Contribution to Zoology, 75 (3/4) – 2006René H.B. Fraaije; Francisco J. Vega; Barry W.M. van Bakel; Luis M. Garibay-Romero: Late Cretaceous dwarf decapods from Guerrero, southern Mexico and their migration patterns

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Vega and Feldmann (1992) described Costacopluma bishopi on the basis of eight carapaces. Because of the extremely small size (average width: 4.9 mm) of their material, these authors interpreted them as juveniles, yet, at the same time, they also noted overall carapace morphology to be closely similar to adults of the related taxa, Costacopluma mexicanaand C. concava . Comparing the mean carapace width of C. bishopi (ten specimens and four fragmentary carapaces known to date), with that of C. mexicana (numerous specimens known) and C. concava (four known specimens) yields ratios of 1:3.5 and ca. 1:4, respectively.

A comparison of average carapace width in Longusorbis quadratus n. sp. and Xanthosia zoquiapensis n. sp. with their closest relatives reveals a striking similarity; that of the former species (two specimens known) is approximately 3.5 times less than of L. cuniculosus (numerous specimens available) and that of the latter (two specimens at hand) equals around 30% of that of X. buchi , around 30% of that of X. socialis (a single specimen known) and around 25% of that of X. semiornata (six specimens).

The above-mentioned consistency in width ratios in all three species described, i.e. all being about 3.5 times smaller than their closest relatives, and overall adult carapace morphology (Fig. 6) suggests interpretation of this decapod assemblage as the first example of brachyuran dwarfism in the geological record. The environmental selection to favour this type of dwarfism in this tectonically active basin are still unknown, and further study is needed. Factors such as limited food supply, size-correlated predators and/or strong fluviatile input/admixture inducing brackish waters might be worthwhile pursuing in detail to understand the observed size abnormality.


Figure 6. Costacopluma mexicanaVega and Perrilliat, 1989. 1. Juvenile specimen, Maastrichtian, Potrerillos Formation, Nuevo León, northeastern México. 2. Adult specimen, Maastrichtian, Potrerillos Formation, Nuevo León, northeastern México. Scale bars equals 5.0 mm.