Bothriolepis

Bothriolepis is a genus placed within the placoderm order Antiarchi. The earliest antiarch placoderms first appeared in the Silurian period of the Paleozoic Era and could be found distributed on every paleocontinent by the Devonian period. Early ontogenetic stages of placoderms had thinner bony plates within both the head and trunk-shield, which allowed for easy distinction between early placoderm ontogenetic stages within the fossil record and taxa that possessed fully developed bony plates but were small by characterization. The last species of Bothriolepis died out, together with the rest of Placodermi, at the end of the Devonian period. == General anatomy ==

, Chicago Head There are two openings through the head of Bothriolepis: a keyhole opening along the midline on the upper side for the eyes and nostrils and an opening for the mouth on the lower side near the anterior end of the head. A discovery regarding preserved structures that appear to be nasal capsules confirms the belief that the external nasal openings lay on the dorsal side of the head near the eyes. Additionally, the position of the mouth on the ventral side of the skull is consistent with the typical horizontal resting orientation of Bothriolepis. It had a special feature on its skull, a separate partition of bone below the opening for the eyes and nostrils enclosing the nasal capsules called a preorbital recess. Jaw A new sample from the Gogo Formation in the Canning Basin of Western Australia has provided evidence regarding the morphological features of the visceral jaw elements of Bothriolepis. Using the sample, it is evident that the mental plate (a dermal bone that forms the upper part of the jaw) of antiarchs is homologous with the suborbital plate found in other placoderms. The lower jawbone consists of a differentiated blade and biting portions. Next to the mandibular joint are the prelateral and infraprelateral plates, which both are canal-bearing bones. The palatoquadrate lacks a high orbital process and was attached only to the ventral part of the mental plate, proving that the ethmoidal region of the braincase (the region of the skull that separates the brain and nasal cavity) was in fact deeper than originally believed. In addition to the above-listed sample from the Gogo Formation, several other specimens have been found with mouthparts held in the natural position by a membrane that covers the oral region and attaches to the lateral and anterior margins of the head. Bothriolepis has a jaw in which the two halves are separate and in the adult are functionally independent. The trunk's outline suggests that there may have been a notochord present surrounded by a membranous sheath, Fins and tail Bothriolepis had a long pair of spine-like pectoral fins, jointed at the base, and again a little more than halfway along. These spike-like fins were probably used to lift the body clear off the bottom; its heavy armor would have made it sink quickly as soon as it lost forward momentum. == Soft anatomy ==

Structures composed of soft tissue are typically not preserved in fossils because they break down easily and decompose much faster than hard tissues, meaning that the fossil record often lacks information regarding the internal anatomy of fossil species. Preservation of soft tissue structures can sometimes occur, however, if sediments fill the internal structures of an organism upon or after its death. Robert Denison's paper titled "The Soft Anatomy of Bothriolepis" explores the forms and organs of Bothriolepis. == Feeding ==

Bothriolepis, as with all other antiarchs, are thought to have fed by directly swallowing mouthfuls of mud and other soft sediments in order to digest detritus, small or microorganisms, algae, and other forms of organic matter in the swallowed sediments. Additionally, the positioning of the mouth on the ventral side of its head further suggests that Bothriolepis was likely a bottom-feeder. The regular presence of "carbonaceous material in the alimentary tract" is believed to indicate that most of its diet consisted of plant material. == Distribution ==

Bothriolepis fossils are found in Middle and Late Devonian strata (from 387 to 360 million years ago). Because the fossils are found in freshwater sediments, Bothriolepis is presumed to have spent most of its life in freshwater rivers and lakes, but was probably able to enter salt water as well, because its range appeared to have corresponded with the Devonian continental coastlines. Large groupings of Bothriolepis specimens have been found in Asia, Europe, Australia (Gogo Formation and Mandagery Sandstone), and all around the world. Catskill Formation site The Catskill Formation (Upper Devonian, Famennian Stage), located in Tioga County, Pennsylvania, is the site of a large sample of small individuals of Bothriolepis. The sample was collected from a series of rock slabs that consisted of partial or complete, articulated, external skeletons. More than two hundred individuals were found packed closely together with little to no overlap. From this sample, much information regarding characteristics of juvenile Bothriolepis can be determined. A morphometric study performed by Jason Downs and co-authors highlights certain characteristics that indicate juvenility in Bothriolepis, including a moderately large head and moderately large orbital fenestra—both of which are characteristics also recognized by Erik Stensio in 1948 in the smallest B. canadensis individuals. == Species ==

Vertebrate paleontology is heavily dependent on the ability to differentiate between different species in a way that is consistent both within a particular genus and across all organisms. The genus Bothriolepis is no exception to this principle. Listed below are a few of the notable species within Bothriolepis; more than sixty species have been named in total, and it is likely that a sizeable proportion of them are valid due to the cosmopolitan nature of Bothriolepis. The external skeleton of Bothriolepis canadensis is made of cellular dermal bone tissue and is characterized by distinct horizontal zonation or stratification. The model fish has an average total length of and an average dermal armor length of , which accounts for 35.6% of the estimated total length. is the Bothriolepis species known from the highest paleolatitude, being described from deposits originally laid down within the Late Devonian Antarctic circle. Remains have exclusively been recovered from a single carbonaceous shale near the top of the Late Devonian, Famennian, Witpoort Formation (Witteberg Group) exposed in a road cutting south of Makhanda/Grahamstown in South Africa. This site, the Waterloo Farm lagerstätte is interpreted as representing a back barrier coastal lagoonal setting with both marine and fluvial influences. Gess observed that Bothriolepis was less abundant at the Waterloo Farm site than at most Bothriolepis-bearing localities, though a full ontogenetic series is represented. The head and trunk armour lengths ranged between which translates, based on the proportions of two of the smallest individuals (in which tail impressions are preserved) into full body lengths varying between . According to original description, Bothriolepis africana was considered to be most closely similar to Bothriolepis barretti from the late Givetian of Antarctica. The similarities between the two have been used to suggest derivation of Bothriolepis africana from an East Gondwanan environment. This species, found in present-day Pennsylvania, was originally described by J. Leidy in 1856. As mentioned above, there is much debate regarding the distinguishability between B. nitida and B. virginiensis, however based on evidence presented by Weems (2004), Characteristics that distinguish B. virginiensis from other species include but are not limited to fused head sutures, fused elements in adult distal pectoral fin segments and long premedian plate relative to headshield length. == References ==