Pyritized in situ trilobite eggs from the Ordovician of New York (Lorraine Group): Implications for trilobite reproductive biology
Thomas A. Hegna1, Markus J. Martin2 and Simon A.F. Darroch3
- Author Affiliations
1Department of Geology, Western Illinois University, 113 Tillman Hall, 1 University Circle, Macomb, Illinois 61455, USA
2371 Pawling Street, Watertown, New York 13601, USA
3Department of Earth and Environmental Sciences, Vanderbilt University, Nashville, Tennessee 37235, USA
Source/Fonte: Sciency Thoughts blog
Abstract
Despite a plethora of exceptionally preserved trilobites, trilobite reproduction has remained a mystery. No previously described trilobite has unambiguous eggs or genitalia preserved. This study reports the first occurrence of in situ preserved eggs belonging to Triarthrus eatoni (Hall, 1838) trilobites from the Lorraine Group in upstate New York, USA. Like other exceptionally preserved trilobites from the Lorraine Group, the complete exoskeletons are replaced with pyrite. The eggs are spherical to elliptical in shape, nearly 200 μm in size, and are clustered in the genal area of the cephalon. The fact that the eggs are smaller than the earliest-known trilobite ontogenetic (protaspis) stage suggests that trilobites may have had an unmineralized preliminary stage in their ontogeny, and that the protaspis shield formed only after hatching. The eggs are only visible ventrally with no dorsal brood pouch or recognized sexual dimorphism. The location of the eggs is consistent with where modern female horseshoe crabs release their unfertilized eggs from the ovarian network within their head. Trilobites likely released their gametes (eggs and sperm) through a genital pore of as-yet unknown location (likely near the posterior boundary of the head). If the T. eatoni reproductive biology is representative of other trilobites, they spawned with external fertilization, possibly the ancestral mode of reproduction for early arthropods. Because pyritization preferentially preserves the external rather than internal features of fossils, it is suggested that there is likely a bias in the fossil record toward the preservation of arthropods that brood eggs externally: arthropods that brood their eggs internally are unlikely to preserve any evidence of their mode of reproduction.
Received 18 July 2016. Revision received 7 November 2016. Accepted 8 November 2016.
© Geological Society of America
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