Aplacophoran traits in the late Ordovician septemchitonid polyplacophorans.

A sample of phosphatized, originally calcareous, mollusk shells from the Katian age uppermost Mójcza Limestone at its type locality yielded a few hundred polyplacophoran plates. The chelodids are very rare among them. Three septemchitonid species dominate. They represent a gradation from underived steep roof-like plates to almost cylindrical ones, leaving only a narrow ventral slit for the foot. Apparently, this represents the first step toward the extremely derived 'segmented clam' Bauplan of the Silurian Carnicoleus, with plates completely closed at the venter except for the mouth and anal openings. To enable growth, the plates became thinner and more flexible (or perhaps resorbed) along the dorsum. The tendency toward reduction of the ventral gap of the plates in the early Paleozoic septemchitonid polyplacophorans implies their lack of ability to cling to the substrate with a muscular foot. In compensation, their plates changed toward a more efficient protective function, covering the animal body sides more and more completely. This may explain the origin of the ventral furrow of extant solenogasters hiding the rudimentary foot. An opposite route was chosen by the coeval Acaenoplax lineage, in which the plates did not contact each other, exposing much of the soft body on the dorsum. In both cases the animals appeared to be worm-like, perhaps representing different ways of evolution from the Paleozoic chitons to the extant aplacophorans.

Protaspis larva of an aglaspidid-like arthropod from the Ordovician of Siberia and its habitat.

A fossil larva lacking segmentation of the calcified carapace, closely resembling the trilobite protaspis, has been found associated with other skeletal elements of an angarocaridid Girardevia species in the mid Darriwilian of central Siberia. The presence of protaspis larvae in the angarocaridids, generally believed to represent a branch of the Aglaspidida, supports their proximity to trilobites and proves a low position on the arthropod phylogenetic tree but does not necessarily contradict the chelicerate affinity. The cephalic appendages of angarocaridids bore massive gnathobases with detachable spines, closely similar to those known in extant xiphosurans and in their probable Cambrian relatives. The stratigraphic succession of the angarocaridids, their phosphatized cuticle pieces being abundant in the Ordovician strata of Siberia, shows a gradual improvement of mechanical resistance of their carapaces, eventually resulting in a honeycomb structure. The associated benthic mollusc assemblage is dominated with the bellerophontids showing high mortality at metamorphosis and only the limpet-like Pterotheca, infaunal bivalves, and scaphopods being able to survive this in a substantial number. This suggests a strong selective pressure from predators equipped with well-skeletonised oral apparatuses able to crush mineralized body covers of their prey. Possibly, these were some of the associated conodonts of appropriate size and co-evolving towards their ability to crush more and more resistant cuticle. Less likely candidates for durophagy are endoceratid or orthoceratid cephalopods. Also the angarocaridids themselves, equipped with robust gnathobases of cephalic appendages, apparently predated on benthic shelly animals.

470-Million-year-old black corals from China.

Phosphatic (possibly secondarily phosphatised) remains of antipatharian coralla, previously unknown in the fossil record, occur abundantly in the early Ordovician Fenxiang Formation in the Hubei Province, southern China. Probably two species (and genera) are represented, which differ in spinosity of branches. The more spinose one, Sinopathes reptans, has its lateral spines bearing regular, longitudinally arranged costellae. The early Floian geological age of this finding, about 470 Ma, supports predictions on the timing of anthozoan phylogeny derived from the molecular phylogenetic evidence. Black corals (Antipatharia) are basal to the scleractinians in the Hexacorallia clade, being more derived than sea anemones and the Zoantharia. Based on calibration of the molecular clock with Mesozoic data, the first split of lineages within the scleractinian hexacorals was proposed to take place approximately 425 million years ago. This implies that the origin of Antipatharia should precede this date. They have not been known in the fossil record because of unmineralised skeleton composed primarily of laminar chitin complexed with a protein. Unlike all recent species, the encrusting basal part of the colony dominated in the Ordovician ones and only occasionally erect branches developed, rather chaotically ramified. This presumably plesiomorphic trait seems consistent with ancient geological age and suggests that some problematic fossils from the Late Cambrian may be their, even less-derived, relatives.

Evolution of morphogenesis in 360-million-year-old conodont chordates calibrated in days.

Highly rhythmic increments of crown tissue are identifiable in conodont oral apparatus elements from the Late Devonian of the Holy Cross Mountains, Poland; individual laminae being of thickness comparable with daily increments of vertebrate tooth enamel and fish otoliths. Abundant occurrence of such specimens enables bed-by-bed (stratophenetic) studies of the process of evolution at the population level and quantitative presentation of the evolution of ontogeny in the sampled geological section covering several million years. The morphologic transformation is expressed as expansion of a juvenile asymmetry to later stages of the ontogeny and in decrease of the mature element width, which was due to a change of the mineral tissue secretion rate. It was not just a simple extension of a juvenile character into the later stage of the ontogeny (heterochrony) but rather a true developmental novelty. The evolution was gradual and very slow. The proposed quantitative approach to growth increments in the mineral skeleton of ancient chordates introduces real-time units to evolutionary developmental studies connected with direct paleontological evidence on the course of evolution.

Gill structure and relationships of the Triassic cycloid crustaceans.

Unusually well-preserved fossils of a Halicyne-like cycloid crustacean frequently occur in the early Late Triassic lacustrine clay bed at Krasiejów in Opole Silesia, southern Poland. Its gill-like structures form a horseshoe-shaped pair of units composed of numerous calcified blades with reverse U-shaped cross-section. Originally, these were parallel slits opening on the ventral surface of the carapace. Lobation of the posterior margin of the carapace, of unusually large mature size for the group, make the animal different from other members of Halicynidae, and the new name Opolanka decorosa gen. et sp. nov. is proposed for it. More completely preserved specimens of cycloids from Vosges, France, and Madagascar show that the slit openings were located above radially arranged coxae of the walking appendages and a reduced abdomen. The disposition and arrangement of the cycloid gills suggest at least close analogy, and possibly homology, with the "respiratory areas" of the Branchiura, serving mostly as ion-exchange organs. It is proposed that they originated, in connection with the body size increase and adaptation to fresh-water environment, as radially arranged infoldings of the respiratory areas cuticle, with strongly calcified rigid dorsal parts suspended from the carapace. At least three ecologically and anatomically distinct lineages were represented in the order Cyclida, which was probably initially confined to marine environments and gradually adapted to life in continental waters. New taxa Schraminidae fam. nov. (with Schramine gen. nov.) and Americlidae fam. nov. (with Americlus gen. nov.) are proposed.

Anatomical information content in the ediacaran fossils and their possible zoological affinities.

Various modes of preservation of Ediacaran fossils in different sediments, quartz sand at Zimnie Gory in northern Russia and lime mud at Khorbusuonka in northern Yakutia, show that the sediment was liquid long after formation of the imprints and that its mineralogy did not matter. A laminated 2 mm thick microbial mat is preserved intact at Zimnie Gory. It stabilized the sediment surface allowing formation of imprints on it. The soft body impressions on the under surface of the sand bed and within it developed owing to formation of a less than 1 mm thin "death mask" by precipitation of iron sulfide in the sediment. Fossils of the same species or even parts of the same organism may be preserved differently. Internal organs either collapsed, their cavities being filled with sediment from above, or resisted compression more effectively than the rest of the body. This allows restoration of the original internal anatomy of Ediacaran organisms. At Zimnie Gory numerous series of imprints of Yorgia on the clay bottom surface with the collapsed body at their end represent death tracks. The environment of formation of the Ediacaran fossils was thus inhospitable to most organisms. Those adapted to it, namely the radially organized frondose Petalonamae (of possible ctenophoran affinities), anchored in the mat with their basal bulbs. They evolved towards sessile life possibly in symbiosis with photo- or chemoautotrophic microorganisms. Vagile Ediacaran organisms belong mostly to the Dipleurozoa (somewhat resembling chordates and nemerteans), characterized by a segmented dorsal hydraulic skeleton, intestine with metameric caeca, and serial gonads. Only a fraction of the actual Precambrian faunal diversity is represented in the Ediacaran biota.

Possible ctenophoran affinities of the Precambrian "sea-pen" Rangea.

The Namibian Kuibis Quartzite fossils of Rangea are preserved three-dimensionally owing to incomplete collapse of the soft tissues under the load of instantaneously deposited sand. The process of fossilization did not reproduce the original external morphology of the organism but rather the inner surface of collapsed organs, presumably a system of sacs connected by a medial canal. The body of Rangea had tetraradial symmetry, a body plan shared also by the White Sea Russian fossil Bomakellia and possibly some other Precambrian frond-like fossils. They all had a complex internal anatomy, smooth surface of the body, and radial membranes, making their alleged colonial nature unlikely. Despite a different style of preservation, the Middle Cambrian Burgess Shale frond-like Thaumaptilon shows several anatomical similarities to Rangea. The body plan of the Burgess Shale ctenophore Fasciculus, with its numerous, pinnately arranged comb organs, is in many respects transitional between Thaumaptilon and the Early Cambrian ctenophore Maotianoascus from the Chengjiang fauna of South China. It is proposed that the irregularly distributed dark spots on the fusiform units of the petaloid of Thaumaptilon represent a kind of macrocilia and that the units are homologous with the ctenophoran comb organs. These superficial structures were underlain by the complex serial organs, well represented in the fossils of Rangea. The Precambrian "sea-pens" were thus probably sedentary ancestors of the ctenophores.