LSC 370 — Vertebrate Zoology
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Vertebrate Diversity
Jawless Fishes & Extinct Groups

(Click on any image for larger version)

Class Cyclostomata — Cyclostomes

Characteristics

  • Elongated, eel-like but lacking lateral fins
  • Round, jawless mouth
  • Primarily supported by notochord; lack complete vertebrae
  • No mineralized tissue (bone)
  • Lack scales

Order Myxiniformes — Hagfishes

  • Mouth surrounded by 6 tentacles
  • 1-15 external gill openings
  • Lack vertebrae: notochord only
  • Single semicircular canal
  • Numerous mucus glands
  • Benthic (bottom) marine environments
Hagfish
Pacific Hagfish, Eptatretus stoutii
Hagfish
Pacific Hagfish, Eptatretus stoutii
See also labeled photo.
Hagfish Specimen
Hagfish, preserved specimen.

Order Petromyzontiformes — Lampreys

  • Incomplete cartilagenous vertebrae along sides of notochord
  • Dorsal fin
  • 2 semicircular canals
  • Adults are external parasites of other verterbrates
  • Larvae are suspension feeders
See also Lamprey Anatomy page
Lamprey specimen
Lamprey, Petromyzon sp., preserved specimen
See also labeled photo.
Ammocoete (Lamprey Larva)
Larval Lamprey Petromyzon sp. ("Ammocoete"), stained slide.
See also labeled photo.

Class Conodonta†

Conodont Fossils
Conodont elements from Chappel Limestone, TX (Mississippian Period)
Conodont Drawing
Reconstruction of possible life appearance of a conodont
Source: Wikipedia

Characteristics

  • Known predominantly from microfossils of complex feeding apparatus
  • Often abundant indicator fossils in marine sediments
  • Few soft-part fossils indicate elongated eel-like body, large eyes
  • Probably more derived than either hagfish or lampreys
  • Lived from late Cambrian to late Triassic (495-200 mya)

"Ostracoderms"

Jawless Fishes Diversity

Characteristics

  • Paraphyletic assemblage of extinct, armored jawless fishes (the term 'Ostracoderm' is not much used anymore, but we will use it here for simplicity to represent several extinct classes)
  • Dermal bone forms body armor
  • Some had paired lateral spines or pectoral fins
  • Most were probably filter feeders or fed on soft benthic invertebrates
  • Lived from Ordovician to Devonian (480-360 mya)
Key:
Jawless Fishes Diversity


Placoderms

Placoderm Diversity

Characteristics

  • Placoderms are a separate clade of early gnathostomes
  • Bony plates on head and anterior body
  • Well-developed jaws, but lacked teeth
  • 2 pairs of paired fins
  • Most were probably bottom dwellers
  • Lived from Silurian to Devonian (430-360 mya); those shown here all date to the late Devonian

Bothriolepis fossil
Bothriolepis canadensis Fossil; Late Devonian, 387-360 mya; North America
Key:
Placoderms Diversity

Class Acanthodii†

Cheiracanthus Fossil
Cheiracanthus sp. fossil (Devonian Period, 450 mya; UK)
See also labeled photo.
Climatius model
Climatius sp. model (late Silurian to early Devonian; Europe and North America)

Characteristics

  • Commonly referred to as "Spiny Sharks" (based on superficial shark-like shape)
  • Cartilagenous skeleton
  • Multiple paired fins (often more than two pairs)
  • Fins usually have an enlarged dentine spine
  • Tail strongly heterocercal
  • Rhomboid scales resemble those of some bony fish (e.g. gars)
  • Phylogenetic position uncertain; may be polyphyletic
  • Lived from Silurian to Permian (430-250 mya)
This page last updated 24 July 2015 by Udo M. Savalli ()
Images and text © Udo M. Savalli. All rights reserved.