Classificació paleoecològica de les formes dels rudistes - una eina per a l'anàlisi paleoambiental

Abstract

As sessile epibenthic suspension feeders, the rudists were only capable of adjusting to their situation in a habitat by shell growth. They therefore show a range of shell forms, the basic features of which can be closely linked with the nature of the habitat occupied. Skelton & & Gili (in press) recognize three broad ecological morphotypes, on the basis of comparison with paradigms orginally defined by Skelton (1979a).
Elevators: the entire growth margin of the attached valve was invoved in upward growth. The elevation index, E, comprising the mean of the highest (α) and lowest (β) declination of the outer wall of the attached valve's growth margin therefore tended in life towards 90°. E could vary, but a lower limit for normal growth is defined at E = 45°. Stabilization was achieved by passive implantation of the attached valve due to the accumulation of surrounding sediment, sometimes assisted by lateral attachment to neighbours. The morphotype was particulary associated with originally soft, usually muddy substrata, with positive net accumulation and only sporadic episodes of sediment removal during the animal's life. Rudists of this morphotype were therefore largely restricted to low to moderate current energy regimes, with or without spasmodic (usually storm-induced) traction removal of sediment. Clingers (originally termed encrusters in Skelton (1979, a): part of the growth margin of the attached valve directly overgrew the substratum, either cementing to it where it was hard (attached clingers) or growing closely over its surface (frictional clingers). Since (β therefore tended to be about O°. E is defined as having been < 45° in shells in life position. Stabilization was achieved by the direct contact of the base of shell with the substratum. This basal contact surface area (A) teded therefore to be maximized, so approximating to the area of the convex polygon mapped onto the substratal surface from the peripheral outline of the shell (A', or virtual area of support). Thus A/A' tended to apprach 1, and is defined as having been >0.75. The morphotype was associated with a range of substrata where net sedimentation was negligible to positive, though with only occasional influxes of sediment, during the animal's life. Sediment accumulation could be matched by upward stepping o' the basal growth surface. Substratal surfaces range from being relatively stable to hard. Associate current conditions might involve frequent or regular inhibition of settling, and even winnowing, of fine-grade sediment though with only rare traction removal of sand and coarse-grade sediment. Recumbents: stabilization was again the key to this morphotype, with a zone of the (unattached) valve margins lying flush against the substratum, so as to produce a broad basal form to the entire shell. E was thus again <45°. Unlike the clinger, however, the area of contact with the (unstable) substratum, A, was of little importance for stabilization. Rather, the shell form resisted being flipped over by currents, or undermined and buried in a scour hollow, by having a widely dispersed arcuate or stellate basal form, with large effective diameters and, hence, maximized A'. Consequently A/A' was much less than 1, and is defined as having been <0.75. This morphotype was associated with mobil lime sand to shell rubble substrata, where deflation by traction currents was frequent and net positive sedimentation negligible.