Sunday 15 January 2017

Preserved Trilobte eggs from the Ordovician of New York State.

Trilobites are the most abundant Arthropods in Palaeozoic deposits, and have been extensively studied form many years. Much of their biology is now well understood, but their reproductive method remains mysterious, with historic claims of copulating Trilobites not taken seriously and claims that structures found unassociated with adult Trilobites are Trilobite eggs impossible to verify. This is a pity, since Trilobites were among the first Arthropods to appear, and an understanding of their reproductive biology could potentially help us to understand the ancestral state in all Arthropods.

In a paper published in the journal Geology on 9 January 2016, Thomas Hegna of the Department of Geology at Western Illinois University, Markus Martin of Watertown in New York and Simon Darroch of the Department of Earth and Environmental Sciences at Vanderbilt University describe in situ eggs associated with the Trilobite Triarthrus eatoni, from the Ordovician Whetstone Gulf Formation at Martin Quarry in New York State.

The eggs were found associated with two Trilobite specimens examined with the North Star Imaging micro-CT scanner housed at Vanderbilt University. This revealed the presence of a series of ovoid structures 159–177 μm in length located on the underside of cephala of the Trilobites. This is similar to the situation in Horseshoe Crabs, the closest living relatives of Trilobites, in which unfertilised eggs are released from a prosomal ovarian network within the head. Several alternative explanations for the objects were considered, however it was thought that they were too large to be preserved microbes, and are in an unlikely location for fecal pellets, small epibionts or localised pyrite growth.

Pyritized specimens of Triarthrus eatoni from the Ordovician Whetstone Gulf Formation (Lorraine Group), upstate New York (USA). (A) Ventrally preserved specimen (Yale Peabody Museum [YPM] 535703) showing nine eggs in the specimen’s left genal angle. (B) Ventrally preserved specimen (YPM 535704) showing four eggs in the specimen’s right genal angle. (C) Close-up of the egg-bearing region from the specimen in (B). (D) Close up of the egg-bearing region from the specimen in (A). (E) Scanning electron microscopy (SEM) image of the eggs from the specimen in (A) and (D). Note that the perspective is twisted ~180° from that in (D). (F) SEM image of an egg from (E). (G) Close-up SEM image of the egg surface from (F). Note the dominant framboids and rare euhedral crystals of pyrite. (H) Close-up of a limb from (A). Note the dominant framboids and rare euhedral crystals of pyrite. (I) SEM image of a disarticulated cranidium (YPM 238366) that was replaced with pyrite. (J) SEM image showing the replacement fabric from (I). Note the dominant euhedral crystals of pyrite. (K)–(M) Dorsal digital reconstruction of the specimen in (A) derived from microcomputed tomography scan data. (L) Ventral reconstruction. (M) Left-ventral reconstruction. Scale bars in (A) and (B) are 5 mm long; scale bars in (C) and (D) are 2 mm long. Hegna et al. (2016).

The presence of eggs in the cephala of Trilobites suggests a similat method of reproduction to that in Horseshoe Crabs, with males clustering around the females as they secrete eggs in order to fertilise them externally. This is consistent with the known anatomy of Trilobites and other Palaeozoic Arthropods in which (with the exception of Eurypterids) in which external genitalia have never been found. Hegna et al. note that a number of recent finds of Palaeozoic Arthropods have shown external brooding of the young. This has led to suggestions that this may have been the most common form of reproduction in early Arthropods, and even the original breeding strategy in the group. However they observe that almost all Palaeozoic Arthropods are preserved by pyritization, which preserves only the external form of the specimens, and is subsequently unlikely to preserve other reproductive strategies such as internal brooding.

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