that are convinced that coelacanths are ‘living fossils’ – have considered morphological differences between extant and fossil coelacanths to be so extensive that they should be grouped in separate genera. In fact, coelacanth body shapes are much more diverse than is generally thought. The vertebral column of coelacanth genera shows considerable variation in the number of neural and haemal arches, as well as in the spacing throughout the abdominal and caudal regions, suggesting that they probably had diverse modes of locomotion. 
In addition, an examination of the skeleton of the fossil genus Macropoma (approximately 70 Ma), the sister group of Latimeria and the only known fossil actinistian record from the Cretaceous to the present, shows some interesting differences. Not only are the extant coelacanths three times larger than their closest extinct relatives (about one and a half metres vs. half a metre), but there are also numerous structural differences. The swim bladder is ossified in Macropoma but filled with oil in Latimeria, indicating they were probably found in different types of environments. There are also noticeable differences in the vertebral column (the post anal region is shorter and ventral spines extend less ventrally in M. Lewesiensis compared with L. chalumnae), and in the attachment bones of the fins. In addition, Macropoma and Latimeria have distinctly dissimilar skull anatomies, resulting in noticeable differences in head morphology. 
Finally, it should not be forgotten that external morphological resemblances can be based on a very different internal anatomical organisation. The most often emphasised resemblance between coelacanths is that they all have four fleshy-lobed-fins. Until recently, the anatomy of the lobed fins of coelacanths was only known in Latimeria, in which the pectoral fin endoskeleton is short and symmetrical. In 2007, Friedman et al. described the endoskeleton of the pectoral fin of Shoshonia arctopteryx, a coelacanth species from the mid Devonian, and therefore contemporary with Miguashaia. They showed that this earliest known coelacanth fin endoskeleton is highly asymmetrical, a characteristic that is probably ancestral since it resembles the condition found in early sarcopterygians such as Eusthenopteron, Rhizodopsis or Gogonasus. This result is additional support, if needed, that extant coelacanths have not remained morphologically static since the Devonian. [1] (Emphasis mine)
Modern coelacanths differ both in size and shape to extinct fossils. 

Figure 1. Comparison of extant and selected extinct actinistians, commonly known as coelacanths. A phylogeny of Actinistia; schematic sketches of body outlines and approximate body length (given in metre) illustrate the morphological diversity of extinct coelacanths: some had a short, round body (Hadronector), some had a long, slender body (Rebellatrix), some were eel-like (Holopterygius) whereas others resembled trout (Rhabdoderma), or even piranha (Allenypterus). Note that the body shape of Latimeria chalumnae differs significantly from that of its closest relative, Macropoma lewesiensis. Adapted from 3, 42, 43. (Source: BioEssays (2013) 35:332-338)
In fact, the concept of a ‘living fossil’ is one that is not helpful at all, and is in many ways a legacy of a fundamentally flawed way of thinking of evolution. As Casane and Laurenti note: 
During the last century, the concept of ‘living fossil’ as a result of evolutionary stasis has been increasingly regarded as misleading as milestones have been added to Darwin's theory of evolution. Hennig, clarified the speciation process (or cladogenesis) by showing that the ancestral species ceased to exist as two new sister species formed. In addition, Gould and Kimura, have reinforced the idea that evolution is a bushy process that has not stopped in any lineage. However, the phylogenetic paradigm, also called ‘tree-thinking’ point of view, has encountered difficulties in spreading outside the community of evolutionary researchers. The most common misunderstanding of tree-thinking and evolution is the remnant misinterpretation of biodiversity as a ‘ladder of progress’, a concept that originated in the pre-evolutionist idea of scala naturae that came from antiquity as it appeared in Aristotle's Historia Animalium.[2]
Evolution? The fossils definitely say yes.


[1] Casane D, Laurenti P “Why coelacanths are not ‘living fossils’. A review of molecular and morphological dataBioEssays (2013) 35:332-338

[2] ibid, p 335-336