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ABSTRACTS

Cohen, A. C., Martin, J. W. and Kornicker, L. S.: Homology of Holocene ostracode biramous appendages with those of other crustaceans: the protopod, epipod, exopod and endopod. Lethaia, Vol., pp., Oslo. ISSN 0024-1164.

Unambiguously biramous appendages with a proximal precoxa, well defined coxa and basis, setose plate-like epipod originating on the precoxa, and both an endopod and exopod attached to the terminal end of the basis are described from several living Ostracoda of the order Halocyprida (Myodocopa). These limbs are proposed as the best choice for comparison of ostracode limbs with those of other crustaceans and 'Lagerstätten' fossil arthropods with preserved limbs, such as the Cambrian superficially ostracode-like Kunmingella (recently redescribed) and Hesslandona. The 2^nd maxilla of Metapolycope (Cladocopina) and 1st trunk limb of Spelaeoecia, Deeveya and Thaumatoconcha (all Halocypridina) are illustrated, and clear homologies are shown between the parts of these limbs and those of some general crustacean models and some of the remarkable crustacean s.s. Orsten fossils.

No living ostracodes exhibit only primitive morphology; all have at least some (usually many) derived characters. Only a few ostracode taxa have the probably more primitive attributes of dorsal trunk segmentation and/or biramous limbs with a basal epipod. Among the Myodocopa only a few halocyprids (including Metapolycope and apparently some Thaumatoconcha) have dorsal trunk segmentation, and several have unambiguous biramous limbs. Within the Podocopa, only Platycopida and Saipanetta (Podocopida) have trunk segmentation; neither of these have unambiguously biramous limbs. Homologies between podocopid limbs and those of the illustrated primitive myodocopid limbs are tentatively suggested. A setose plate-like extension, often attached basally to a podocopid protopod, is probably homologous to the myodocopid epipod, which was present at least as early as the Triassic (illustrated in Triadocypris, a fossil with some myodocopid body parts). Somewhat more distal and less setose and plate-like extensions, present on some podocopid limbs (e.g. mandible), perhaps may be homologous instead to the exopod (clearly present on myodocopid mandibles). The coxa (or precoxa) is by definition the most basal part of the crustacean and ostracode limb, i.e. the part attached to the body. A molar-like tooth is present proximally on the mandibular protopod of many ostracodes (many Podocopa and primitive Halocyprida); it is the coxal endite and projects medially from the coxa (or proximal protopod when the protopod is undivided).

The Ostracoda is probably a monophyletic crustacean group composed of Myodocopa and Podocopa. All have a unique juvenile (not a larva) initially with three or more limbs, and except that juveniles lack some setae and limbs, with morphology similar to the adult, i.e. with each limb pair different and completely enclosed within a bivalve carapace lacking growth lines. Thus the following suite of characters in all instars may be considered a synapomorphy uniting all Ostracoda: 1. Each pair of limbs is uniquely different from the others. 2. The whole body is completely enclosed within a bivalved carapace that lacks growth lines. 3. No more than nine pairs of limbs are present in any instar. 4. The body shows little or no segmentation, with no more than ten dorsally defined trunk segments. No other crustaceans have this suite of characters. A probable synapomorphy uniting the Podocopa is a 2nd antenna with exopod reduced relative to the endopod. Ostracodes are a large crustacean taxon with probably about 8,000 described living species and perhaps as many as 25,000 extant species including undescribed ones. Thaumatoconcha) have dorsal trunk segmentation, and several have unambiguous biramous limbs. Within the Podocopa, only Platycopida and Saipanetta (Podocopida) have trunk segmentation; neither of these have unambiguously biramous limbs. Homologies between podocopid limbs and those of the illustrated primitive myodocopid limbs are tentatively suggested. A setose plate-like extension, often attached basally to a podocopid protopod, is probably homologous to the myodocopid epipod, which was present at least as early as the Triassic (illustrated in Triadocypris, a fossil with some myodocopid body parts). Somewhat more distal and less setose and plate-like extensions, present on some podocopid limbs (e.g. mandible), perhaps may be homologous instead to the exopod (clearly present on myodocopid mandibles). The coxa (or precoxa) is by definition the most basal part of the crustacean and ostracode limb, i.e. the part attached to the body. A molar-like tooth is present proximally on the mandibular protopod of many ostracodes (many Podocopa and primitive Halocyprida); it is the coxal endite and projects medially from the coxa (or proximal protopod when the protopod is undivided).

The Ostracoda is probably a monophyletic crustacean group composed of Myodocopa and Podocopa. All have a unique juvenile (not a larva) initially with three or more limbs, and except that juveniles lack some setae and limbs, with morphology similar to the adult, i.e. with each limb pair different and completely enclosed within a bivalve carapace lacking growth lines. Thus the following suite of characters in all instars may be considered a synapomorphy uniting all Ostracoda: 1. Each pair of limbs is uniquely different from the others. 2. The whole body is completely enclosed within a bivalved carapace that lacks growth lines. 3. No more than nine pairs of limbs are present in any instar. 4. The body shows little or no segmentation, with no more than ten dorsally defined trunk segments. No other crustaceans have this suite of characters. A probable synapomorphy uniting the Podocopa is a 2nd antenna with exopod reduced relative to the endopod. Ostracodes are a large crustacean taxon with probably about 8,000 described living species and perhaps as many as 25,000 extant species including undescribed ones.

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