Vertebrate origins Flashcards
Vertebrata total clade
All taxa more closely related to Cyclostomata + Gnathostomata than to Urochordata
Deuterostomia
Ambulocrania (hemichordates and echinoderms) +
historically united on derivation of mouth from secondary opening, radial cleavage and enterocoely (coelom forms in mesoderm) during early embroyonci stages.
All characters have homoplastically evolved in some protostomes
Now unambiguously united by presence of pharyngeal slits (lost in Echinoderms)
Chordata
(Vertebrata + Tunicata) + Cephalochordata
Dorsal hollow nerve chord- nervous system along A-P axis
Notochord formed from chordamesoderm, midline flexible support structure below nerve chord
Endostyle- ciliated, mucous-producing organ on floor of pharynx
Pharyngeal slits
Muscular, postanal tail
Sigmoidal muscles arrangement
Cephalochordata
Early Cambrian-Recent. Extant are burrowers
Oral cirri- flexible, food gathering structure
Notochord extends along length of body
Tunicata
Early Cambrian-Recent
Sessile adult stage
Covering sheath (tunic)
Closer to vertebrates than cephalochordate- clear in genome
Synapomorphies present in larval stage
Attach to substrate and degenerate, filter feed for adult life
Vertebrata
Neural crest, neurogenic placodes, duplicated Hox clusters, distinct head, ventral, lobed heart, endocrine glands, mineralized tissue, kidneys
Chengjiang biota, Maotianshan Shale, China
Lower Cambrian
See exceptional preservation of soft tissue
eg. Tunicata: U-shaped digestive tract, branchial basket, siphons
Cephalochordates: paired myomeres and gill slits
Pikaia gracilens
Burgess shale fauna, middle Cambrian
Previously considered stem cephalochordate or possible stem vertebrate based on notochord, dorsal nerve chord, post-anal tail, sigmoid-shaped myomeres, pharyngeal slits
But subsequent study suggested that don’t actually have these characters, probably stem chordate
Haikouichthys ercaicunensis
Maotianshan shale
Stem vertebrate, paired eyes and nasal placode, and what might be vertebrae
Metaspiggina walcotti
Burgess Shale
stem vertebrate: stem eyes, nasal placodes, branchial basket consists of bipartite bars
Problem with laggerstatten
Sequence of tissue decay in dead cephalochordates and cyclostomes mirrors hypothesised evolutionary character acquisition in early vertebrate phylogeny
Therefore hard to differentiate between species, depends on how long rotting for
Hox duplication
Vetebrates- 2 duplication events from ancestral chordate Hox family
Additional duplications in teleost fish and independently in hagfish
Potential mechanism for increasing morphological complexity, gives increased control and variability in turning on and off molecular systems
Neural crest
derived from ectodermal epithelium
delaminates from ectoderm during neurulation, becomes mesenchyme and migrates to the sides to positions of derivative structures
Segregated from embryonic ectoderm by differential concentrations of signalling proteins (BMP) and TFS (Snail)
eg. high BMP => Epidermal
Mid BMP and Snail => neural crest
Low BMP => Neural
Cranial neural crest
Cartilages, bones, cranial neurones, glia, connective tissues of the dermal skull
Migrates to pharyngeal arches to form thymic cells, odontoblasts of tooth primordia
Trunk neural crest
Dorsal root ganglia, sympathetic ganglia, melanocytes
