New morphometric parameters for assessment of body size in the fossil freshwater crab assemblage from the Acheulian site of Gesher Benot Ya'aqov, Israel

Abstract

The faunal assemblage of Gesher Benot Ya'aqov (GBY) in the northern Jordan Valley, Israel, a waterlogged Early–Middle Pleistocene Acheulian site (0.79 Ma) correlated with Oxygen Isotope Stage (OIS) 20.2–18.2, includes over 5000 fragments (1–40 mm) of fossil freshwater crab of an undetermined species suspected to be Potamon sp., based on the morphology of its pincers. Most of the fragments (92.6%) were identified to crab body part, of which 74.6% are pincers. The fossil crabs of GBY demonstrate four distinct pincers, as is also typical of the present-day species in the region, Potamon potamios. In this study we propose for the first time a series of 22 easily measurable morphometric parameters that permit assessment of the pincer size of fossil freshwater crabs from measurable fragmented pincers (35.5% of fragments). We assume that the pincer length represents the crab body size, as in the recent species in the region. The study was restricted to GBY Area C layer V-6, which yielded 91.6% of the crab fragments found in Area C. It has been found that the measurable parameters are significantly correlated to the length of the related pincer ( r = 0.671 – 0.955 , P < 0.0025 ). The same parameters measured for comparison in 208 specimens of recent Potamon potamios, mainly from the same region, resulted in higher correlations ( r = 0.806 – 0.990 , P < 0.0002 ). Consequently, even if fossil crab remains are very fragmentary, it is still possible to reconstruct the body size of the crab. Regression line equations are given as an implicative tool for future assessment of the pincer size of fragmentary fossil crabs from the region. The GBY pincer length data indicate that the broad size range seen in living populations of Potamon potamios also exists in the GBY fossil crabs. A minimum number of individuals (MNI) resulted in a very high density of 668 specimens (321 specimens per m 3). In cluster analyses for each of the 22 parameters, the mean of the GBY crab population measurements is consistently smaller than that of the recent crabs, in which females display intermediate values and males are the largest. However, when allometry is also taken into consideration, cluster analysis groups together the recent males and the recent females and distinctly separates them from the GBY crabs. Further allometric analyses of fossil crab populations from earlier and later prehistoric sites in the northern Jordan Valley will enable a better understanding of the significance of these differences and possible evolutionary trends in the fossil crab population's taxonomy.