Bothriolepis

Bothriolepis Temporal range: Ma PreꞒ Ꞓ O S D C P T J K Pg N
Cast of fossil of B. canadensis
Reconstruction of B. africana, reconstructed after anatomy of B. canadensis
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	†Placodermi
Order:	†Antiarchi
Family:	†Bothriolepididae
Genus:	†Bothriolepis Eichwald, 1840
Species
Holotype: B. ornata Eichwald, 1840 B. africana Long et al., 1997 B. alexi Young, 1988 B. alvesiensis Stensio, 1948 B. amankonyrica Malinovskaja, 1988 B. antarctica Woodward, 1921 B. askinae Young, 1988 B. babichevi Malinovskaja, 1992 B. barretti Young, 1988 B. bindareii Long, 1983 B. canadensis Whiteaves, 1880 B. cellulosa Pander, 1846 B. ciecere Lyarskaja, 1974 B. coloradensis Eastman, 1904 B. cristata Traquair, 1895 B. cullodenensis Long, 1983 B. dairbhrensis Dupret et al., 2023 B. darbiensis Denison, 1951 B. dorakarasugensis Moloshnikov, 2009 B. evaldi Lyarskaja, 1986 B. favosa Agassiz, 1844 B. fergusoni Long, 1983 B. gigantea Traquair, 1888 B. gippslandiensis Hills, 1929 B. grenfellensis Johanson, 1996 B. groenlandica Heintz, 1930 B. hayi Miles, 1968 B. heckeri Luksevics, 2001 B. hicklingi Miles, 1968 B. hydrophila Agassiz, 1844 B. jani Luksevics, 1986 B. jarviki Stensio, 1948 B. jazwicensis Szrek, 2004 B. jeremijevi B. karawaka Young, 1988 B. kassini Malinovskaja, 1988 B. kohni Young, 1988 B. kwangtungensis Pan, 1964 B. laverocklochensis Miles, 1968 B. leptocheira Traquair, 1893 B. lochangensis Pan, 1964 B. lohesti Leriche, 1931 B. macphersoni Young, 1988 B. macrocephala Egerton, 1862 B. maeandrina B. markovskii Moloshnikov, 2010 B. mawsoni Young, 1988 B. maxima Gross, 1933 B. meandrina Horrmann, 1911 B. minor Newberry, 1889 B. nielseni Stensio, 1948 B. nikitinae Malinovskaja, 1988 B. nitida Leidy, 1856 B. niushoushanensis Pan & Wang, 1980 B. obesa Traquair, 1888 B. obrutschewi Gross, 1934 B. ornata Andrews, 1982 B. panderi Lahusen, 1880 B. paradoxa Agassiz, 1845 B. pavariensis Lyarskaja, 1974 B. perija Young & Moody, 2002 B. portalensis Young, 1988 B. prima Gross, 1934 B. rex Downs et al., 2016 B. sanzarensis Moloshnikov, 2010 B. shaokuanensis Liu, 1963 B. sibrica Obruchev, 1941 B. sinensis Chi, 1940 B. sosnensis Moloshnikov, 2003 B. stevensoni Miles, 1968 B. tastenica Malinovskaja, 1988 B. tatongensis Long & Werdelin, 1985 B. taylori Miles, 1968 B. traquairi Bryant, 1924 B. trautscholdi Jaekel, 1927 B. tungseni Chang, 1965 B. turanica Obruchev, 1939 B. verrucosa Young & Gorter, 1981 B. virginiensis Weems et al., 1981 B. volongensis Lyarskaja & Luksevics, 1998 B. vuwae Young, 1988 B. warreni Long, 1983 B. wilsoni Miles, 1968 B. yeungae Johanson, 1998 B. yunnanensis Liu, 1962 B. zadonica Obrucheva, 1983

Bothriolepis (from

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.[7] 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.^[8] 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.^[9] Bothriolepis has a jaw in which the two halves are separate and in the adult are functionally independent.^[9]

Trunk

Bothriolepis had a slender trunk that was likely covered in soft skin with no scales or markings. The orientation that appears to have been mostly stable for resting was the dorsal surface up, evidenced by the flat surface on the ventral side.

The dermal skeleton is organized in three layers: a superficial lamellar layer, a cancellous spongiosa, and a compact basal lamellar layer. Even in early ontogeny, these layers are apparent in specimen of Bothriolepis canadensis. The compact layers develop first.[10] The superficial layer is speculated to have denticles that may have been made of cellular bone.^[11]

Fins and tail

Bothriolepis had a long pair of spine-like

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.^[7] These internal structures were preserved when different types of sediments surrounding the exterior of the animal-filled the internal carapaces (only organs that communicate with the exterior could be preserved in this manner). Three different sediment types were identified within the different sections of Bothriolepis: the first a pale greenish-gray medium-textured sandstone largely consisting of calcite; the second similar but finer sediment which preserves many of the organ forms; and the third distinct, fine-grained siltstone consisting of quartz, mica and other minerals but no calcite.^[7] These sediments helped preserve the following internal elements:

Alimentary system

In general, the

It is inferred that the gills of Bothriolepis are of the primitive type, though their structure is still not well understood. Laterally, they are enclosed by an opercular fold and are found in the space beneath the lateral part of the head shield, extending medially underneath the neurocranium. Compared to the gills of normally-shaped fish, the gill region of Bothriolepis is considered to be placed more dorsally, is anteriorly more crowded, and in general is relatively short and broad.^[7]

Paired ventral sacs

Extending posteriorly from the trunk carapace are paired ventral sacs that extend to the anterior end of the spiral intestine. The sacs seem to originate at the pharynx as a single median tube, which then broadens posteriorly and eventually splits into two sacs that may be homologous to the

Despite the original interpretation presented by Denison in 1941, not all paleontologists agree that placoderms like Bothriolepis actually possessed lungs. For example, in his paper "Lungs" in Placoderms, a Persistent Palaeobiological Myth Related to Environmental Preconceived Interpretations, D. Goujet suggests that although traces of some digestive organs may be apparent from the sedimentary structures, there is no evidence supporting the presence of lungs in the samples from the Escuminac formation of Canada upon which the original assertion was based. He notes that the worldwide distribution of Bothriolepis is restricted to strictly marine environments, and thus believes that the presence of lungs in Bothriolepis is uncertain. Further investigation of the fossils is likely necessary to reach a conclusion about the presence of lungs in Bothriolepis.^[15]

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.^[7]

Distribution

Bothriolepis fossils are found in Middle and Late

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.[3]

Bothriolepis canadensis

Bothriolepis canadensis is a taxon that often serves as a model organism for the order Antiarchi because of its enormous sample of complete, intact specimens found at the Escuminac Formation in Quebec, Canada.^[1] Because of the vast sample size, this species is often used to compare growth data of newly acquired specimens of Bothriolepis, including those found in the Catskill Formation mentioned above. This comparison allows researchers to determine if newly found samples represent juvenile individuals or new "Bothriolepis" species.

B. canadensis was first described in 1880 by J.F. Whiteaves, using a limited number of disfigured samples. The next to propose a reconstruction of the species was W. Patten, who published his findings in 1904 after a discovery of several specimens that were well preserved in 3-D. In 1948, E. Stensio released a detailed depiction of B. canadensis anatomy using an abundance of material, which eventually became the most widely accepted description of this species. Since Stensio's publication, many others have provided reconstructed models of B. canadensis with modified aspects of the anatomy, including Vezina's modified single dorsal fin and more recently, reconstructions by Arsenault et al from specimens with little taphonomic distortion. Presently, the model of Arsenault et al. is regarded to be the most accurate, while there is still much debate about various aspects of this species' external anatomy. Despite the uncertainty, B. canadensis is still classically considered one of the most well-known species.^[13]

The external

Gondwanan environment.^[17]

Bothriolepis coloradensis

First described by Eastman in 1904, this species was found localized in present-day Colorado. There is a possibility that this species is similar, if not identical, to B. nitida, however because the material available regarding B. coloradensis is fragmented, it is impossible to compare the two species with any degree of certainty.^[18]

Bothriolepis nitida

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),^[18] there are several distinguishable traits specific to each species. B. nitida has a maximum headshield length of 65 millimetres (2.6 in), a narrow and shallow trifid preorbital recess, has an anterior-median-dorsal (AMD) plate that is wider than it is long and a ventral thoracic shield that has convex lateral borders.^[18]

Bothriolepis rex

Originally described by Downs et al. (2016), Bothriolepis rex is from the Nordstrand Point Formation of Ellesmere Island, Canada. B. rex's body length is estimated at 1.7 metres (5.6 ft) and is, therefore, the largest known species of Bothriolepis. Its armor is especially thick and dense even when taking its size into account. Downs et al. (2016) suggest that this may have both protected the animal from large predators and served as ballast to prevent this large bottom-dweller from floating to the surface.

Bothriolepis virginiensis

Originally described by Weems et al. in 1981, this species, Bothriolepis virginiensis, is from the "Chemung", near Winchester, Virginia. Several traits found in B. virginiensis can also be found in other species of Bothriolepis, (especially B. nitida), including posterior oblique cephalic sensory line grooves that meet relatively far anteriorly on the nuchal plate, relatively elongated orbital fenestra and a low anterior-median-dorsal crest.^[18] 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.^[18]

Currently, there is much debate regarding whether the species B. virginiensis and B. nitida can actually be distinguished from one another. Thomson and Thomas state that five species of Bothriolepis from the United States (B. nitida, B. minor, B. virginiensis, B. darbiensis and B. coloradensis) are unable to be consistently distinguished from one another.^[4] Conversely, Weems asserts that there are several traits that distinguish the species from one another, including several listed above.^[18]

Bothriolepis yeungae

This species is described in 1998, from Mandagery Sandstone in Canowindra, where is known from high numbers of placoderm specimens gathered at one place. Bothriolepis is one of the most common fish in Canowindra site alongside Remigolepis, over 1,300 individuals are discovered by 1998. This species is differed from all other species by having a reduced anterior process of the submarginal, separated from the posterior process of the submarginal by a wide, open notch. The head and trunk armour lengths ranged between 77.6–190 millimetres (3.06–7.48 in).^[16]

References

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23. ^ YOUNG, G.(1984). Reconstruction of the jaws and braincase in the Devonian placoderm fish Bothriolepis. Palaeontology 27, 635–661.

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