Bothriolepis

Bothriolepis (from Greek: βόθρος bóthros, 'trench' and Greek: λεπίς lepis 'scale') was a widespread, abundant and diverse genus of antiarch placoderms that lived during the Middle to Late Devonian period of the Paleozoic Era. Historically, Bothriolepis resided in an array of paleo-environments spread across every paleocontinent, including near shore marine and freshwater settings. Most species of Bothriolepis were characterized as relatively small, benthic, freshwater detritivores (organisms that obtain nutrients by consuming decomposing plant/animal material), averaging around 30 centimetres (12 in) in length. However, the largest species, B. rex, had an estimated bodylength of 170 centimetres (67 in). Although expansive with over 60 species found worldwide, comparatively Bothriolepis is not unusually more diverse than most modern bottom dwelling species around today.

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. The earliest members of Bothriolepis appear by the Middle Devonian.

Antiarchs, as well as other placoderms, are morphologically diverse and are characterized by bony plates that cover their head and the anterior part of the trunk. 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. Placoderm bony plates were generally made up of three layers, including a compact basal lamellar bony layer, a middle spongy bony layer and a superficial layer; Bothriolepis can be classified as a placoderm since it possesses these layers.

Placoderms were extinct by the end of the Devonian. Placodermi is a paraphyletic group of the clade Gnathostomata, which includes all jawed vertebrates. It is unclear exactly when gnathostomes emerged, but the scant early fossil record indicates that it was sometime in the Early Palaeozoic era. The last species of Bothriolepis died out, together with the rest of Placodermi, at the end of the Devonian period.

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.

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.

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 trunk's outline suggests that there may have been a notochord present surrounded by a membranous sheath, however, there is no direct evidence of this since the notochord is made up of soft tissue, which is not typically preserved in the fossil record. Similar to other antiarchs, the thoracic shield of Bothriolepis was attached to its heavily armored head. Its box-like body was enclosed in armor plates, providing protection from predators. Attached to the ventral surface of the trunk is a large, thin, circular plate marked by deep-lying lines and superficial ridges. This plate lies just below the opening to the cloaca.

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. The superficial layer is speculated to have denticles that may have been made of cellular bone.