Bothriolepis
Bothriolepis 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, averaging around in length. However, the largest species, B. rex, had an estimated bodylength of. Although expansive with over 60 species found worldwide, comparatively Bothriolepis is not unusually more diverse than most modern bottom dwelling species around today.
Classification
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.
General anatomy
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 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 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.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 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.Dermal skeleton
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.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. It may also have used its pectoral fins to throw sediment over itself. In addition to the pectoral fins, it is originally considered to have two dorsal fins, but existence of a low, elongated anterior dorsal fin was denied in 1996, and now it is considered to have only a high rounded posterior dorsal fin. The caudal tail was elongated, ending in a narrow band, but is unfortunately rarely preserved in fossils. Bothriolepis lacked pelvic fins. Early antiarchs like Parayunnanolepis had pelvic fins, which implies secondary loss of pelvic fin in Bothriolepis.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. These internal structures were preserved when different types of sediments surrounding the exterior of the animal-filled the internal carapaces. 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. These sediments helped preserve the following internal elements:Alimentary system
In general, the alimentary system of Bothriolepis –which includes the organs involved in ingestion, digestion, and removal of waste– can be described as simple and straight, unlike that of humans. It begins at the anterior end of the organism with a small mouth cavity located over the posterior area of the upper jaw plates. Posteriorly from the mouth, the alimentary system extends into a wider and dorso-ventrally flattened region called the pharynx, from which both the gills and lungs arise. The esophagus, which is also characterized as a dorso-ventrally flattened tube, extends from the mouth into the stomach and leads to a flattened ellipsoidal structure. This structure may be homologous to the anterior end of the intestine found in other fish. The flatness of these structures may have been exaggerated when the fossil specimens experienced tectonic deformation through geologic time. The intestine begins narrowly on the anterior end, expands transversely, and then again narrows posteriorly towards the cylindrical rectum, which terminates just within the posterior end of the trunk carapace. While the alimentary system is primitive in nature and lacks an expanded stomach region, it is specialized by an independently acquired complex spiral valve, comparable to that in elasmobranchs and many bony fish and similar to that found in some sharks. A single fold of tissue rolled upon its own axis forms this specialized spiral valve.Gills
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.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 lungs of certain dipnoans and tetrapods. It has been hypothesized that these lungs, coupled with the jointed arms and rigid, supportive skeleton, would have allowed Bothriolepis to travel on land. Additionally, as Robert Denison states because there is no evidence of a connection between the external naris and mouth, Bothriolepis likely breathed similarly to present-day lungfish, i.e., by placing the mouth above the water's surface and swallowing air.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.