Trilobite Limbs, Antennae and Guts: Raymond (1920)

Trilobites are amongst the most popular of all extinct animals. Most commonly found are disarticulated fragments of their calcareous exoskeletons. Occasionally, when the animals were abruptly killed and buried, complete exoskeletons were preserved. Most exceptionally of all, in extremely rare circumstances, early precipitation of minerals around the freshly buried trilobite carcasses preserved a record of the animals' limbs, antennae and even features of the internal anatomy such as the digestive system.

Charles Doolittle Walcott made the first discovery of trilobite limbs, announced in 1876 and summarized in Walcott (1881). His paper compiled seven years of work studying thin sections of enrolled specimens of Calymene (now Flexicalymene) senaria, Ceraurus pleurexanthemus, Isotelus gigas and Acidaspis sp. The material came from the Ordovician Trenton Limestone of New York State. Photographs of some of these original thin sections are reproduced in Bonino & Kier (2010).

The next major discovery came in 1892 when W.S. Valiant, curator of the Museum of Rutgers College, discovered a thin layer bearing hundreds of complete pyritized trilobites in the Ordovician Utica Shales near Rome, New York. Limbs and antennae were spectacularly preserved.

Percy E. Raymond dedicated his major 1920 work, "The Appendages, Anatomy and Relationships of Trilobites", to Charles Beecher, who had done the first detailed work on the astonishingly complete trilobites discovered by Valiant. I had a chance to see some of these specimens in the mid 1980s when John Almond at the University of Cambridge showed me two trays of Triarthrus individuals that he had been working to expose with air abrasives.

Raymond's 1920 book summarizes the state of knowledge at that time, including details of the anatomy of limbs, antennae and digestive organs. The figure shown below illustrates the two-part (biramous) limbs of trilobites, with a coxa (1) from which extend a walking limb ("endopodite") and an "exopodite" decorated with filamentous gills. The six segments of the endopodite (named in the figure) terminate in three sharp little claws. Spines at the bases of the walking leg segments are not shown in this drawing, but are indicated in the reconstruction of the whole trilobite in his Figure 10.

Here below is a view of such limbs preserved in an actual fossil.

This specimen was prepared, using air abrasive techniques, by Markus Martin, who, like a modern day W.S. Valiant, discovered a new occurrence of pyritized trilobites in New York State in 2005. He tells the story of the search and eventual discovery here.

Following the Triarthrus discoveries of the 1890s specimens of Neolenus (now Olenoides) showing preservations of limbs were found by Walcott in the Middle Cambrian of British Columbia (Walcott, 1911, 1912) and these were also incorporated in Raymond's (1920) paper.

In the decades since, cases of exceptional preservation of the soft parts of trilobites have come to light in the Devonian of Germany (e.g. Bartels et al. 1998), the Lower Cambrian of China (Hou et al. 2009), The Lower Cambrian of Australia, the Middle Cambrian of the USA (Robison & Babcock 2011) and the Lower Ordovician of Morocco (van Roy et al. 2010). Fatka et al. (2013) recently described further examples of indications of guts in Upper Ordovician trilobites from the Czech Republic. A selection of specimens from many of these occurrences can be seen here.