Abstract
Representatives of the subgenus Dicrorygma Poag are found in Lower Cretaceous beds in Texas, England, and Denmark.Two new Lower Cretaceous species have been described from England and Denmark, respectively. Besides, an ostracod closely related to the English species has been described from Danish beds.Species of Dicrorygma (Orthorygma) nov. subgen. occur in beds of the Kimmeridgian, Portlandian, and Purbeckian Ages from Denmark. Frequent occurrences of the species in the Purbeckian beds indicate marine ingressions in these otherwise brackish deposits. A number of formerly described Upper Jurassic species from NW. Europe are considered to belong to the genus Dicrorygma. Three new Upper Jurassic species are described.
Highlights
By searching the more fine-grained portion of washed samples from the Kiamichi Formation in Northern Texas PoAG discovered a small ostracod, which, it was common in the samples, had so far been overlooked
Species closely related to Dicrorygma mullinsi PoAG have been found in beds of Hauterivian from England and from Denmark, D. (Dicrorygma) speetonensis nov. sp. and D. aff
By examination of well-preserved specimens of D. mullinsi POAG has found that there is a tendency towards development of a small toothlike projection on the posterior part of the hinge on the right valve
Summary
By searching the more fine-grained portion of washed samples from the Kiamichi Formation in Northern Texas (uppermost sect ion of the Frederiksburg Group, Lower Cretaceous) PoAG discovered a small ostracod, which, it was common in the samples, had so far been overlooked. D. (Dicrorygma) mullinsi POAG 1962 is known only from black shales in the lower section of the Kiamichi Formation Species closely related to Dicrorygma mullinsi PoAG have been found in beds of Hauterivian from England and from Denmark, D. Of these species the first mentioned ostracods originate from brackish water and the last ones have a marine connexion. As the species occurs in a number of samples throughout the lower part of the Jydegaard Formation, together with the above-mentioned fauna of brackish water ostracods, it is reasonable to conclude that marine ingressions in the brackish Upper Purbeckian beds was a frequent phenomenon during the time of sedimentation. A number of ostracods from the Upper Jurassic and the Lower Cretaceous generally described as belonging to the limnic, maybe Cenozoic, genus Limnocythere 1) For subspecies, notice the footnote to the table p. 13
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