Reconstruction of Priscomyzon
in dorsal (top) and left lateral (bottom) views. b
, Macropthalmia stage of Lampetra
showing anterior location of orbit and smaller oral disc, both positioned in front of the branchial region. The total length of the specimen is 116 mm. Drawings in a
are scaled to show equivalent head lengths: from anterior limit of the oral disc to rear of the branchial region. Horizontal bars indicate the anterior?posterior span of the oral disc in each species.
The life of a parasite must be a good one, and often successful; the creature at the top of the drawing above is a primitive lamprey from the Devonian, 360 million years ago, and the similarities with the modern lamprey (at the bottom) are amazing. It’s less eel-like and more tadpole-like than modern forms, but it has the same disc-shaped mouth specialized for latching on to the flank of its host, it has similar circumoral teeth for rasping through scales and skin for its blood meal, the same pharyngeal adaptations for a life spent clamped to a fish.
I’ve put a photo of the fossil and a cladogram below the fold.
This 360-million-year-old lamprey is the earliest example known in the fossil record, showing most of the specialized feeding structures present in modern forms. a
, Part (a
) and counterpart (b
) of holotype AM5750. The total length of the specimen is 42 mm. c
, Interpretive drawing of the holotype. ac, annular cartilage; blb, bi-lobed structure; bra, branchial arch; brb, branchial basket; cm, circular mouth; ct, circumoral teeth; df, dorsal fin; hyb, hypobranchial bar; h/eb, hypotrematic/epitrematic bar; oc, otic capsule; od, oral disc; ol, outer lip; or, orbital region; sc, styliform cartilage; 1?7, positions of gill pouches.
It’s not a very exciting animal, but put it in context. This creature was swimming about in the late Devonian, in the waning years of the great agnathan radiation. It shared the seas with those exotic armored jawless fishes that are all gone now, with just the hagfish and lampreys remaining. If you’d been alive in those days, would you have picked the lineage of those annoying soft and slimy parasites as the only members of that diverse group that would survive the next few hundred million years? A mere degenerate ostracoderm, thin-skinned and leech-like, is the line with the greatest longevity…perhaps because it was suitably adapted to take advantage of the gnathostome radiation, rather than compete with it.
(click for larger image)
Lampreys and hagfishes are the only two living groups of jawless vertebrate. The 360-million-year-old lamprey Priscomyzon
(green) discovered by Gess et al
. is very similar to modern lampreys, even though it dates from the twilight age (grey area) of the armoured jawless vertebrates (known as ostracoderms, in red) that were once considered to be ancestors of hagfishes and lampreys. The evolutionary tree now proposed by Gess et al
. (simplified version shown here; crosses indicate extinction dates) agrees with the current consensus that ostracoderms are more closely related to jawed vertebrates than to lampreys or hagfishes. This suggests that living jawless vertebrates and their forerunners never developed an extensive bony skeleton, and that their origin must lie among early Palaeozoic jawless vertebrates that lacked scales and bone, such as Euphanerops (blue).
Gess RW, Coates MI, Rubidge BS (2006) A lamprey from the Devonian period of South Africa. Nature 443(7114):981-984.