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Dorcadospyris alata (Riedel , 1959)

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Brachiospyris alata Riedel, new species
Plate 1, figures 11-12


Description: Shell thick-walled, nut-shaped, tuberculate, with slight sagittal stricture externally and subcircular to circular pores irregularly arranged and separated by wide intervening bars. Two feet, circular in section, widely divergent, proximally at 180° or sometimes more, subsequently curved downward to a greater or lesser extent, in occasional specimens approximately semicircular. The outer edges of the feet of most specimens bear short conical spinules. This species is distinguished from all other members of the genus by the pronounced divergence of the feet and the tendency to the development of accessory spinules on the feet.


Dimensions (based on twenty specimens): Length of shell 58-68μ; of feet 285-930μ; of accessory spinules 3-13μ. Breadth of shell 68-78μ. Localities and stratigraphic range: Chubasco core 24, 16-20 cm. (locality of the holotype, U . S . N . M . no. 563362);
Globorotalia fohsi barisanensis zone of the Cipero formation, Dumfries Road, Palmiste Estate, near San Fernando, Trinidad. This species has been found at only two localities, Bolli's Globorotalia fohsi barisanensis zone and a Pacific sediment believed to be of approximately the same age as that zone.


Remarks: Judging from the data at present available, Brachiospyris alata is the last representative of a remarkable phyletic series of two-footed and three-footed zygospyrids (Triospyrididae according to Campbell, 1954) that arose during the Upper Eocene, attained their acme during the Oligocene and Lower Miocene, and apparently became extinct at approximately the end of the Lower Miocene or in the Middle Miocene. Although the species reported in this paper do not include all members of this zygospyrid group - some of Haeckel's species were not met with during the present study - it seems probable that most of the more common and widespread members are included here. The group includes
Tristylospyris triceros, Cantharospyris ateuchus, Hexaspyris papilio, Dipodospyris forcipata, Dorcadospyris dentata, Brachiospyris simplex, and Brachiospyris alata. This series of forms, with an indication of the time of maximum development of each, is illustrated diagrammatically in text-figure 3.

During the Upper Eocene, Tristylospyris triceros, with three primary feet and usually with three less strongly developed secondary feet, arose apparently from a form in which six basal feet were equally developed, possibly Hexaspyris bütschlii Haeckel or a related species. During the Oligocene, species arose in which the number of primary feet was reduced from three to two, with retention of the secondary feet (Cantharospyris ateuchus and Hexaspyris papilio). Species in which the secondary feet had disappeared, leaving only two strongly developed primary feet, first appeared in the Upper Oligocene, and reached their maximum development during the Lower Miocene (Dipodospyris forcipata and Brachiospyris simplex). Late in the Lower Miocene species arose in which the two feet were ornamented by accessory spinules (Dorcadospyris dentata and Brachiospyris alata); these youngest species tend to have shells reduced in size, with relatively small, sparse pores. Although it is not yet possible to trace in detail the phylogenetic development of each of these species, there can be little doubt that the reduction in number of the primary feet, and the later development of accessory spinules, are evolutionary trends within this group of species.

These species with thick-walled, tuberculate, generally nut-shaped shells appear to form a genetically related group which cuts across the taxonomic boundaries set up in Haeckel's system. When sufficient data are available for a revision of Haeckel's system, this is the type of grouping of species on which genera should be based.

Riedel 1959


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