Taphonomy and palaeobiology of echinoderms in the Lower Devonian of the Argentine Precordillera.

HAUDE, R. Y STERREN, A.F.

Córdoba, Argentina

Congreso; IX Congreso Argentino de Paleontología y Bioestratigrafía; 2006

Asociación Paleontológica Argentina

Relatively well preserved echinoderms from the Precordillera de San Juán, Argentina (Talacasto Formation, Lower Devonian), are of general interest because, (a) completeness is prerequisite for reliable phylogenetic and palaeobiologic interpretation; (b) non-disarticulation suggests special taphofacies, which (c) may provide clues for interpreting the original biotope (Ausich 2001, Waisfeld 1989). The Talacasto Formation is a monotonous series of almost 650 m mudstones with intercalated beds of fine sandstone and siltstone corresponding to a muddy shelf depositional system (Astini 1991). The mudstones with a typical ichnofauna (mainly Zoophycos) are considered to represent background sedimentation below normal weather wave base. Some of the sand- and siltstones suggest a tempestitic origin with weak to stronger distalward transport suggested by lenticular cross section and current marks. Rarely, they contain diverse echinoderm faunulas of more or less complete specimens, or segments of skeletons: mostly crinoids (about 15 species: 5 camerates, 7 cladids, 2 disparids, 1 flexibles), and ophiuroids (9 species); rarely carpoids (2), edrioasteroids (1), blastoids (1 species); asteroids (2), and holothurians (2 species). The distinction of taphofacies according to lithologic criteria of proximality in siliclastic shelf environments ranging from high to low hydrodynamics, i.e. from proximal positions above wave base of average storms, to distal positions below wave base of major storms (Seilacher & Aigner 1991, and related to echinoderm Lagerstätten by Brett et al. 1997) may be useful for deduction of biotopes of the Talacasto echinoderms. Several observations on these occurrences are indicative of detachment or uplift by higher energy hydrodynamics, transport, and gradual deposition of specimens: (1) a general lack of autochthonic criteria; (2) three-dimensional complete crowns ‘swimming’ in rather thick siltstones which may show current marks; (3) occurrence of same species in thick layers with small scale channelling, and in thin beds with lateral lenticular thinning or fingering out. On the other hand, observations of the occurrence of some ‘exotic’ forms in thin argillaceous layers, and of local accumulations of fragments together with quite delicate intact forms may suggest also adaptations to low energy conditions. Hence, according to taphonomic observations, several of these occurrences may correspond to criteria of average storm wave base, i.e. their crinoids probably have been transported for considerable distances within suspensions of sediment from more proximal positions. A more current-exposed (proximal?) position may also be reflected by functional morphology of large crinoids with well branched arms, and a strong but flexible stem. Such structures can be indicative of rheophilic adaptations in agitated environments. Acanthocrinus benedettoi is a typical rheophilic form to represent a more proximal ecotype. Other members of this ecotype (only crinoids identified) are Apurocrinus cf. sucrei and Pterinocrinus australis as permanent to facultative fixosessile, higher level parabolic filter-feeders. On the other hand, according to criteria of mayor storm wave base, some echinoderms apparently are parautochthonic in relatively distal and deeper positions on the shelf platform, thus having been submitted only to minor hydrodynamics. A distal position may also be reflected by forms with structures suggesting rather rheophobic autecologic adaptations. The odd Manocrinus maldigitus may represent this ecotype which would contain also small crinoids like Radicalcarocrinus huenickeni, blastoids, and vagil echinoderms like the carpoids, asteroids and holothurians. Some relatively frequent ophiuroids are apparently occupying, either, both environments, e.g. Argentinaster, or living commensalistic on rheophilic crinoids, like a new Eospondylus.