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Lecture 5

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ZOO 2090
Fred Laberge

ZOO*2090 Lecture 5 September 23, 2013 Diversity and evolution of fishes Objectives - describe major trends in fish evolution - describe how bone and jaws enabled fished to expand their ecological niches Agnathans: “Jawless” fishes - mouth present, but no biting apparatus derives from pharyngeal arches - two extant groups: hagfish and lamprey (together often referred to as cyclostomes) - groups were already highly specialized by 330 mya and display features likely representative of ancestral agnathans Hagfishes - scavengers of the deep sea - prey on invertebrates - use a “rasping tongue” – cartilaginous plate bearing horny teeth-like processes - characteristics: o osmoconformers – they will adapt to their seawater salinity o single nostril o multiple venous hearts o tentacles near the mouth o large slime glands along the body o lack bone Lamprey - anadromous (freshwater) - many are parasitic - use round mouth to cling to live prey while sucking blood, others use their mouth to cling to substrate - characteristics: o metamorphosis of ammocoete larva o single nostril o seven gill openings o cartilage associated with notochord o lack bone Conodonts (extinct) - were known only from hard tooth-like elements for a long time - so abundant they are used for stratigraphic dating - fossils found in early 1980s highlighted vertebrate characters (dentin and enamel, notochord, myomers, postanal tail) - complex feeding basket in the pharynx and large eyes suggest that they were active feeders - monophyly of conodonts is uncertain Ostracoderms (extinct) - small jawless fishes - displayed a lateral line system and an outer exoskeleton made up of dermal bone plates ZOO*2090 Lecture 5 September 23, 2013 - armous innovation  good fossils - they appeared in the Ordovician, radiated most in the Silurian and were extinct by the end of the Devonian (>100 mya) - four major groups are recognized: o Pteraspiodomorphs (extinct)  most had a head shield made of fused bone plates  the caudal exoskeleton was made up of small plates and scales, allowing movement of their single caudal fin  spines were often present, presumable protection from predation o Anaspids (extinct)  lived in Silurian  showed armour reduction  a laterally compressed body  a hypocercal tail  paired fins suggestive of open- water swimming  their single nostril and many branchial openings have been used to suggest a relationship with lampreys o Thelodonts (extinct)  scattered assemblage of primitive ostracoderms or a clade of their own  characterized by an exoskeleton made up of small dentine scales  which has been used to identify species  some species displayed a distinctive forked tail o Osteostracans (Cephalaspids) (extinct)  diverse clade part of a larger group that includes Galeaspida and Pituriaspida  all characterized by a massive head shield and gills that opened ventrally  dorsoventrally flattened shape suggests they lived at th
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