Changes in the chalk heart-urchinMicrasterInterpreted in relation to living forms

Abstract

The fossil genusMicrasterfrom 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 purpureas, 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. raschiandE. cordatum, was observed.S. purpureuscan 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 tube-feet 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 thatE. pennatifidumandE. flavescensalso build single sanitary tubes, whereasB. lyriferabuilds 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. raschiploughs 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 particleor 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, burrow-building, 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 ofE. 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 ofMicrasterand the closely related subgenusIsomicrasterare 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 theMicrasterandIsomicrasterseries. There is a progressive increase in number in the main Micraster line with time, while theIsomicrasterseries 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 asS. raschi, and this is also supported by other features, particularly the reduction in ciliary currents in the subanal region. In the mainMicrasterphylum 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. WhenMicrasterappears in thecuvierizone asM. leskeiit 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 (corbvis), while features suggesting deeper burrowing gradually become more common and more pronounced on the other (cortestudinarium). At the top of theplanuszone the shallow-burrowing forms die out, but the deeper burrowers persist up to thetestudinariuszone, 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-calledIsomicrasterline, and the main line and at the most it should only be considered subspecifically distinct from the micrasters. Similarly, in themucronatazone of Norfolk intermediates occur betweenglyphusand 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. thatstolleyiis an immigrant), or whether both these forms have re-invaded the English area after becoming subspecifically distinct outside it.