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Slow evolution in the Holectypidae, a family of echinoids
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Tubercles and spines of 33 species of extant irregular echinoids were studied to provide data for interpreting tubercle structure and arrangement in fossil echinoids. Tubercle morphology is analysed functionally. It is possible to infer much about the posture, movement and function of the associated spine from tubercle morphology. The development and direction of areole enlargement and the structure of the platform are the most important features in this respect. Surface stereom porosity is used to differentiate tubercles and miliaries, which bear either spines or pedicellariae, from granules, which have no such appendages. Tubercles of regular echinoids are usually radially symmetrical and can be broadly separated into fixed-pivot and sliding-pivot systems. In irregular echinoids, spines are usually modified for a particular function and tubercle morphology is correspondingly varied. The power stroke of the oral spines is radial in pygasteroids and holectypoids, whereas, in clypeasteroids and cassiduloids, it is posterior. This is reflected in their different burial and locomotory behaviour. Oral spine and tubercle arrangement in Cassidulus resembles that found in spatangoids and this again is reflected in its behaviour. Spine and tubercle differentiation becomes quite pronounced in certain clypeasteroids. It is argued that lunules and notches are modifications which allow sand dollars to feed on the organic-rich surface layer of sediment while remaining infaunal. Tubercle and spine diversity is most pronounced in the spatangoids and is described in detail for Echinocardium. Each group of spines with a particular function is associated with morphologically distinct tubercles. Tufts of spines are readily recognizable from the tubercle arrangement. The considerable variation in the arrangement of spines and tubercles within the anterior ambulacrum is thought to reflect the varying emphasis placed on adoral transportation of sediment. Fasciole arrangement shows no correlation with depth of burial or grain size of the substratum. Aboral tubercle density is correlated with sediment grain size. To maintain a water-filled space between the tip of the spines and the test surface, echinoids increase spine density and develop a uniform covering of spines with either distally swollen or spatulate tips. In spatangoids, a more effective coverage is achieved by development of curved or oblique aboral spines, and the aboral mucous coat prevents fine particles from falling between spines and clogging the burrow. The structure and arrangement of tubercles can be extremely useful in palaeobiological studies.
S ummary
The Holectypoida are restored to the rank of an order of the Echinoidea Irregularia. A classification, somewhat modified from that proposed by Gregory ( 50 ), is given, and revised diagnoses of the families, subfamilies, and genera are drawn up. A comparative study of the morphology of the skeletal structures of typical genera of the group is given; and, in the light of the results of this study, the course of evolution both within and beyond the limits of the order is indicated.
The Holectypoida are regarded as an annectant group of the Irregular Echinoids, whose characters retain a considerable uniformity owing to a persistent retardation of evolution. At various periods offshoots from the Holectypoid stock appeared, which, usually with a relatively accelerated differentiation, developed into the various orders and families of the Irregularia. The order commenced in the Liasic period, and became extinct at the end of the Cretaceous. Two of the groups of Echinoids now living retain many features that were characteristic of the Holectypoida (the Echinoneidæ and the Fibulariidæ). The other groups of Irregular Echinoids show a much greater departure from the primitive character, but they all possess some features which indicate their Holectypoid anptzy.
Two new names are introduces in the Systematic Part:— Megapygus as a subgenus of Pygaster , corresponding with the Pygaster (sens. str.) of Pomel (37), with type M. umbrella ; and Conulopsis , a genus including the “ Echinoconus ” of Desor (21), with type C. roemeri d'Orbigny. The latter group will be studied in greater detail in a forthcoming paper.
Thesis--Paris. Includes index.
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