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Lecture notes Br26820 (BR26820)
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Vertebrate Zoology notes - Metazoan Phyolgeny
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Metazoan Phylogeny Chordata: Haeckel, 1874, Latin from neuter plural of Latin chordates “having a
(spinal) cord”, from chorda “cord” string”
Putnam et al. 2008, The amphioxus genome & the evolution of the chordate
karyotype
Of the living lineages, the cephalochordates diverged first, before the split
between the morphologically diverse urochordates & vertebrates
To a remarkable extent, the amphioxus genome appears to be a good
surrogate for the ancestral chordate genome
Together, the vertebrates, urochordates & lancelets (also known as
cephalochordates) constitute the phylum Chordata, descended from a last
common ancestor that lived perhaps 550 million years ago
Delsuc et al 2006, Tunicates & not cephalochordates are the closest living
relatives of vertebrates
Tunicates should therefore no longer be considered as “primitive” but rather
as derived chordates with highly specialized lifestyles & developmental modes
Non-vertebrate Chordates
Free swimming larval “tunicate” (Subphylum “Tunicata” or “Urochordata”)
Sessile adult “tunicate”
“Lancelet” or “Amphioxus” (Subphylum “Cephalochordata)
Notochord = Elastic rod tip to tail of body
Sea-squirts also known as class Ascidiacea
Ascidians are soft-bodies animals, for this reason their fossil record is almost
entirely lacking
Earliest reliable ascidians are Shankouclava shankouense from Lower
Cambrian Maotianshan Shale (South China)
2 enigmatic species from Ediacaran period with some affinity to the ascidians
– Ausia from Namibia & Burykhia from Northern Russia
About 3000 species of tunicate exist in the world’s oceans, living mostly in
shallow water
32 species of cephalochordates = <5cm fishlike marine animals
Greek “amphi” = both, “oxy” = sharp – lacks distinct head. Lancelets, or
amphioxus are small worm-like marine animals that spend most of their lives
buried in the sea floor, filter-feeding through jawless, ciliated mouths. Marine
waters of continental shelves
Latin for chord – notochord is present
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