Extraordinary Ordovician trilobite Fantasticolithus gen. nov. from Peru and its bearing on the trinucleimorph hypothesis

Abstract

Fantasticolithus isabelae, a new genus and species of trinucleid trilobite from the Lower Ordovician of Peru, developed anterior ventral cephalic 'props' that are unique in the Trilobita. Its broad and finely pitted genal prolongations are unlike those of any previously known trinucleid, and closely resemble those invariably present in the unrelated Family Harpetidae, supporting the hypothesis that the two families shared a similar mode of life, as 'trinucleimorphs'. Fantasticolithus was probably derived from an early trinucleid with a marginal girder, although it resembles younger genera such as Marrolithus in its enlarged anterolateral pits.The new genus provides a natural experiment to test the hypothesis of trinucleimorphs as filter feeders that used a ventral feeding chamber. The role of the pitted marginal fringe as an egress for feeding currents is discussed. Fantasticolithus had an unusual anterior vent that provided a simpler exit mechanism for feeding currents than the pitted fringe. However, the pits were probably important for trinucleids that lacked an anterior arch.

Hydrodynamic function of genal prolongations in trinucleimorph trilobites revealed by computational fluid dynamics

“…Importantly, the hydrodynamic function of the genal prolongations described above does not preclude the existence of other adaptive functions. For example, the ‘snowshoe’ hypothesis (Fortey and Gutiérrez-Marco, 2022) suggests that, thanks to the genal prolongations, trinucleimorphs with broader genal prolongations were less prone to sinking into the sediment due to their larger surface area. These genal prolongations may have also served as predatory deterrents, like for other trilobite spines (Fortey and Owens, 1999; Pates and Bicknell, 2019).…”

“…Members of the not closely related trilobite families Dionididae, Harpetidae, Ityophoridae, Brachymetopidae, Raphiophoridae, and Trinucleidae (and rare representatives of other groups) (Chatterton et al, 1994; Fortey and Gutiérrez-Marco, 2022; Fortey and Owens, 1990; Hughes, 2007), are counted among these ‘trinucleimorphs’. Generally considered filter feeders (Adrain et al, 2004; Fortey, 2014; Fortey and Owens, 1999) but see (Pearson, 2017), trinucleimorphs are thought to have been slow moving benthic trilobites that lived on soft substrates (Fortey, 2014; Fortey and Gutiérrez-Marco, 2022; Miller, 1972). The order Trinucleida (Adrain, 2011; Bignon et al, 2020) exemplifies this trinucleimorph form (Hughes et al, 1975), and trinucleids are characteristic members of Ordovician trilobite faunas (see Adrain et al, (2004); distributions in Hughes et al, (1975)).…”

“…In comparison, trinucleids appear to have lived in predominantly deeper waters, having often lost the visual structures (Hughes et al, 1975). Trinucleids are frequently found in black shales and fine sandstones (Miller, 1972; Fortey and Gutiérrez-Marco, 2022), though some might have lived slightly shallower (Waisfeld et al, 2011), or are preserved in distal turbidites (Ghobadi Pour, 2022).…”

“…In comparison, the function of the prominent genal prolongations has received less attention. Genal prolongations in the South American trinucleid Fantasticolithus isabelae were suggested to have acted as ‘snowshoes’ that prevented sinking into the substrate (Fortey and Gutiérrez-Marco, 2022), conducive with the ‘mud-shoe’ hypothesis proposed for the broad harpid, though not trinucleid, brim (Miller, 1972). Elongate postero-lateral and axial spines in other trilobites have been suggested to perform an antipredatory role (Fortey, 2014; Lynch et al, in press; Pates and Bicknell, 2019; Rustán et al, 2011), particularly when coupled with enrolment behaviour (Chipman and Drage, 2023; Esteve et al, 2011).…”

Distinct causes underlie double-peaked trilobite morphological disparity

“…A large part of trilobite diversity and disparity of forms is manifested in their cephalic morphometries; the shapes and sizes of the cephala of different groups. The variation in functional morphology of the cephalon is intrinsically linked to the varied life modes of trilobites; different cephalic shapes and structures have been suggested to be adaptations to different feeding modes (e.g., Fatka and Szabad, 2011; Fortey and Gutiérrez-Marco, 2022; Fortey and Owens, 1999; Hegna, 2010; Hughes, 2000; Pearson, 2017), life modes (e.g., Bault et al, 2023b; Cherns et al, 2006; Esteve et al, 2021; Fortey, 2014), or specific behaviours (e.g., Drage, 2019; Henningsmoen, 1975; Suárez and Esteve, 2021). However, the evolution and extent of disparity of the trilobite cephalon remains unclear, with uncertainty around the unstable high-level trilobite taxonomy (Adrain, 2013, 2011; Paterson, 2019), the potential homology of cephalic structures (Du et al, 2023; Hughes, 2003; Park and Kihm, 2017), and the adaptation of cephalic shape to hypothetical life mode.…”