Ambulacraria Hypothesis

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Question 1
1.1) Hemichordates share some characters with echinoderms and chordates. Pharyngeal slits they share with chordates. Hemichordates are the sister taxon to echinoderms as described by Ambulacraria hypothesis, then gill slits are an ancestral feature for deuterostomes. These gill slits are supposedly lost in in lineage but some researchers have found evidence for gill slits in extinct carpoid echinoderms.
The Ambulacraria hypothesis unites echinoderms and hemichordates on the basis of a shared diffuse epidermal nervous system. It’s therefore unlikely that the short dorsal, somewhat hollow, nerve cord in the collar zone of hemichordates is homologous to the dorsal hollow nerve cord of chordates.
Another phylogenetically important character
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Some fishes and reptiles lack sex chromosomes altogether. In these groups sex is determined by non-genetic factors such as temperature and behaviour. In crocodiles, many turtles and lizards, the incubation temperature of the nest determines the sex ratio, by indirectly activating/suppressing genes that direct development of the animals’ sex organs. Evidence suggests that temperature regulates the expression of the DMRT1 gene, which is expressed in higher doses in male embryonic testes than in female embryonic ovaries. In turtles DMRT1 gene is higher at low temperatures and promotes development of males. In contrast alligator eggs incubated at low temperatures promote …show more content…
Blue light is filtered by the overlaying yellow pigment and thus appears green. Many frogs can adjust their colour to match their background and thus to camouflage themselves.
(Integrated Principles of Zoology, 15th edition, Hickman Roberts Keen, McGraw-Hill, 2011, P558)

4.2) The structure of the skull roof permits us to identify three majour groups of amniotes that diverged in the Carboniferous period of the Palezoic era.
Synapsids, anapsids and diapsids.
The synapsids group which includes, mammals and their ancestors, has a pair of temporal openings in the skull associated with attachment of jaw muscles. They were the first amniote group to diversify widely into terrestrial habitats.
The anapsid group is characterised by solid skulls and includes some of the earliest amniotes.
The diapsids have two pairs of temporal openings in the skull and contain dinosaurs, lizards, snakes, crocodilians, birds and their ancestors.
(Integrated Principles of Zoology, 15th edition, Hickman Roberts Keen, McGraw-Hill, 2011,

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