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Interrelationships of the Osteostraci

The main characteristics supporting the nodes of this phylogeny are:

• Node 1: Narrow-based paired fins, oralobranchial cavity closed ventrally by large dermal platelets
• Node 2: Anterior dorsal fin reduced to a large median dorsal scute
• Cornuata: Cornual processes
• Node 3: Large median dorsal scute lost.
• Node 4: First canal leading from the inner ear to lateral fields branches close to the orbit.
• Node 5: Posterior ventral myodome of the orbit (for recti eye muscles) reduced.

Containing group: Vertebrata

Introduction

The Osteostraci, or osteostracans, are a major clade (about 200 species) of fossil, armored jawless vertebrates which lived from the Early Silurian (about 430 million years) to the Late Devonian (about 370 million years). Most of them have a characteristic horseshoe-shaped head, which consists of a massive endoskeletal skull, covered with a shield of dermal bone. On the dorsal surface of the head are the closely-set eyes, a pineal foramen, and a median, keyhole-shaped nasohypophysial opening. In addition, there are peculiar "fields" (in fact, depressions of the braincase, covered with loose platelets of dermal bone), which have been regarded as either sense organs or electric organs. The mouth and gill openings are, like in the Galeaspida, situated on the ventral side of the head. Osteostracans also have large, pad-shaped paired fins. Most osteostracans are about 20 to 40 cm. in total length, but some species could be extremely small (about 4 cm in length). The largest species is about one metre in length.

Osteostracans, also known as "cephalaspids", played an important role in the history of vertebrate palaeontology, as they were the first fossil jawless fishes whose internal anatomy has been described in detail, thereby raising heated debates about the organization of the primitive vertebrate head.

Most osteostracans lived in marginal, marine environments, some of them being regarded as fresh water. Their horseshoe-shaped headshield suggests a benthic mode of life, but it is possible that some of them, such as the Boreaspididae or the Kiaeraspidida were more active swimmers. They are most diverse in the Early Devonian and become very rare in the Middle and Late Devonian. Osteostracans are widely distributed in North America, Europe, Siberia, and Central Asia, north of the Tian-Shan.

Characteristics

Osteostracans are characterized by:

• Two lateral and one median depression in the dorsal surface of the endoskeletal headshield, referred to as "cephalic fields". These fields are covered by polygonal, exoskeletal platelets and are connected to the inner ear by means of large branching canals. They have been variously interpreted as having housed electric organs, expansions of the labyrinth, or special organs of the sensory-line system. To date, their function remains unknown.
• A peculiar, leaf-shaped, horizontal fin underlying the caudal fin. This may be a modified anal fin.

Osteostracans resemble the Galeaspida and Pituriaspida in having a massive endoskeletal headshield, made up by cartilage (sometimes with globular calcifications) lined with a thin layer of cellular perichondral bone. As in the latter two groups, this endoskeletal headshield is hollowed ventrally by a large oralobranchial cavity, the roof of which shows impressions of the gills and velum. It is undecided whether this headshield only represents the braincase, or also includes part of the visceral and scapular skeleton. This endoskeletal headshield is covered with an exoskeletal headshield, generally made up by polygonal platelets of cellular bone and bearing tubercles of mesodentine. The oralobranchial cavity is closed ventrally by either minute scales or larger dermal platelets. The mouth and gill openings (8 to 10) open ventrally, along the rim of the oralobranchial cavity. Dorsally, the headshield is pierced by two closely-set orbits, between which opens a pineal foramen (often piercing a small, separate plate), and a median, keyhole-shaped nasohypophysial opening. The nasohypophysial opening is the external opening of the small olfactory organ and the blind hypophysial tube. This remarkable similarity to lampreys has long suggested that the two groups were close relatives and were included, along with the Anaspida in a clade Cephalaspidomorphi. Current theories of craniate interrelationships suggest, however, that this characteristic is homoplastic.

Thanks to the perichondral ossification of the endoskeletal headshield, the internal anatomy of osteostracans could be studied in great details. It shows the brain cavity, with a paired recess for the cerebellum, the orbits, with myodomes for the eye muscles, the labyrinth cavity, with two vertical semicircular canals, and the canals for the cranial nerves and blood vessels. There have been heated debates about the path of the cranial nerves, in particular those innervating the gill arches. Some considered that the maxillary branch of the trigeminus nerve innervated a premandibular gill arch. Others considered that the gill anatomy of osteostracans was similar to that of larval lampreys, the foremost gill being that of the hyoid arch. The most intriguing feature of osteostracan anatomy is the large, branching canals which lead from the labyrinth to the lateral and dorsal cephalic "fields". Some interpret then as expansions of the membranous labyrinth, containing endolymph, whereas others regard them as having housed either sensory or electromotor nerves.

Osteostracans have paired fins which contained a musculature and were attached to the posterolateral region of the endoskeletal headshield.

The body of osteostracans is massive, covered with large, diamond-shaped scales, and bears primitively two dorsal fins (reduced to one or none in the more advanced forms). The tail is epicercal (i.e. the lobe containing the notochord points posterodorsally, as in sharks). The caudal fin web is underlain by a peculiar horizontal, leaf-shaped, fin web, which may represent a modified anal fin. Sensory-line canals are developed on the head and part of the body.

Discussion of Phylogenetic Relationships

The Osteostraci include a major clade, the Cornuata, characterized by pointed lateral processes of the headshield, in front of the paired fins (homoplastic with the cornual processes of the Galeaspida and Pituriaspida), and a few primitive genera (Ateleaspis, Aceraspis, Hirella, Hemicyclaspis) which represent "basal" osteostracans. The Early Silurian Ateleaspis is regarded as the most generalized osteostracan.

The Cornuata comprise five major taxa: the Cephalaspidida, Zenaspidida, Benneviaspidida, Kiaeraspidida, and Tyestiida. There are a few other minor taxa of uncertain affinities, such as the Tannuaspidida from Central Asia (Tuva).

The Cephalaspidida are characterized by broad and flattened cornual processes, onto which extend the lateral fields. Cephalaspis is the classical representative of this group which, however, includes many other forms, such as the large Parameteoraspis, whose headshield can reach 45 cm in breadth.

The Zenaspidida have a more massive headshield, often bearing a posterior median crest or spinal process. They have a characteristic ornamentation of large tubercles, surrounded by smaller ones.

The Benneviaspidida have a flattened headshield and their exoskeleton has lost the polygonal pattern of the platelets, to form a continuous layer. Some benneviaspidids, such as the Boreaspididae, develop a long rostral process, like some galeaspids and pituriaspids. One of them, Tauraspis, is unique in having a pair of forward pointing "horns".

The Kiearaspidida are small osteostracans whose headshield extends posteriorly into a long abdominal division and whose cornual processes are extremely reduced or lost. Some of them, the Acrotomaspididae, are very specialized forms, with extremely reduced cephalic fields and their mouth opens anterodorsally.

The Thyestiida are also small osteostracans, characterized by the special structure of their sensory-line canals. They include forms, the Tremataspididae, which have lost the paired fins and cornual process, their headshield becoming olive-shaped. This may represent an adaptation to burrowing habits.

Among all "ostracoderms", osteostracans (possibly along with pituriaspids) share the largest number of uniquely derived characteristics with the gnathostomes: cellular bone, sclerotic ring in eyes, paired fins containing musculature, and epicercal caudal fin.

References

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Afanassieva, O. (1991). (The cephalaspids of Soviet Union). Trudy Paleontologicheskogo Instituta Akademia Nauk, 248, Moscow. (In Russian.)

Afanessieva, O. and Janvier, P. (1985). Tannuaspis, Tuvaspis and Ilemoraspis, endemic osteostracan genera from the Silurian and Devonian of Tuva and Khakassia (USSR). Géobios, 18, 493-505.

Janvier, P. (1981). Norselaspis glacialis n.g., n.sp. et les relations phylogénétiques entre les Kiaeraspidiens (Osteostraci) du Dévonien inférieur du Spitsberg. Palaeovertebrata, 11(2-3), 19-131.

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