Titanoprosoma edgecombei, Bicknell & Kimmig & Smith & Scherer, 2024

Bicknell, Russell D. C., Kimmig, Julien, Smith, Patrick M. & Scherer, Torsten, 2024, An enigmatic euchelicerate from the Mississippian (Serpukhovian) and insights into invertebrate preservation in the Bear Gulch Limestone, Montana, American Museum Novitates 2024 (4008), pp. 1-16 : 4-9

Titanoprosoma edgecombei , gen. et sp. nov.

Figures 1–3

centic ( Tollerton, 1989; Anderson and Selden, 1997), with fewer examples of ovate ( Eldredge,

1974; Selden et al., 2019; Lustri et al., 2021) or rounded ( Lustri et al., 2021) shapes. Further, prosomal sections commonly comprise ~30% of the exoskeleton. To date, the combination of an ovate prosoma consisting of half the body length is

FIGURE 2. Proposed reconstruction of Titanoprosoma edgecombei . Image credit: Katrina Kenny.

unknown, distinguishing Titanoprosoma edgecombei , gen. et sp. nov., from other euchelicerates. Nonetheless, comparisons with other Paleozoic euchelicerates are required. The new taxon is distinct from Eurypterida (sea scorpions) as T. edgecombei lacks a demonstrable metasoma, excluding the material from Eurypterida ( Snodgrass, 1952; Dunlop and Lamsdell, 2017). Although SMNK-PAL 1271 is superficially comparable to Xiphosurida (true horseshoe crabs), the lack of an opisthosoma fused into a plate (= thoracetron) excludes the new material from xiphosurids ( Dunlop and Lamsdell, 2017; Bicknell and Pates, 2020; Bicknell et al., 2021). Chasmataspidida is another possibility. However, the lack of a bucklerlike morphology along the opisthosoma ( Marshall et al., 2014; Lamsdell, 2020) and nine postabdominal segments ( Dunlop and Lamsdell, 2017) excludes the Bear Gulch material from the chasmataspidids. We can also consider the synziphosurines ( Bicknell and Pates, 2020)—a paraphyletic assemblage containing basal crown-group euchelicerates ( Giribet and Edgecombe, 2019). Morphologically, T. edgecombei is most comparable to synziphosurine taxa ( Bicknell and Pates, 2020). However, synziphosurine species have at least eight expressed tergites, a pretelson, and lack ovate prosomas. This suggests SMNK-PAL 1271 is not a synziphosurine ( Bergström, 1975; Moore et al., 2005; Dunlop and Lamsdell, 2017). Alternatively, if this animal is a synziphosurine, the morphology is distinct from other forms. Finally, the ovate prosomal morphology contrasts the cooccurring Anderella parva that has a much more crescent-shaped prosoma, excluding SMNK-PAL 1271 from the known Bear Gulch synziphosurine (fig. 4). In sum, SMNK-PAL 1271 is distinct from all other Paleozoic euchelicerates. An additional consideration is that the specimen has a broad, horseshoe shape akin to Lunataspis Rudkin et al., 2008 , followed by the thoracetron and then a pretelson region ( Rudkin et al., 2008; Lamsdell et al., 2023). The holotype does not preserve evidence for these morphologies.

ffle euchelicerate fossil record contains forms that have not been placed within a higherorder grouping due to aberrant morphologies. fflis includes genera such as Bembicosoma Laurie, 1899 , Maldybulakia Tesakov and Alekseev, 1992 , and Offacolus Orr et al., 2000 . We have taken a similarly conservative position in our placement of Titanoprosoma edgecombei within Euchelicerata as the holotype shows seven opisthosomal tergites; a morphology not observed in any marine Paleozoic euchelicerate groups ( Dunlop and Lamsdell, 2017). One conservative perspective is that the additional tergites were telescoped under the large prosoma. However, more material of this rare animal is needed to confirm this assumption and additional specimens are likely to shed more light on the taxonomic position of this species.

One final possibility is that Titanoprosoma edgecombei could belong within Aglaspidida. At least Brachyaglaspis singularis Ortega-Hernández et al., 2016 , shows a larger cephalic region and a reduced number of trunk tergites. However, our material shows no evidence of postventral plates, a morphological characteristic observed in all aglaspidids ( Van Roy, 2005; Ortega-Hernández et al., 2013, 2016; Lerosey-Aubril et al., 2017; Siveter et al., 2018). As such, we can confidently exclude T. edgecombei from Aglaspidida.

Figures

FIGURE 1. Titanoprosoma edgecombei, gen. et sp. nov., holotype from the Bear Gulch Limestone, Carboniferous (Serpukhovian), Montana. (A–C) SMNK-PAL 1271, part. A. Specimen imaged under ethanol and LED light. B. Specimen imaged dry under UV light. C. Interpretative drawing of part showing three main exoskeletal divisions. Numbers indicate expressed tergites. (D–F) SMNK-PAL 1271, counterpart. D. Specimen imaged under ethanol and LED light. E. Specimen imaged dry under UV light. F. Specimen imaged under ethanol and UV light. Abbreviations: app, appendage; dub, doublure. Image credit: Mathias Vielsäcker.

FIGURE 3. SEM micrograph and SEM-EDS elemental maps of Titanoprosoma edgecombei holotype from the Bear Gulch Limestone, Carboniferous (Serpukhovian), Montana. A. SEM micrograph of SMNK-PAL 1271, part. B–I. SEM-EDS elemental maps of Ca, Al, P, Si, Fe, O, Mg, and S, respectively. Scale bar in A applies to all panels.