Moy 4 eggs evolution series#

Hence, from the perspective of living taxa, a whole suite of characteristics unite Vertebrata, including an axial skeleton, brain complexity, cranial sensory complexity, neural crest cells (see Glossary, Box 1) and their many derivatives, and perhaps also a predatory (as opposed to a filter feeding) lifestyle. Phylogenomic (see Glossary, Box 1) data convincingly place the Urochordata, not Cephalochordata, as more closely related to Vertebrata ( Delsuc et al., 2006).

Moy 4 eggs evolution series#

The immediate invertebrate relatives of vertebrates are also chordates: the Urochordata (ascidians and other tunicates see Glossary, Box 1) and the Cephalochordata (amphioxus and allies see Glossary, Box 1), both of which have been the subject of other articles in this Evolutionary crossroads in development biology series ( Bertrand and Escriva, 2011 Lemaire, 2011). However, this clade implies a phenotypically complex ancestral vertebrate, broadening the gulf in bodyplan organisation distinguishing vertebrates from their spineless invertebrate relatives, and making the challenge of explaining its origin in developmental evolution all the more formidable. A monophyletic cyclostome clade thus provides two experimental models that, in contrast with gnathostomes, can reveal the nature of the ancestral vertebrate. Perhaps most significantly, contradictory morphological evidence has eroded, particularly through new insights into hagfish development, which have demonstrated that they have lost, not primitively lacked, many of the characteristics used previously to diagnose a lamprey-gnathostome clade. Support for cyclostome theory is now overwhelming, with the recognition of novel families of non-coding microRNAs that are shared exclusively by hagfish and lampreys ( Heimberg et al., 2010). Delarbre et al., 2002 Furlong and Holland, 2002 Kuraku et al., 1999). The competing hypothesis, which unites lampreys and hagfish as sister taxa in the clade Cyclostomata, thus equally related to gnathostomes, has enjoyed unequivocal support from phylogenetic analyses of protein-coding sequence data (e.g. However, only comparative morphology provides support for this phylogenetic hypothesis. This phylogenetic scheme is commonly seen in text books and is appealing because it implies a gradual assembly of vertebrate characters, and supports the hagfish and lampreys as experimental models for distinct craniate and vertebrate evolutionary grades (i.e. One places lampreys as most closely related to jawed vertebrates in the clade Vertebrata (see Glossary, Box 1), with hagfish more distantly related all three together then comprise Craniata (see Glossary, Box 1). Three arrangements of these lineages are possible, and two have been widely championed in recent years. 1), the precise relationship between the lamprey, hagfish and jawed vertebrate lineages has been more controversial. The latter are accordingly known as jawed vertebrates, or gnathostomes (see Glossary, Box 1), and several other characteristics support this separation, perhaps most notably the presence of paired fins/limbs and a mineralised skeleton in the gnathostomes and their absence in cyclostomes.Īlthough there is general consensus that living gnathostomes comprise a clade ( Fig. Together, lampreys and hagfish are usually referred to as the cyclostomes, ‘agnathans’ or jawless vertebrates (see Glossary, Box 1), and, as these names imply, they lack the hinged jaws characteristic of other living vertebrates. They lack clear vertebrae allowing them to tie their body in a knot, and they can produce huge quantities of slime when provoked.

Hagfish, by contrast, are typically deep-sea scavengers, feeding on sunken carcasses by burrowing inside via an orifice or wound. Rasping, tooth-like structures then grind into host flesh for feeding. Most lampreys are ectoparasites on fish, using a circular, sucker-like mouth to clamp onto their hosts. Lampreys and hagfish are unusual animals, and you are not likely to forget them if you have seen them.