Homology of Holocene ostracode biramous appendages with those of other crustaceans: the protopod, epipod, exopod and endopod

Abstract

Kornicker, L.S. 1998 09 15: Homology of Holocene ostracode biramous appendages with those of other crustaceans: the protopod, epipod, exopod and endopod. Lethaia, Val. 31, pp. 251-265. 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 fossil arthropods with preserved limbs, such as the Cambrian superficially ostracode-like Kunmingella and Hesslandona. The 2nd 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 as well as 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. Few have the probably primitive attribute of trunk segmentation (two genera of halocyprid Myodocopa, one order plus one genus of Podocopa, and the problematic Manawa); unambiguously biramous limbs are limited to a few halocyprids. 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. Somewhat more distal, less setose, and plate-like extensions, present on some podocopid limbs (e.g., mandible), 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 limb. A molar-like tooth is present proximally on the mandibular protopod of many ostracodes; it is the coxal endite and projects medially from the coxa (or proximal protopod). The Ostracoda is probably a monophyletic crustacean group composed of Myodocopa and Podocopa. All have a unique juvenile (not a larva) initiallywith three or more limbs. Except that juveniles lack some setae and limbs, they are morphologially similar to the adult. 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. OOstracoda, Myodocopa, Crustacea, Podocopa, homology, morphology. Anne ...

An exceptionally preserved myodocopid ostracod from the Silurian of Herefordshire, UK

“…Yamaguchi & Endo 2003;Regier et al 2005) and supported by the use of a morphological approach (e.g. Cohen et al 1998; see Horne et al 2005). The fossil record of myodocopes, which begins with one (latest) Ordovician species (Gabbott et al 2003), is generally poor, in part due to their typically weakly calcified carapace.…”

On the origin of the putative furca of the Ostracoda (Crustacea)

“…Obviously, both the great variation in structure and the varying position of the 'furca' in relation to the anus within the different ostracod lineages have defied any attempt of homologisation. Morin & Cohen (1991), Cohen et al (1998) and Cohen (2005) (Oct. 10 2005, informal communication to Ostracon, the discussion list on the Ostracoda on the Internet) and Kempf (1986Kempf ( , 1995 for the Cytherelloidea Here I intend to show that:…”

“…In recent years a number of attempts have been made to homologise cephalic and postcephalic limbs throughout the Ostracoda and also limbs of ostracods with those of other crustaceans (Boxshall, 1997(Boxshall, , 2004Cohen et al, 1998;Horne, 2005;Newman, 2005). The so-called furca, however, was excluded from these analyses.…”

“…Although their classification is still the subject of debate (Martens, 1992;Cohen et al, 1998;Park & Ricketts, 2003;Horne, 2005;Liebau, 2005), modern systematists generally recognize extant ostracods to belong to five lineages classified as orders (Table 1).…”

Ostracoda from Johnston Atoll, Pacific Ocean, and Proposal of a New Tribe, Bruuniellini (Myodocopina: Cylindroleberididae)

“…Myodocopid Ostracoda have not been described previously from the atoll. The appendage designated fifth limb herein is called second maxilla by some authors (Cohen et al 1998). …”