Szabó János szerk.: Fragmenta Mineralogica Et Palaentologica 24-25. 2007. (Budapest, 2007)

Middle Miocene micromorph brachiopods Size-frequency distributions of brachiopods • ••••• • * **** • * • ******** • • • c • • • £, * * * " t s o 4 1 1 1 1 0 12 3 4 length :mm; Figure 5 — Scatter diagram of length-width relationships for specimens of Argyrotheca cuneata (Risso, 1826), Bánd. 0 -I ! 1 , 1 0 12 3 4 length I mm Figure 6 — Scatter diagram of length-width relationships for specimens of Argyrotheca cordata (Risso, 1826), Bánd. Size-frequency distributions of brachiopods are frequently used to study population dynamics. High juvenile mortality is characteristic for most marine invertebrates in Recent assemblages (e.g. NOBLE & LOGAN 1981), but TADDEI RUGGlERO (1996) demonstrated that Neocrania anomala has stable population structure and much more adult specimens in protected cave environment than elsewhere. LOGAN (1975) published width-frequency histograms of reef-dweller Argyrotheca berniudana, which generally showed normal distribution. THAYER (1975) suggested that unimodal size-frequency distributions may indicate patchy larval dispersion. STEWART (1981) correlated differences in population structure with environmental condilions. BlTNER (2002) have studied the size-frequency distributions for 28 assemblages of four species from different Middle Miocene environments. These assemblages showed variation even within a single species, but different species from the same environment gave similar sizefrequency distributions. The assemblages collected from reef cavities produced bell-shaped distributions both in A. cuneata and A. cordata (BlTNER 2002). It suggests that the protected and stable cryptic habitats are characterised by lower juvenile mortality and enable the brachiopods to reach larger size (BlTNER 2002). Size-frequency histograms are very useful but several factors can influence the results (inadequate sampling, taphonomic processes, diagenetic solution, mechanical destruction, prédation) (see detailed discussion in BlTNER 2002). The studied Hungarian Middle Miocene brachiopods contain three species from which Megathiris detruncata is present in small number (3 specimens). However, the other two species are sufficiently numerous at Bánd locality (A. cuneata: 147 specimens; A. cordata: 275 specimens) for size-frequency studies. The specimens are derived from bulk samples, washed on 0.5 mm sieve, therefore the sampling method is adequate. A very significant part of the material contains complete shells (64% at A. cuneata and 69% at A. cordata) and the numbers of isolated dorsal and ventral valves are nearly the same (25 vs. 28 at A. cuneata and 39 vs. 45 at A. cordata). It suggests that the studied assemblage is more or less autochthonous, without significant transportation, selective removal of smaller specimens or mechanical destruction. The associated mollusc fauna yielded well-preserved calcitic and aragonitic shells, so diagenetic dissolution did not influence the composition of the fauna. The prédation was also a limited affecting factor, only 3.4% of A. cuneata and 3.6% of A. cordata specimens were drilled by predatory gastropods (see below). All the above mentioned circumstances justify that the size-frequency distributions of these two Argyrotheca species give reasonable results. All Argyrotheca specimens are rather small and their sizes are concentrated in very narrow interval, mainly between 1.5 and 3.0 mm (Figures 5—6). Generally, A. cuneata has slightly smaller average size than A. cordata. Both Argyrotheca species show bell-shaped size-frequency histograms in the Bánd material (Figures 7-8). The lack of juvenile peak indicates low juvenile mortality. These results correspond well with the above referred literature data. The geological context and the associated fauna suggest that a small patch reef existed at Bánd in the Early Badenian (HEGEDŰS 1970, OOSTERBAAN 1990). The micromorphic Argyrotheca specimens lived in sheltered, hidden places of the reef; they attached to the lower surfaces of colonial corals, mollusc shells or other hard substrates. Patchy larval distribution is common in these microenvironments, and unimodal, bell-shaped size distribution suggests a stable community without high juvenile mortality. ASGAARD & BROMLEY (1991) have found three peaks for Recent A. cordata: at 0.5 mm for newly settled larvae,