T. Mccormick et Ra. Fortey, INDEPENDENT TESTING OF A PALEOBIOLOGICAL HYPOTHESIS - THE OPTICAL DESIGN OF 2 ORDOVICIAN PELAGIC TRILOBITES REVEALS THEIR RELATIVE PALEOBATHYMETRY, Paleobiology, 24(2), 1998, pp. 235-253
Evidence that can be used to interpret the life habits of extinct orga
nisms usually takes three forms: functional analysis, analogy with liv
ing organisms, and geological evidence. Independent quantitative tests
for habit are rarely available. A theory of optimum eye design origin
ally derived for living aquatic arthropods provides quantitative data
that are used to test previous suppositions about the life habits of t
wo Ordovician pelagic trilobites: that the telephinid trilobite Caroli
nites was epipelagic while the cyclopygid Pricyclopyge was mesopelagic
. Optimum compound eye design theory uses measured lens diameters and
interommatidial angles to determine the ''eye parameter'' (p), which c
an be used to gauge approximate optimum level of illumination for the
eyes of these trilobites. The eye parameter provides an independent te
st for their relative paleobathymetry. Values of the eye parameter mea
sured in the dorso-ventral direction across two eyes of Carolinites ki
llaryensis utahensis were found to have medians of 2.13 and 3.24. Valu
es measured in the antero-posterior direction have medians of 3.17 and
4.86 for the two eyes. Values measured in the dorso-ventral direction
across two eyes of Pricyclopyge binodosa have medians of 4.23 and 4.9
8, while values measured in the antero-posterior direction have median
s of 7.06 and 8.31. Eye parameters are higher in Pricyclopyge than in
Carolinites, the difference statistically significant at p = 0.05. The
eyes of Pricyclopyge are optimally designed for lower levels of illum
ination than are those of Carolinites. This accords with the previous
interpretations of the former trilobite as mesopelagic and the latter
as epipelagic.