The English Silurian Fossil Placocystites forbesianus and the Ancestry of the Vertebrates

• Published in: Philosophical transactions of the Royal Society of London. Series B, Biological sciences Biological sciences, 1978-03, Vol.282 (990), p.205-323
• Main Authors: Jefferies, R. P. S., Lewis, D. N.
• Format: Article
• Language: English
• Subjects: Brain; Canals; Ganglia; Nerves; Pharynx; Rectum; Sediments; Skeleton; Tunicates; Vertebrates
• ISSN: 0962-8436; 0080-4622; 1471-2970; 2054-0280
• DOI: 10.1098/rstb.1978.0013

Placocystites forbesianus de Koninck, from the Silurian Dudley Limestone, near Dudley, West Midlands, is here interpreted as a primitive chordate with a calcite skeleton of echinoderm type. This agrees with earlier papers by the senior author and disagrees with the work of Ubaghs (1968 etc.). Applying Hennig’s terminology, Placocystites probably belongs to the stem group of the vertebrates and therefore throws light on primitive vertebrate anatomy. It also belongs to the group Calcichordata, set up by one of us as a subphylum (Jefferies 1967). The Calcichordata, however, are not comparable in phylogenetic position with the living chordate subphyla, so the word calcichordate will henceforth be used only informally, for any chordate with a skeleton of echinoderm type. Ubaghs, who has developed a totally different interpretation, assigns Placocystites to the subphylum Homalozoa of the phylum Echinodermata. In assigning it to that phylum, Ubaghs’s work is more traditional than ours. Within the calcichordates, Placocystites forbesianus belongs to the more advanced group known as mitrates. These are distinguished from more primitive calcichordates (cornutes) by having right gill slits in addition to left ones. Within the mitrates it is possible to suggest the stem groups, in the Hennigian sense, of acraniates, tunicates and vertebrates. The term standard vertebrate is proposed to denote vertebrates in the usual sense, as contrasted with those stem vertebrates included in the mitrates. The two obvious parts of a calcichordate, formerly called theca and stem, or body and tail, are best called head and tail by homology with standard vertebrates. Mitrates correspond to the tunicate-tadpole-like protovertebrate of ‘antisegmentationist’ morphologists such as Froriep, Starck and Romer. The uniformly segmented protovertebrate of 'segmentationist’ morphologists such as Goodrich would represent a real but later stage in the ancestry of standard vertebrates, descended from a mitrate. The somites of standard vertebrates and acraniates can be plausibly identified inside calcichordates. The premandibular and mandibular somites would be located in the head, along with the buccal cavity, pharynx, gill slits and viscera. The left and right mandibular somites were probably represented in mitrates by the left and right anterior coeloms. The paired premandibular somites would be represented by a crescentic body situated in the posterior part of the head just in front of the brain.