Morphological similarity between echinoid taxa as a result of convergent and parallel evolution

Abstract

Convergence is observed in groups, which are phylogenetically remote. A flat test is typical for some representatives of the order Cassiduloida, such as Jurassic and Early Cretaceous species of the genera Pygurus and Clypeus, and also many Cenozoic «sand dollars,» i.e., echinoids of the order Clypeasteroida. Both usually inhabit coarse sandy grounds of shallow areas. The superorder Spatangacea includes the so-called Echinocorys life form, which is characterized by an oval test with superficial nonpetaloid or subpetaloid ambulacra, marginal or inframarginal periproct, and absence of fascioles. These are the following genera: Early Cretaceous Corthya (family Collyritidae), Late CretaceousLate Paleocene Echinocorys (family Holas-teridae), Paleocene Isaster, Recent Isopatagus (family Isasteridae), Recent Scrippsechinus (family Palaeotro-pidae), and Recent Urechinus (family Urechinidae). In contrast to the majority of spatangaceans with the burrowing mode of life, these genera dwell on the substrate surface. In the Cenozoic, the monobasal apical system appears in some genera of the order Spatangoida, most genera of the order Cassiduloida, all groups of the orders Clypeasteroida and Oligopygoida, and in the genus Echinoneus (order Holectypoida). The paral-lelism is revealed in groups connected by remote relationships. At the end of the Middle Jurassic (Callovian) and, especially, Late Jurassic, the so-called disasterid echinoids (superorder Spatangacea) show a distinct trend to the loss of contact between ocular plates I and V and apices of the posterior ambulacra with the periproct, which are shifted to the anterior part of the apical system (genera Collyrites, Collyropsis, Cyclolam-pas). At the same time, the peristome of some genera was displaced to the anterior margin of the test, which became bilaterally symmetrical in outline. However, in the Jurassic, all spatangaceans remained disasterid echinoids, i.e., had a disjunct apical system, which can be interpreted as a somewhat “abnormal” state. This trend disappeared only at the beginning of the Cretaceous, when “normal” forms with a joint apical system appeared, that is, the families Holasteridae (genera Eoholaster and Holaster, order Holasteroida) and Toxas-teridae (genus Toxaster, order Spatangoida). Interesting examples of synchronous parallelism are provided by the appearance of meridosternous (diasternal) plastron in two collyritid genera (Tetraromania and Corthya) in the Barremian, whereas in the holasterid genus Holaster, this type of plastron apparently appeared in the Valanginian (heterochronous parallelism). The ethmolitic type of the apical system appeared at the end of the Cretaceous and Paleocene at least in five families: Schizasteridae, Paleopneustidae, Brissidae, Spatangidae, and Loveniidae.