*Palæos:*		Unit 20: Craniata
*The* *Vertebrates*		100: Craniata

Page Back	Unit Back	Unit Home	Unit References	Unit Cladogram	Glossary	Taxon Index
Page Next	Unit Next	Vertebrates Home	Vertebrate References	Vertebrate Cladograms	Bones	Time

Craniata

Hagfishes, Lampreys, and Anaspids

Abbreviated Cladogram

CHORDATA
|
Craniata
|--Myxinoidea
`--Vertebrata
   |--CONODONTA
   `--+--Hyperoartia 
      |  |--Endeiolepis
      |  `--Petromyzontiformes
      `--+--Anaspida
         |  |--Pharyngolepis 
         |  `--+--Pterygolepis
         |     `--Rhyncholepidida 
         `--+--PTERASPIDOMORPHI
            `--"THELODONTI"

20.000: Overview
20.100: Craniata
References

Anaspida

The Anaspida lack the extensive bony plates of their ostracoderm cousins. These jawless fishes had a laterally-compressed scale-covered body. They range from Early Silurian to the Latest Silurian, and are limited to the Euramerica Province. All were quite small, about 10 or 15 cm in length.

The genera Jamoytius, from the Early to Middle Silurian of Scotland, and Endeiolepis, Euphanerops and Legendrelepis, from the Late Devonian of Quebec, are traditionally referred to as anaspids by, among other features, their strongly hypocercal tail, a characteristic which is now known to occur in other vertebrate groups (e.g. thelodonts). In fact, they are apparently more closely related to the lampreys.

The Anaspida are all narrow-bodied jawless vertebrates characterized by triradiate postbranchial spines. They possess rod-like body scales (when not naked), large median dorsal scales or scutes, broad-based paired fins, and a strongly hypocercal (downward- lobed) tail. Their exoskeleton (scales, dermal plates) are made up of acellular bone (aspidin). They may have been awkward, inefficient swimmers, since they lacked the basic stabilizing fins of

An interpretation of the paired fins of the anaspid Pharyngolepis as a slow displacement device (arrows indicate the direction of the undulation of the fin).
Illustration from Janvier (1996).

other fishes -- even many other jawless fishes. A row of crested scales ran along the back, and a pair of bony spines projected from the pectoral area. There was a well-developed anal fin, and the tail was downturned. But none of these features would have stabilized these animals body in the water. It is quite possible however that the small paired fins in forms such as Pharyngolepis were undulated as a slow displacement device (see diagram below) to provide stabilization.

Their feeding method was probably to plough through the bottom sediment, head-first, scooping up tiny food particles into the small, rounded mouth.

Anaspids are rare as fossils and are apparently restricted to the Silurian. The earliest record consists of isolated scales from the Llandovery of Arctic Canada, and the latest from the Late Silurian of the Baltic area. The Anaspida occur mostly in Norway and Scotland, with some sparse records from eastern Canada, the Baltic area, and a single perhaps dubious record of a "birkeniid" from China.

One of the possible phylogenies of the Anaspida suggests a tendency towards the reduction of the number of gill openings, enlargement of the median dorsal scutes, reduction of anal fin, but is neither corroborated nor refuted by stratigraphy, since most anaspid taxa are of the same age.

Descriptions

Craniata: Myllokunmingia

Range: From the Tommotian

Phylogeny: Chordata::: Haikouella + *: Myxinoidea + Vertebrata.

Characters: Neural crest cells, somites, gills with cartilagenous or bone supports (absent in Myllokunmingia?), tripartite brain and paired cranial sense organs, cranium with anterior notochord, semi-circular canal(s), 1 heart, kidneys.

Note: The description by Shu et al. (1999) of Myllokunmingia from the Lower Cambrian of South China is generally what one might have expected of a very primitive craniate. The surprising exception is the probable presence of paired fin folds, as well as a definite dorsal fin. None of the fins have detectable radials. The state of preservation is also insufficient to state whether Myllokunmingia had eyes, a single heart, an anterior notochord, or even a brain, so that the exact position of this fossil is uncertain. Shu describes it as a vertebrate, but the evidence is unclear. Perhaps it is better treated as a pre-vertebrate craniate since, as Shu states, it is not part of the crown group vertebrates, defined as the LCA of lampreys and gnathostomes.

Links: Waking Up to the Dawn of Vertebrates, Science News Online (11/6/99); BBC News | Sci/Tech | Oldest fossil fish caught. ; Craniata; UW-Green Bay Cofrin Arboretum Center for Biodiversity - - ...; Craniata.

References: Janvier (1999); Shu et al. (1999); Zimmer (2000). APW040314.

Myxinoidea: Hagfish (Myxine, Neomyxine, Paramyxine, Eptatretus).

Range: from the Late Carboniferous

Phylogeny: Craniata: Vertebrata + *.

Characters: Anterior nasal opening; "taste" per unique neural and sensory system; mouth surrounded by ring of tentacles; rasping tongue with keratinous teeth; velum; water inhaled through separate nasopharyngeal opening; cartilagenous braincase and branchial basket; poorly developed eyes, with no lenses; gills asymmetric; no vertebrae, finrays absent; no muscles in caudal web fin; scales absent; numerous dermal mucous glands; no lateral line neuromasts; primitive kidney; aneural heart & accessory hearts; isomolal body fluid; benthic and nektobenthonic; marine (deep shelf); ties itself into slip knot to remove food; scavengers; very low metabolism.

Links: link: UC Berkeley; OceanLink: OceanInfo - The Hagfish Page; Hyperotreti; Agnatha.Head.pdf; Hagfish At Home; Pirålar. APW040314.

Vertebrata: Used here as crown group, i.e. LCA lampreys + gnathostomes.

Range: from the Lower Cambrian

Phylogeny: Craniata: Myxinoidea + *: Conodonta + (Hyperoartia + (Anaspida + (Pteraspidomorphi + Thelodonti))).

Characters: Well-developed eyes; extrinsic eye muscles attached to the eyeball and orbital wall; 2-3 semi-circular canals; muscular, perforated pharynx; endoskeleton with at least cranium, visceral arches, limb girdles, and 2 pairs of appendages; repeating endoskeletal elements flanking the spinal cord, primitively two pairs in each metamere (interdorsals and basidorsals); radial muscles in fins; integument with two layers, epidermis and dermis; lateral line canals with true neuromasts; atrium and ventricle of heart closely-set; nervous regulation of heart by vagus nerve; blood with (a) red blood cells (erythrocytes) containing hemoglobin, and leukocytes; mesodermal muscle fibers in gut; hox gene duplications; neural crest cells (arguably the basis for most other important vertebrate characters); sensory placodes (neuroectoderm discs giving rise to nose, lens of eye, ears and lateral line system).

Links: Vertebrata; Subphylum Vertebrata; Introduction to Vertebrates; Vertebrata -- The Dinosauricon; LAB 13 71.125; Phylum Chordata Overview; Subphylum Vertebrata; Chordata: Subphylum Vertebrata; Chordata: Subphylum Vertebrata; À la recherche du premier vertébré; PNAS -- Cameron et al. 97 (9): 4469; Subphylum Vertebrata; Basal Vertebrata.

References: Cameron et al. (2000). APW040314.

Hyperoartia: Lampreys and their ancestors. Haikouichthys?, Hardistiella, Mayomyzon, Pipiscius.

Range: From the Early Cambrian?

Phylogeny: Vertebrata:: (Anaspida + (Pteraspidomorphi + Thelodonti)) + *: Endeiolepis (?) + Petromyzontiformes.

Characters: Eel-shaped aspect; oral sucker with keratinous "teeth"; sucker reinforced by annular cartilage; sucker with the pumping velum; piston cartilage in lingual apparatus with pectinate keratinous "teeth" which rotate on the tip of the retractable piston cartilage; eyes relatively large, with lens, but no intrinsic muscles; extrinsic eye muscles almost same as gnathostomes; $ single median dorsal nasohypophysial opening for both the olfactory organ and a blind hypophysial tube including the hypophysis (hypophysial tube may be remnant of primitive nasopharyngeal duct, but see The Basisphenoid); translucent pineal spot; true cartilagenous braincase plus cranial cartilage plates and bars; labyrinth with two vertical semicircular canals; seven gill openings; gill pouches large and posteriorly placed; spine-shaped process on gill arches; unjointed "branchial basket" for gills; arches external to gill pouches and branchial circulation; small cartilaginous dorsal arcualia (basidorsals and interdorsals) on notochord; loss or absence of anal fin; dorsal and caudal fins with thin cartilaginous radials associated with radial muscles; tail slightly hypocercal; loss or absence of mineralized exoskeleton; no scales; large neuromasts present; ammocete larva and extensive larval stage; anadromous; genome probably has undergone one round of duplication from ancestral chordate.

Links: Hyperoartia; Introduction to the Petromyzontiformes; Hyperoartia; Neuroethology: Swimming in the Lamprey; Evolution der Fische (German); Phylogénomique.

References: Janvier (1999); Shu et al. (1999).

Note:
Haikouichthys is
A fish from Haikou, China --
Not verse from Japan. APW040314.

Endeiolepis: E. aneri

Range: Late Devonian.

Phylogeny: Hyperoartia: Petromyzontiformes + *.

Characters: tail strongly heterocercal; no mineralized exoskeleton

Image: from John Pojeta, Jr.

Links: Vertebrata; Anaspida; DEVONIANO vegetais primitivos (Portuguese); Page 1. APW040314.

Petromyzontiformes: fr lwC. Lampreys

Phylogeny: Hyperoartia: Endeiolepis + *.

Characters: Body anguilliform; round, suckerlike jawless subterminal mouth filled with rows of keratinous "teeth"; rasplike "tongue"; internal ring of cartilage supports rim of mouth; 1 median dorsal nasal opening; 2 semicircular canals; 7 gills similar to anaspids arranged behind the orbit; vertebrae present; 2 dorsal fins; no bone; often anadromous; ammocoete larvae 3-7 yrs as sedentary filter feeders filtering particles from mucous secretions; short-lived, parasitic adult form.

Links: Introduction to the Petromyzontiformes; Svenska fiskar: havsnejonöga (Swedish); Geol 437 jawless fishes (NOT \AGNATHA\), Fall, 1995; Lampreys (Petromyzontiformes); Petromyzontiformes (mtDNA equence data); Petromyzontiform.

Image: Right: unidentified lamprey from Fishbase. Left: Cross-section of an ammocete larva at the level of the gills from BIODIDAC. Note the external gill supports that have become a key issue in Mallat's theory of the origin of jaws. The (even more significant) internal gill supports are not readily identifiable in this image. APW040314.

Anaspida:

Range: Silurian.

Phylogeny: Vertebrata::: (Pteraspidomorphi + Thelodonti) + *: Pharyngolepis + (Pterygolepis + Rhyncholepidida).

Characters: 15 cm. Minnow-like, slender with no head shield; some naked; upper & lower margins of round mouth bore dermal bones which may have been used a sort of dentition; orbits large & surrounded by dermal bone, but sclerotic ring absent; single medial pineal and hypophyseal openings; 6-15 gill openings (trend is reduction of #), usually in a slanting line, behind orbits; $ at least one large, tri-radiate spine behind the series of the gill openings; body covered in elongated scales arranged in chevrons which may reflect underlying myomeres (if so, body musculature extended far ant, perhaps overlapping gills); anal fin usually in far post; primitively, pectorals extended from gills to anal fin; muscles & radials for pectorals inferred from scale structure; no dorsal or separate pelvic fins, but dorsal scutes common & trended to larger, curved or hooked shapes; strongly hypocercal tail, but with large dorsal fin web (possibly this is dorsal fin); acellular, laminar bone similar to aspidine; no dentine & no enameloid caps. Relationship to Petromyzontiformes and to Devonian forms unclear.

Links: Anaspid model ; Anaspid cast; Geol 437 jawless fishes; Anaspida. APW040314.

Pharyngolepis:

Range: Silurian of Europe

Phylogeny: Anaspida: (Pterygolepis + Rhyncholepidida) + *.

Characters: Head covered with relatively large scales compared to other anaspids; distinctive keeled oral plate on ventral surface just under mouth; about 15 branchial openings in postero-ventrally slanting line, terminating in tri-radiate spine diagnostic of anaspids; elongated body; median dorsal ridges small; no dorsal fin (as all anaspids); anal fin present and posteriorly placed; long ventrolateral "fin-fold" from branchial spine to anal fin; "fin-fold" apparently lacks fin rays and perhaps musculature; folds covered with minute scales in thin rows with thicker basal row of scales.

Links: Pharyngolepis oblongus; Annual Conference Abstracts 1999; Anaspida; Volume 16 (V2); Fiskarnas evolution.

Note: believed to be primitive form based on (a) unreduced anal fin (b) relatively large number of branchial openings (c) small dorsal ridge scales. Overall appearance strongly reminiscent of classical thelodonts, but scale histology is supposedly distinct. APW040314.

Pterygolepis: