Heads and Tails: A Chordate Phylogeny

Abstract

A cladistic analysis of chordates is presented, based on some 320 nested characters. All the principal higher taxa are defined by synapomorphies, including extinct acanthodians and placoderms. The data base draws broadly from adult anatomy (including osteological data for Recent and fossil taxa), embryology, physiology, and biochemistry. A conventional sequence of chordate higher taxa is generated (hemichordates, urochordates, cephalochordates, craniates). Among the craniates, cyclostomes are considered paraphyletic. Gnathostomes are monophyletic, but two fossil "agnathan" groups (galeaspids, osteostracans) are regarded as stem gnathostomes. Chondrichthyans and osteichthyans are monophyletic. New arguments for osteichthyan affinity of acanthodians are presented. The phylogenetic position of placoderms is still problematic, but they can no longer be perceived as stem chondrichthyans or even as "elasmobranchiomorphs." Recent dipnoans and tetrapods are sister groups, but new paleontological discoveries refute many of their supposed osteological synapomorphies, thereby reopening the possibility of a closer relationship between tetrapods and osteolepiform rhipidistians. CLADISTICS[VOL. 2 204 CLADISTICS[VOL. 2 of motor nerves in the ventral "roots" of cephalochordates (Flood, 1966) are probably primitive conditions. Contrary to Jefferies (1979), I do not think muscle innervation patterns in tunicates, echinoderms, and cephalochordates offer any clear indication of their interrelationships, and it may be that motor nerve end plates arose independently in adult thaliacean tunicates and in craniates, whereas the remaining similarities between tunicates and craniates are most parsimoniously regarded as primitive.The supposed presence of chitin in the cephalochordate branchial endoskeleton suggested to Ldvtrup (1977:94) that chordates are not monophyletic. R&r (1982) presented convincing evidence that although the branchial skeleton of Amphioxus is not collagenous, neither is it chitinous; instead the branchial rods probably consist of acid mucopolysaccharides and proteins. Ldvtrup's (1977) hypothesis of chordate non-monophyly is not substantially helped whether cephalochordate branchial bars consist of chitin or mucopolysaccharides; their structure in the first case could be primitive; in the second case it is most economically regarded as a synapomorphy of cephalochordates. Other possible cephalochordate synapomorphies include the tentacular oral languets, ciliated wheel organ, and Hatschek's nephridium. Hatscheck's pit may be the homolog of the craniate pituitary (Goodrich, 1917;De Beer, 1925, 1926, but nothing is known of cephalochordate endocrine function (Barrington, 1968).Although accepting cephalochordates as the sister group of craniates, Janvier (1981: 122) stated that they "show hardly any precise and surely homologous synapomorphy . . . sufficient to reject comparisons between the Chordata and other groups."However, cephalochordates and craniates are united by the following characters that have not been identi...

Expression and phylogeny of claudins in vertebrate primordia

“…The number of distinct claudin genes found to date in each clade is given in brackets. The appearance of those vertebrate characters that we found to be associated with claudin expression is indicated beside the stem lineage (13). Arrows indicate two presumptive genome duplications (16); a third genome duplication may have taken place in ray-finned fishes alone (15,47).…”

“…4), when tight junctions replaced septate junctions (1). Like many other gene families (16), the claudins then expanded as the vertebrate body plan evolved from the chordate stem configuration through the addition of novel structures such as the cranium, paired sensory organs, and paired appendages (13,14). A more precise specification of this model will require exhaustive screens for claudin genes in the nongnathostome clades shown in Fig.…”

“…Sensory placodes and the pronephros, the domains of cldna and cldnb expression, are characters of vertebrate embryos that arose along the chordate stem lineage (13). Tight junctions, the cellular structures in which claudins occur, are likewise characteristic of chordates (1).…”

“…During the evolution of vertebrates, for example, the chordate body plan was retained while novel structures such as the cranium, paired sensory organs, and paired appendages were added (13,14). This increase in anatomical complexity likely reflected an increase in genetic complexity as many gene families expanded by retrotransposition, tandem duplication, and duplication of whole genomes (15,16).…”

Evidence for independent Hox gene duplications in the hagfish lineage: a PCR-based gene inventory of Eptatretus stoutii

“…On morphological grounds, lamprey and hagfish are often considered paraphyletic to jawed vertebrates ( Fig. 1a) (Maisey, 1986). In contrast, most molecular phylogenies suggest that lampreys and hagfish form a monophyletic clade which is the sister taxon to the jawed vertebrates The "vertebrate theory" assumes that lampreys and gnathostomes are a clade, the vertebrates, whose sister group is hagfishes.…”