Class Myxinoidea — Hagfishes
Pacific Hagfish, Eptatretus stoutii?
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Hagfish, preserved specimen.
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Characteristics
- Elongated, eel-like but lacking lateral fins
- Jawless mouth surrounded by 6 tentacles
- 1-15 external gill openings
- Lack vertebrae: notochord only
- No mineralized tissue (bone)
- Single semicircular canal
- Numerous mucus glands
- Benthic (bottom) marine environments
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Class Petromyzontoidea — Lampreys
Lamprey, Petromyzon sp., preserved specimen
See also labeled photo.
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Larval Lamprey Petromyzon sp. ("Ammocoete"), stained slide.
See also labeled photo.
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Characteristics
- Elongated, jawless, scaleless, lacking paired fins
- Incomplete cartilagenous vertebrae along sides of notochord
- Dorsal fin
- 2 semicircular canals
- External parasites of other verterbrates
See also Lamprey Anatomy page
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Class Conodonta†
Conodont elements from Chappel Limestone, TX (Mississippian Period)
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Reconstruction of possible life appearance of a conodont
Source: Wikipedia
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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)
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"Ostracoderms"
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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)
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Key:
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Placoderms
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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
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Key:
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