BIOMETRICS OF CAMBRO-ORDOVICIAN AGNOSTOIDS: RELEVANCE TO OXYGEN DURING EXTINCTION AND BIODIVERSIFICATION EVENTS

Two aspects of agnostoid arthropod biometrics assume new significance given recent attention to fluctuating oxygen conditions in the Cambro-Ordovician oceans, and the relation of those conditions to serial mass extinction and biodiversification events.

In field collections from the U.S. Great Basin, a single species or species group of the genus Pseudagnostus persists across the Steptoean-Sunwaptan stage boundary (=Pterocephaliid-Ptychaspid), one of a series of recurring Late Cambrian extinction boundaries. New three-dimensional biometrics show that growth trends of pygidial volumes of the boundary-crossing species significantly exceed the growth trends of non-persisting species. Given published descriptions of Late Cambrian agnostoid limbs, and interpretation of club-shaped appendage structures, greater volumes of an enrolled/semi-enrolled test allowed significantly increased respiratory surface area, and thus greater selective advantage in times of rapidly shifting oxic/anoxic marine conditions. Maximizing oxygen intake may also have accelerated feeding behavior, enabling survivors to take advantage of the end-of-SPICE plankton revolution.

A larger biometric data set from agnostoids in the Natural History Museum of London defines a latitudinal size cline in the cosmopolitan Ordovician genus Geragnostus, confirming earlier work based on monographs. The genus exhibits at least 60% increase in linear dimensions over a range from near equatorial to high Southern latitudes in the Early Ordovician. Similar clines are known for modern invertebrates, but have rarely been documented for fossils. Considering modern amphipod analogues, it appears that metabolic slowing from cooler temperatures was more than balanced by partial pressure of oxygen and oxygen solubility at higher latitudes.

In both cases, volume increase mitigated the oxygenating challenges of a nearly enclosed agnostoid carapace. In deuterolobate forms, accessory furrow displacement provided greater volume in areas occupied by the presumed respiratory structures. Thus some morphologic innovations and size changes contributed to survivorship during Late Cambrian oxygen-related extinctions and to adaptability as greater oxygenation supported Early Ordovician biodiversification.