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Genetic Studies Weigh In

Earlier molecular studies supported the hypothesis of modern coelacanths as slow evolving. They examined HOX gene clusters which revealed low mutation rates.

However, later studies have criticized this work as the researchers looked at specific loci in the Latimeria genome rather than looking at a greater portion of the sequence. Critiques have cited that various factors such as the recombination rate, distance from the beginning site of replication in the DNA, and inconsistencies across species as reasons why the entire genome must be considered as variation in mutation rates will exist from one locus to another.

Population size and selective pressures also have an impact on mutation rates. IntroductionSince the discovery of extant populations off the coast of Africa, coelacanths have been labeled as the classic example of a living fossil. A living fossil is a species whose form has undergone few, if any, changes over geological time. They are considered to be literally living representations of specimens that have been collected in the paleontological record. An example of another living fossil is the Lingula brachiopod which we have had the opportunity to examine in class. Living fossils are also considered to be the only living example of their kind, with no extant living relatives available to compare forms.Though this term had begun to fall out of favor in academia, recent genetic studies on the modern coelacanth species has sparked a revival in the usage of the term due to preliminary results suggesting that the modern coelacanth evolves too slowly. Coelacanths refer to any member of the order Actinistian. They are most closely related to other modern tetrapods and the lungfish. Limb morphology is what sets them apart from other fish. Morphology in the Fossil RecordHere, we examine the structures of various actinistians to see just how much their structures have varied over time. In order for a species to truly classify as a living fossil, very little change must occur in its morphology over time. Living Fossils: An examination of the ActinistiansAlejandro TorresBibliographyConclusionThough extant populations of L. chalumnae and L. menadoensis may not be undergoing rapid genomic changes, this alone is insufficient justification for bringing back the term living fossil to describe the two species. The fossil record shows numerous examples of actinistians expressing a variety of body forms that do not exist in todays actinistians. The fact alone that L. chalumnae and L. menadoensis have been given their own species distinction separates them from their ancestral relatives. Living fossil is an incorrect interpretation of earlier work.DiscussionCoelacanth MorphologyTwo surviving members of actinistians exist into the present day. Latimeria chalumnae and Latimeria menadoensisNo fossil specimens of extant Latimeria exist..Variety in the shape of fossil specimens and overall fossil preservation suggest coelacanths occupied different environmental niches in the geological past.

Fig 1. The body forms of various extinct actinistians are compared to the modern coelacanth, Latimeria. Evidently, the shape of Latimeria differs greatly even from ancestral forms. (From Casane and Laurenti 2013)Various genetic studies confirm that Latimeria show lower mutation rates as compared to other vertebrates. The slow mutation rate present in both extant coelacanth species has been explained as a consequence of low population numbers, and little selective pressure. DNA studies are limited to modern coelacanth. For a discussion on whether or not to label an extant species as a living fossil, one must also incorporate information from the fossil record before drawing conclusions as far too much valuable information would otherwise go ignored.The fossil record presents an abundance of evidence that contradicts the notion that coelacanths have experienced little to no morphological change over time.

Amemiya, Chris T., et al. "The African Coelacanth Genome Provides Insights into Tetrapod Evolution."Nature496.7445 (2013): 311-6. Casane, Didier, and Patrick Laurenti. "Why Coelacanths are Not 'Living Fossils'."BioEssays35.4 (2013): 332-8. Dutel, Hugo, et al. "The Giant Cretaceous Coelacanth (Actinistia, Sarcopterygii) Megalocoelacanthus Dobiei Schwimmer, Stewart & Williams, 1994, and its Bearing on Latimerioidei Interrelationships."PLoS ONE7.11 (2012): 1-27.Graf, John. "A New Early Cretaceous Coelacanth from Texas."Historical Biology24.4 (2012): 441-52. Soto, Matas, et al. "Coelacanth Remains from the Late Jurassic?Earliest Cretaceous of Uruguay: The Southernmost Occurrence of the Mawsoniidae."Journal of Vertebrate Paleontology32.3 (2012): 530-7. Wendruff, Andrew J., Mark V. H. Wilson, and Hans Sues. "New Early Triassic Coelacanth in the Family Laugiidae (Sarcopterygii: Actinistia) from the Sulphur Mountain Formation Near Wapiti Lake, British Columbia, Canada."Canadian Journal of Earth Sciences50.9 (2013): 904-10.Wendruff, Andrew J., and Mark V. H. Wilson. "A Fork-Tailed Coelacanth, Rebellatrix Divaricerca , Gen. Et Sp. Nov. (Actinistia, Rebellatricidae, Fam. Nov.), from the Lower Triassic of Western Canada."Journal of Vertebrate Paleontology32.3 (2012): 499-511.. Palaeontologists have thus far identified and classified several coelacanth fossil specimens. The differences between each fossil species as compared to the modern coelacanth show enough morphological differences in body type and fin structure to justify classifying fossil specimens into their respective genera. Coelacanth species even differ greatly in size as the Latimeria are nearly three times larger than their nearest relative, Macropoma.

Fig. 3: Comparison of the skeletal structures of Latimeria and Macropoma. A and C represent Latimeria while B and D are Macropoma. E compares the pectoral fins of L. chalumnae (above) to Shoshonia arctopteryx (below). (From Casane and Laurenti 2013)

Fig 2: A look at the arrow-shaped caudal fin of the coelacanth, Belemnocerca prolata. It differs greatly from the caudal fin shape of both extant Latimeria species. (From Wendruff and Wilson 2013).1