Vol 50, No 2 (2016)

Every aspect of biological orderliness is a result of evolution, which expresses the systemic reorganization of organismal body plan, along with the way of its ontogenetic formation. Phyletic changes in the developmental rates (heterochronies) experienced by the organism or its structures exemplify just a kind of such consequences. The current belief that heterochronies are the causes of evolutionary events is based on the assumption that evolution of ontogeny proceeds in the same way as the ontogeny itself, i.e., from a germ cell to adult state. This premise (termed here “the central dogma”) is the cornerstone of traditional ideas of the evolutionary mechanism, regardless of whether it is perceived in terms of gene mutations or “embryonic modes.” In fact, the directions of two transformations compared are opposite each other. An evolutionary change in the body plan results from reorganization of the developmental system, which comes in response to disturbance of stability of the system’s terminal (adult) state. Realized by selection, this change starts immediately from the terminal state and then spreads in generations towards early ontogenetic stages. Heterochronies show just the same dynamics of events irrespective of whether they reflect the acceleration or delay of development. Empirically, such course of evolutionary changes was grounded most evidently by Severtsov in the early version of his concept of the phylembryogenesis. The theoretical basis of the same regularity is provided by the Schmalhausen–Waddington’s theory.

Early development stages imprinted on the shells of Cambrian brachiopods from the class Linguliformea (orders Paterinida, Lingulida, Acrotretida) were studied with scanning electron microscope based on a large collection from the Siberian Platform. Some specimens of all three orders preserved protegulum (embryonic shell); their brephic (juvenile) shells, also were studied. Many of them might lack larval development stage as it is known for the recent representatives of the family Lingulidae. But unlike recent lingulids, the surface of the juvenile dorsal valves of all studied linguliformeans bore two (rarely three) pairs of gentle elevations accommodating bundles of setae, which sometime preserve as groups of fine imprints along their margins. Recent Lingula and Glottidia lack setae in this stage. On the other hand, recent classes Craniformea and Rhynchonelliformea have setae, but they develop in the larval stage. Most of brachiopod groups have the setae in the adult stage but these setae have different origin, are short, located on the lateral and anterior ends of the shell, and grow from the marginal cells of the mantle.

Two unusual Middle Ordovician crinoid genera from the St. Petersburg Region, Pentamerocrinus Jaekel, 1918 and Grammocrinus Eichwald, 1860, are redescribed. A new species, Pentamerocrinus kosovi sp. nov., is described based on new material and the cup structure in the type species Grammocrinus lineatus, which has previously been known only from stem fragments, is reconstructed. The system of ridges and canals on the internal surface of the cup plates in both genera is described, allowing the reconstruction of the aboral nervous system. It is in general similar to extant crinoids, but the two Ordovician genera differ considerably in structural details. In Pentamerocrinus, ten nerve cords diverge interradially from the ganglionated nerve ring at the cup base and soon are fused in pairs inside the rays. In Grammocrinus, a similar nerve ring also gives rise to ten cords, but they are inside the rays and become fused in the interrays. This distinction in morphology of the nervous systems is correlated with the interradial arrangement of the axial canal angles of the stem in the first case and radial in the second. Therefore, despite general similarity in the structure and arrangement of calyceal plates, the genus Grammocrinus is assigned to the disparid crinoid family Iocrinidae, while the monotypic family Pentamerocrinidae Jaekel, 1918 is regarded as a fam. inc. sed. of inadunate crinoids.

Based on the study of the Trlica faunas (Montenegro), the paleozoogeographical aspects of the history of faunas of the Central Balkans and Europe in the late Early Pleistocene and beginning of the Middle Pleistocene (1.8–0.4 Ma) are analyzed.