Anaspida

The main characteristics supporting the nodes of this phylogeny are:

• Node 1: Ten or less than ten external branchial openings
• Node 2: Reduced anal fin, short-based paired fins, large median dorsal scutes
• Node 3: Loss of anal fin, larger and hook-shaped median dorsal scutes

Introduction

The Anaspida, or anaspids, are a group of fossil, jawless vertebrates which lived in the Silurian (-430 to -410 million years ago). Anaspids differ from most other "ostracoderms" by their dermal skeleton not forming a massive headshield, although their head is covered by both large bony plates and minute scales. Also, contrary to most other "ostracoderms", they have a rather slender and laterally compressed body, with the gill openings extending behind the eyes in a slanting row, as in lampreys. The tail of anaspids is strongly hypocercal (or reverse heterocercal); that is, the part containing the notochord dips posteroventrally. On the top of their head, there is a median hole in the skull-roof, which is interpreted as a nasohypophysial opening similar to that of lampreys, yet there is no information of their internal anatomy. They differ, however, from lampreys in having paired fins. All these characteristics make anaspids roughly look like lampreys and they have long been regarded as their ancestors. Current phylogenies, however, imply that anaspids, along with other "ostracoderms", are more closely related to the gnathostomes than to lampreys. Yet, their phylogenetic position fluctuates considerably.

Anaspids lived in marine, coastal environments. They are rarely found as articulated fossils, since their skeleton was comparatively fragile. The best preserved anaspids are from the Silurian of Norway and Scotland.

Characteristics

• At least one large, tri-radiate spine behind the series of the gill openings.
  

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  Anaspids are characterized by a large, tri-radiate spine (red) posteriorly to the series of branchial openings. Typical anaspids are restricted to the Silurian but some doubtful forms occur in the Late Devonian. It is assumed that the most primitive anaspids, such as Pharyngolepis (top), possessed a long, ribon-shaped, ventrolateral fin-fold (green). More advanced forms, such as Rhyncholepis (bottom), possessed a shorter paired fin-fold (green) and enlarged, spine-shaped, median dorsal scutes. (After Ritchie 1964, 1980.)

All typical anaspids are Silurian in age. Several forms of debated affinities have also been included in this group: the Silurian Jamoytius, and the Late Devonian Endeiolepis and Euphanerops. Although the latter two genera share with the Anaspida a strongly hypocercal tail, a characteristic which is unknown in other groups, they have no mineralized exoskeleton and, therefore show no tri-radiate postbranchial spine. Both Jamoytius and Euphanerops display a very long series of more than twenty gill openings, and a circular structure around the mouth, which may be an annular cartilage, and thus would suggest affinities with lampreys (see Hyperoartia).

Anaspids have a laterally compressed body, a large dorsal web and a hypocercal notochordal lobe in the caudal fin, and an anal fin. They clearly possess thin, sometimes ribon-shaped paired fins, but it is uncertain whether these contained musculature and radials. The body is covered with rod-shaped scales arranged in chevrons, and the head is covered with minute scales and platelets forming the skull-roof. The gill opnings (8 to 15 in number) are minute and aligned in slanting row behind the eye. The skull-roof displays a median pineal opening, between the two orbits, and in front of it there is a peculiar keyhole-shaped opening interpreted as a dorsal nasohypophysial opening. However, since nothing is known of the internal anatomy of anaspids, there are still debates as to whether it corresponds to a blind nasohypophysial complex of lamprey type, or to an inhalent duct leading to the pharynx.

The exoskeleton of anaspids is made up of aspidine and contains no dentine. Some anaspids (e.g. Lasanius), however, are almost naked and retain only the postbranchial spines and the large median dorsal series of hook-shaped scute. Their endoskeleton was probably cartilaginous and is not preserved. However, some naked Silurian and Devonian forms often referred to as anaspids (Jamoytius, Endeolepis, Euphanerops) show imprints of a branchial skelton which may have been more or less similar to the "branchial basket" of lampreys.

Discussion of Phylogenetic Relationships

Anaspids are classically regarded as close relatives or ancestors of lampreys. However, all the characteristics which were considered as supporting this relationship (dorsal nasohypophysial opening, gill opening in posteriorly slanting row, hypocercal tail) occur in some other fossil "ostracoderm" taxa. Only their slender body shape is somewhat suggestive of that of lampreys. Current character analyses show anaspids in various phylogenetic positions (sister-group to all other "ostracoderms" + gnathostomes or sister-group to galeaspids + osteostracans + pituriaspids + gnathostomes). This is largely due to the lack of information about their internal anatomy.

The most generalized anaspids are exemplified by Pharyngolepis. More derived forms include the Rhyncholepididae and Birkeniidae, in which the anal fin is reduced or lost and the median dorsal ridge scales considerably enlarged. The birkeniid Lasanius has lost most scales, with the exception of the tri-radiate postbranchial spines and the median dorsal ridge scales.

References

Arsenault, M. and Janvier, P. (1991). The anaspid-like craniates of the Escuminac Formation (Upper Devonian) from Miguasha (Québec, Canada), with remarks on anaspid-petromyzontid relationships.In Early vertebrates and related problems of evolutionary biology (ed. M. M.Chang, Y. H. Liu, and G. R. Zhang) pp. 19-40, Science Press, Beijing.

Janvier, P. (1996). Early vertebrates. Oxford Monographs in Geology and Geophysics, 33, Oxford University Press, Oxford.

Kiaer, J. (1924). The Downtonian fauna of Norway. 1. Anaspida. Skrifter utgitt af det Norske Videnskapsakademien, 1, matematisk-naturvidenskapslige Klasse, 6, 1-139.

Parrington, F. R. (1958). On the nature of the Anaspida. In: Studies on fossil vertebrates (ed. T. S. Westoll), p.108-128, The Athlone Press, London.

Ritchie, A. (1964). New light on the morphology of the Norwegian Anaspida. Skrifter utgitt av det Norske Videnskaps-Akademi, 1, Matematisk-Naturvidenskapslige Klasse, 14, 1-35.

Ritchie, A. (1968). New evidence on Jamoytius kerwoodi White, an important ostracoderm from the Silurian of Lanarkshire, Scotland. Palaeontology, 11, 21-39.

Ritchie, A. (1980). The Late Silurian anaspid genus Rhyncholepis from Oesel, Estonia, and Ringerike, Norway. American Museum Novitates, 2699, 1-18.

Ritchie, A. (1984). Conflicting interpretations of the Silurian agnathan, Jamoytius. Scottish Journal of Geology, 20, 249-256.

Stensiö, E.A. (1964). Les Cyclostomes fossiles ou Ostracodermes. In: Traité de paléontologie (ed. J. Piveteau), Vol. 4(1), pp. 96-383, Masson, Paris

White, E. I. (1946). Jamoytius kerwoodi, a new chordate from the Silurian of Lanarkshire. Geological Magazine, 83, 89-97.