Experimental biomechanics of trinucleid fringe pits (Trilobita)

The morphometric uniqueness of the trinucleid family of fossil arthropods, known as the trilobites, has led to a considerable amount of attention in paleontology literature. In particular, the distinctive hourglass-shaped pits that dot their anterior have been the subject of debate for over a century. Though anatomically well understood, their function remains unknown. Many proposals have been suggested, including its use as a sieve for filter-feeding, a strong shield for defense, and a sensory mechanism to compensate for their blindness. Despite the wide range of speculations, no study has attempted to model these hypotheses experimentally. Flume experiments and mechanical strength tests using a tenfold scale, 3D-printed model of a trinucleid head suggest that the dominant theories for over a century, filter-feeding and skeletal strengthening, are not well supported. It is proposed that the results suggest that the pits are an ontogenetic signature that optimize the cephalon’s growth to be maximal, providing trinucleids with an excellent mechanism for plowing through fine-grained silts and clays. ! Introduction and Background ! Anatomical Background ! The asaphid trilobite family Trinucleidae were benthic dwellers in the Ordovician oceans (Campbell 1975). They are best characterized by their wide, perforated head shield located on the cephalon (Fig I). These perforations have traditionally been referred to as “fringe pits,” or “pits,” while the fringe has lent itself to a popular nickname for the family: “lace collar trilobites.” Previous work has suggested that the fringe pits were added to the outermost layer of the cephalon, the cranidium, over several molts in a consistent order. The addition of new pits ceased before segment accretion ceased, meaning adult molts see no change in pit placement or arrangement (Whittington 1959, 1968; Chatterton et al. 1994). The glabella, the large, pear-shaped feature in the center of the cephalon (Fig II), expands towards the inner Page ! of ! 1 36 Fig I: C. tesselatus, a trinucleid, with a pitted cephalic fringe. (Image courtesy of Fossilera, 2015) 5 mm Oberlin College, Depts. of Geology and Biology Spring 2017 margin of the fringe where it makes contact with the innermost row of pits, which in turn extend toward the back and sides (posterolaterally) behind the cephalon. The pits increase in size as they move towards the periphery. In order to describe the location of any particular row of fringe pits, each is referred to by a list number: the “first internal list” describes the concentric arc of rings closest to the glabella, and increases as one moves outward (Hopkins and Pearson, 2016) Previous studies have shown that the number of pits in each list can change within ontogenetic stages and between variations of species (Whittington, 1968), noting that the specific placement of pits is not homologous across specimens. ! In sagittal section, each pit resembles an hourglass (Fig IV) with a flared surface opening of approximately 400 μm narrowing to a funnel of 100 μm midway between the inner and outer surfaces of the fringe. A suture used for molting runs parallel to the surface of the fringe, bisecting the hourglassshaped pits at their narrowest section—a Page ! of ! 2 36 Fig II: An illustration of a generic trilobite showing its three major lateral divisions: the cephalon (1), the thorax (2), and the pygidium (3). The name “trilobite” comes from its three longitudinal lobes: the right pleural lobe (4), the axial lobe (5), and the left pleural lobe (6). (Graphic by Sam Gon III, 2011, Wikimedia Commons) Fig III: An illustration of a trilobite cephalon, showing the ventral sutures. (Graphic by Sam Gon III, 2011,