Hypotheses on the origin and early evolution of chordates in the light of recent zoological and palaeontological evidence

Abstract

Both new palaeontological and embryological evidence on chordates require a reassessment of traditional ideas on their early phylogeny and on the evolution of their Bauplan and tissues. A revision of the available data from both palaeontological and zoological points of view is useful both to clarify the better justified possible hypotheses and to point to the more profitable lines of investigation for further advancing our understanding of these problems. Recent advances in the knowledge of the morphology of Cambrian protochordates and of conodonts provide significant evidence for the understanding of the earliest phases of development of typical vertebrate morphology. It appears that, in the oldest lineages related with the evolution of vertebrates, mineralized tissues may have been restricted to the bucco-pharyngeal region and be of ectodermal and ectomesenchymal origin. The development of dermal and visceral skeletal structures in living vertebrates further supports the overall significance of ectodermal-ectomesenchymal interactions as the first step towards the origin of the vertebrate skeleton.

An Odontogriphid from the Upper Permian of Australia

“…We regard the case for Bowengriphus at least being an undulating swimmer as untenable. Simonetta et al (1999) accepted the dorsoventral compression of the body of Odontogriphus as a primary feature, and the preservation of Bowengriphus perphlegis agrees with this interpretation for odontogriphids in general, as both specimens indicate that the stable orientation for burial is dorsoventral.…”

“…5), this homology is dif®cult to accept. Further, as noted by Simonetta et al (1999), the dorsoventral compression of the odontogriphid body is not readily interpreted in a chordate context. The placement of Odontogriphidae ( Odontogriphus Bowengriphus) within the Metazoa remains problematic; its status as a bilaterian is perhaps the only incontrovertible assignment that can be made.…”

“…Although transverse furrows in the paratype of Bowengriphus perphlegis have a regularity of spacing and symmetry that suggests segmentation, the holotype of that species, as well as Odontogriphus, shows only an irregular annulation. Unlike Simonetta et al (1999), we are thus reluctant to accept odontogriphids as metameric, though perhaps the most suggestive evidence of metamerism is the series of dark ovate structures preserved in Odontogriphus. The most complex structure preserved in the fossils is the feeding apparatus.…”

“…Bowengriphus perphlegis also lacks a tail and preserves no structures corresponding to Dzik's (1993) branchial openings. Dzik (1995 and Simonetta et al (1999) expanded upon chordate comparisons, suggesting that the dark oval structures (`lateral muscles' of Conway Morris), rather than being branchial openings, were instead homologous with organs identi®ed as gonads in the Early Cambrian chordate Yunnanozoon (Chen et al 1995). Given that the latter structures exhibit distinct bilaterally asymmetry (Chen and Li 1997;contra Dzik 1995, ®g.…”

False teeth: conodont-vertebrate phylogenetic relationships revisited

“…Although the imprinting of external cells of overlying soft tissue on (dermal) hard parts is possible in many animals, the patterns seen in many conodont elements are diff erent from those in vertebrate scales and teeth. Th ese impressions were used as an argument for conodonts being vertebrates by Simonetta et al (1999) but, as Reif (2006 also showed, cell impressions are not exclusive to the surface of enamel nor can they be used as conclusive identifi cation for enamel in conodonts. For comparison, Märss (2006) reviewed micro-ornaments in a wide variety of vertebrate scales with surfi cial enamel, enameloid, and dentine.…”