Coelacanths
Lobe-Finned Fishes of the Western Interior Sea
Compared with earlier geological periods, coelacanth diversity had declined quite a lot by the Late Cretaceous. However, despite this they continued to occupy a role within marine ecosystems alongside other vertebrates inhabiting the Western Interior Sea. Fossil evidence indicates that one of the largest known coelacanths of all time, Megalocoelacanthus, was found in the Interior Sea at this time. These animals would have been fearsome, albeit fairly slow moving, predatory animals who preyed upon smaller fishes and even mid-sized marine reptiles.
Anatomy
Skeletal Anatomy
Coelacanths have a series of strong bones at the bases of their fins which support the muscles of their fin lobes. Some of these bony structures contain the bones which are homologous with the bones in the arms and legs of tetrapods. Most coelacanths also keep a large spinal notochord throughout adulthood. Unlike the fully developed vertebral columns of most modern bony fishes, the coelacanth's notochord remains as a fluid-filled, supportive rod running along the body in place of a bony spine. At the front of their skulls they have a special rostral organ which allows these fishes to pick up electrical signals left by their prey. The outer parts of the bodies of coelacanths are covered in a coat of mineralized scales. Much of their skeleton consists of cartilage rather than dense bone, reducing weight while maintaining structural support. Their tails are tri-lobed and have a small central extension along the middle of the caudal fin set known as the epicaudal lobe.
(BELOW) A labeled diagram of the skeleton of a coelacanth. Note the presence of a very large notochord running along the length of the animal in the place of a spine. This notochord is an unusual feature among members of the bony fish group
Evolutionary History
Lobe-finned fishes are a group of vertebrate animals with bony ossified skeletons and whose earliest members had/have fleshy, bone-supported paired fins. These fins each have a muscular lobe at their bases instead of having fins composed only of sets of long thin rays as in ray finned actinopterigian fishes. The main three groups of lobe-finned fishes are the coelacanths, the lungfishes, and the tetrapods, including most terrestrial vertebrates. Coelacanths are the best known type of lobe-finned fishes. Although they are only found in deep waters today, their predecessors lived in shallow water and riparian environments for millions of years. This lineage first showed up during the early Devonian but they suffered a massive drop in species diversity at the end of the Cretaceous period. Lungfishes are another type of lobe-finned fish. These animals are able to breathe air for short periods of time. Despite this air breathing ability these lungfishes are not ancestral to the other group of air-breathing lobe-finned fishes, the tetrapods. Tetrapods, including all reptiles, amphibians, birds and mammals, are all descended from aquatic lobe-finned fish ancestors. As such this group is by far the most abundant lineage of lobe-finned fish.
(BELOW) The main living groups of lobe-finned fishes include coelacanths, lungfishes and tetrapods. This technically means that all terrestrial vertebrates are part of the lobe-finned fish clade.
Megalocoelacanthus
Temporal Range: 87 million years ago - 66 million years ago
Geographic Range: Kansas
Species: M. dobei
Diet: Carnivorous, fish, cephalopods, marine reptiles
Megalocoelacanthus was a very large genus of coelacanths that lived in the Late Cretaceous seas of North America. It has been estimated to have grown to be up to 15 feet long, which would make it one of the largest coelacanths known from the fossil record. Fossils recovered from the Niobrara Formation of Kansas show that these animals had a narrow parietonasal shield bordered by unusually large openings on the sides of its head that housed parts of the supraorbital sensory canal system. A similar skull arrangement is seen only in a few closely related genera, including the genus Libys from the Jurassic period. Megalocoelacanthus also seems to have been largely edentulous, meaning that it probably had very few teeth. Some mawsoniid coelacanths also have this toothless feature but these two animals seem to be fairly distantly related to one another and Megalocoelacanthus is currently positioned as a closer relative of the living coelacanth genus Latimeria.
Suggested References: Coelacanths
Brito, Paulo M.; Meunier, François J.; Clément, Gael; Geffard-Kuriyama links, Didier (2010). "The histological structure of the calcified lung of the fossil coelacanth Axelrodichthys araripensis (Actinistia: Mawsoniidae)". Palaeontology. 53 (6): 1281–90.
Dutel, H., Maisey, J., Schwimmer, D., Janvier, P. and Clément, G. (2011). “Giant coelacanth Megalocoelacanthus dobiei from the Upper Cretaceous of North America and its bearings on the phylogeny of Mesozoic coelacanths”. Journal of Vertebrate Paleontology 31
Everhart, M.J. (2017). “Oceans of Kansas - A Natural History of the Western Interior Sea,” Second Edition. Indiana University Press, 460 pp.
Fricke, H.; Plante, R. (1988). "Habitat requirements of the living coelacanth Latimeria chalumnae at grande comore, Indian Ocean". Naturwissenschaften. 75 (3): 149–51.
Schwimmer, D. R., Stewart, J.D. and Williams, G.D. (1994). “Giant fossil coelacanths of the Late Cretaceous in the eastern United States”. Geology. 22:503-506.