Micraster

Abstract: The fossil genus Micraster from the Chalk gives an excellent example of continuous evolution of individual characters. The changes, however, appear to be independent of changes in those environmental conditions which have left their mark in the formation of the sediment (changes in depth at which it was deposited, particle size, temperature, etc (Nichols 1956). Consequently, these changes in the urchins probably reflect either a change in niche or an improvement in the adaptation of the animal in an effectively unchanged niche. To discover the significance of the changes, the following seven living species of British irregular echinoids have been studied in detail, both morphologically and functionally. Order Spatangoida: Spatangus purpureus, S. raschi, Echinocardium cordatum, E. pennatifidum, E. flavescens, Brissopsis lyrifera. Order Clypeastroida: Echinocyamus pusillus. The ciliary currents on the tests of these urchins are in general similar, except at the fascioles. Features not previously reported are, first, the existence in all the urchins of an apical eddy of currents to ensure that the genital products are not wafted into the mouth, and secondly, forwardly directed currents into the mouth flowing over the lip. The normal burrowing of three of the urchins, S. purpureus, S. raschi and E. cordatum, was observed. S. purpureus can burrow with its dorsal surface some 5 cm below the surface of the substratum, with no visible connexion with the sea water; water currents for respiration are apparently obtained through the interstices of the gravel particles in which it lives. The prehensile tube-feet of its subanal region build a double sanitary tube, each element originating at one lobe of the bilobed subanal fasciole. E. cordatum, observed both in aquaria and in the field, burrows to a maximum depth of about 18 cm, and maintains contact with the sea water via a respiratory funnel built by the tubefeet of the dorsal part of the anterior ambulacrum. A single sanitary tube is built, originating at the shield-shaped subanal fasciole. From the configuration of their subanal regions it is inferred that E. pennatifidum and E. flavescens also build single sanitary tubes, whereas B. lyrifera builds a double one. The main factor governing the number of tubes for sanitation is suggested to be the coherence of the substratum, those living in shell gravel and mud requiring only the same number of tube-feet to build two tubes as those living in sand require to build one; in consequence the shell-gravel and mud forms can attain a larger size. Some experimental evidence is presented for this hypothesis. S. raschi ploughs through the substratum with about half of its corona exposed above the surface; it attains a larger size than any other urchin studied, yet apparently builds only one subanal tube. The reason for this seems to be that since the respiratory tube-feet are not confined in a burrow, there is no need for a large soak-away for the respiratory water, and the sanitary device can be correspondingly smaller in cross-section. The feeding of all six spatangids was observed; in general they grasp a particle or cluster of particles of the substratum with the oral prehensile tube-feet, convey the material with its covering of organic matter to the mouth, and scrape it off against the spines surrounding the peristome so that it passes into the gut. Selection of food particles by weight occurs in those urchins normally inhabiting shell gravel (S. purpureus, E. pennatifidum, E. flavescens). The functional significance of the following morphological features of all the urchins is discussed: (i) number and arrangement of the fascioles providing augmentation of the ciliary currents on the test where they are specially needed; (ii) shape and arrangement of spines, for use in scraping, locomotion and burrow-building; (iii) division of labour among the tube-feet for feeding, burrowbuilding, respiration and sensation; (iv) depth of the anterior groove providing a channel for the passage of food currents from the dorsal surface to the mouth; (v) arrangement of the respiratory organs in the most efficient way for the type of burrow. The morphology of the various tube-feet of E. cordatum, and the pores from which they arise, is described. The disks of the feeding tube-feet have a velvet-like pile of papillae for collecting the sediment, whereas those of the burrow-building tube-feet tend to allow space in the centre so that the fringe of papillae can fold in during retraction. The feeding tube-feet, extending at all angles to the test, have wide bases to their stems, whereas those whose extensions are mainly perpendicular to the test are much narrower. The main features of Micraster and the closely related subgenus Isomicraster are described, and the work of Rowe and Kermack on the evolution of the group is reviewed. The probable effect of other animals living in or feeding on the sea bottom at the time of these fossils is described. The number of pore-pairs in the petaloid parts of the paired ambulacra (those which gave rise to the respiratory tube-feet) have been examined in specimens from successive stages of the Micraster and Isomicraster series. There is a progressive increase in number in the main Micraster line with time, while the Isomicraster series have many more than the highest number of these. The increase in the main line is interpreted as enabling successive populations to burrow more and more deeply and is confirmed by changes in other features, particularly the increase in surface area by ornamentation to increase the concentration of cilia. The higher number in the isomicrasters is interpreted as enabling a reduction in size of the tube-feet as a countermeasure to predation, thus paralleling such living forms as S. raschi, and this is also supported by other features, particularly the reduction in ciliary currents in the subanal region. In the main Micraster phylum changes in the peristomial region, in the position of the mouth, degree of development of the labrum and the depth of the anterior groove suggest by comparison with the mode of life of living urchins a gradually increasing reliance on a ciliary mode of feeding at the expense of that of using tube-feet. When Micraster appears in the cuvieri zone as M. leskei it is a rare urchin. Its features suggest that it burrowed only shallowly. In the next two zones the shallow-burrowing features are retained by extremes on one side (corbovis), while features suggesting deeper burrowing gradually become more common and more pronounced on the other (cortestudinarium). At the top of the planus zone the shallow-burrowing forms die out, but the deeper burrowers persist up to the testudinarius zone, the features that suggest deeper burrowing becoming even more marked (coranguinum). At the same time, following the disappearance of the shallow burrowers, a partial burrower, senonensis, appears, probably by immigration. Considerable interbreeding occurs between this, the so-called Isomicraster line, and the main line and at the most it should only be considered subspecifically distinct from the micrasters. Similarly, in the mucronata zone of Norfolk intermediates occur between glyphus and its contemporary member of the isomicrasters, stolleyi, though from the small sample available and because of the lack of evidence from intermediate strata it is not possible to say whether the situation parallels the micrasters and isomicrasters of the south of England (i.e. that stolleyi is an immigrant), or whether both these forms have re-invaded the English area after becoming subspecifically distinct outside it.

Abstract: I. General Considerations. The Micrasters dealt with in this paper were collected from the sections exposed at Gravesend, the Thanet Coast, Dover, Beachy Head, the Dorset Coast, and Beer Head, in the zones indicated above. Two thousand examples have been measured and analysed, and no specimen has been included in the summary unless its zonal origin is accurately determined. Six hundred photomicrographio negatives of the special features of the test have been made, in order that mere conjecture may play no part in the enquiry, and that these important aids to specific and zonal determination may be placed on a permanent and scientific footing. The field-work has been extended over as wide an area as possible, so that conclusions should not be vitiated by being drawn from any one isolated district. In this paper, the term ‘low-zonal’ will apply to the zones of Rhynchonella Cuvieri, Terebratulina gracilis, Holaster planus, Micraster cor-testudinarium, and the lower third of the zone of M. cor-anguinum (80 feet); and the term ‘high-zonal’ will refer to the remainder of the zone of M. cor-anguinum, and to the zones above it Though all the accessible literature on the subject has been studied, the conclusions here set forth have not been influenced by previous publications, for rigid zonal collecting has been the paramount object, and the facts brought out by analysis have been allowed to speak for themselves. The scope of the present paper will not admit of the discussion of questions of nomenclature, which must be left

Abstract: During the Late Cretaceous, both Mangyshlak and the Opole area were part of the North European Province, and irregular echinoid faunas during the Turonian and Coniacian in both areas were dominated by holasteroids and spatangoids, in particular by Plesiocorys , Echinocorys and Micraster . Almost 1,000 specimens from both areas have been studied. Taxonomically more varied are the faunas from Mangyshlak (15 species in 6 genera); the Opole assemblages comprise 9 species in 3 genera. One species, Micraster ( Micraster ) praerogalae , a transitional form between Micraster ( Micraster ) cortestudinarium (GOLDFUSS) and Micraster ( Micraster ) rogalae NOWAK, is described as new. Micraster ( Micraster ) rogalae is markedly diachronous within the North European Province, first appearing in the late Coniacian in Mangyshlak and reaching central and western Europe during the late Early Santonian. A similar pattern is revealed by a large morphotype of Echinocorys ex gr. scutata LESKE. Palaeoecological and actualistic data show the taxa studied to have been shallow infaunal ( Catopygus ), shallow epifaunal [ Conulus ( Conulus ) subrotundus ], deeper infaunal [ Micraster ( Micraster )], deeper semi-infaunal [ Micraster ( Gibbaster ), Plesiocorys ( Sternotaxis ) plana ], deeper epifaunal [ Echinocorys , Plesiocorys ( Plesiocorys ) placenta ], and deeper infaunal ( Hemiaster ). Echinoid bioevents described originally from the Salzgitter-Salder section (Lower Saxony, Germany) can also be recognised in the Opole area, but not further afield in Mangyshlak.