(Chelicerata: Chasmataspidida) from North America (Silurian, Bertie Group, New York State) Is the Oldest Species of Diploaspis

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Geol. Mag. 154 (1), 2017, pp. 175–180. c Cambridge University Press 2016 175 doi:10.1017/S0016756816000662 RAPID COMMUNICATION

The ﬁrst diploaspidid (Chelicerata: Chasmataspidida) from North America (Silurian, Bertie Group, New York State) is the oldest species of Diploaspis

∗ ∗ JAMES C. LAMSDELL ‡† & DEREK E. G. BRIGGS § ∗ Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, CT 06511, USA ‡Current address: Division of Paleontology, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024, USA §Yale Peabody Museum of Natural History, Yale University, New Haven, CT 06511, USA

(Received 10 March 2016; accepted 6 June 2016; ﬁrst published online 21 July 2016)

Abstract 2004), and Diploaspididae, comprising the remaining nine genera (Achanarraspis, Diploaspis, Dvulikiaspis, Forfarella, A single specimen of a new species of the chasmata- Heteroaspis, Loganamaraspis, Nahlyostaspis, Octoberaspis spidid Diploaspis Størmer, 1972 is described from the up- and Skrytyaspis). Diploaspidids are known from the Devo- per Silurian (Pridoli) Phelps Member of the Fiddlers Green nian of Germany (Dunlop, Poschmann & Anderson, 2001; Formation (Bertie Group) in Herkimer County, New York Poschmann, Anderson & Dunlop, 2005), Scotland (Dunlop, State, USA. Diploaspis praecursor sp. nov. is distinguished Anderson & Braddy, 1999; Anderson, Dunlop & Trewin, by the shape of the posterolateral margins of the buckler, 2000) and Russia (Dunlop, 2002; Marshall et al. 2014), and, which are drawn out into angular epimera, and by the lack to date, just one Silurian occurrence has been reported, from of elongate tubercles on the postabdomen. This discovery Lesmahagow, Scotland (Tetlie & Braddy, 2004). Chasmata- increases the taxonomic diversity of the Bertie Group by ex- spididae share several similarities with xiphosurans, while tending the geographic extent of Diploaspididae into North Diploaspididae more closely resemble eurypterids (Tetlie & America. D. praecursor pre-dates previously known species Braddy, 2004): Chasmataspis has a broad, semicircular car- of Diploaspis by more than 10 million years. apace and potentially chelate prosomal appendages in contrast to diploaspidids, which have a narrower carapace with Keywords: arthropod, Diploaspididae, Diploaspis prae- the sixth pair of prosomal appendages modified into broad cursor, Bertie Group, Silurian. ‘paddles’. The sole known exception to this is the Silurian representative, Loganamaraspis, which was described by Tet- 1. Introduction lie & Braddy (2004) as a diploaspidid even though they in- terpreted it as having a sixth pair of prosomal appendages The upper Silurian Bertie Group, which crops out across modified as walking legs. Tetlie & Braddy (2004) considered upper New York State, USA (Fig. 1), has been a source of Loganamaraspis as intermediate between Chasmataspis and remarkably preserved arthropods for nearly 200 years. The the remaining diploaspidids. The only specimen, however, is Lagerstätten within this unit are dominated by a diversity of not well preserved, and the morphologically similar Dvuliki- eurypterids, but the assemblages also include xiphosurans, aspis from the Devonian of Russia preserves clear evidence scorpions, crustaceans and other arthropods as well as naut- of paddles (Marshall et al. 2014). These uncertainties make a iloids and early land plants (Nudds & Selden, 2008). Here we full phylogenetic treatment of the group premature, although describe the first representative of a rare group of Palaeozoic a number of evolutionary trends within it have been hypo- chelicerates discovered in the Bertie Group, a chasmataspidid thesized (Tetlie & Braddy, 2004; Marshall et al. 2014). which we assign to a new species of Diploaspis, a genus pre- Despite a number of new chasmataspidid discoveries since viously unknown from North America. the turn of the century, these chelicerates remain enigmatic Chasmataspidids are presently known from only 11 and poorly understood. Resting traces exhibiting a possible species assigned to ten genera (Marshall et al. 2014). chasmataspidid affinity suggest Cambrian origins (Dunlop, Despite previous doubts as to the validity of the taxon Anderson & Braddy, 2004), but only two definitive pre- Chasmataspidida (Bergström, 1980; Tetlie & Braddy, 2004), Devonian chasmataspidids have been described (Dunlop, it is now considered to be a monophyletic group diagnosed Anderson & Braddy, 2004; Tetlie & Braddy, 2004). Fur- by the possession of an ankylosed four-segmented ‘buck- thermore, while Chasmataspis is known from North Amer- ler’. This group resolves phylogenetically as sister to a ica, diploaspidids were previously only known from Europe clade comprising eurypterids and arachnids (Lamsdell et al. and Russia. Here, we describe a new species of Diploaspis 2015; Selden, Lamsdell & Liu, 2015; Lamsdell, 2016). from the late Silurian (Pridoli) Bertie Group of New York Two families of chasmataspidid are recognized: Chasmata- State. This is the oldest known Diploaspis, which is other- spididae, comprising the single genus Chasmataspis from wise represented by two species from the Devonian of Ger- the Ordovician of Tennessee (Dunlop, Anderson & Braddy, many (Dunlop, Poschmann & Anderson, 2001; Poschmann, Anderson & Dunlop, 2005), and it is the first record of Dip- †Author for correspondence: [email protected] loaspididae from North America. The new species therefore

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tie Group (Silurian, Pridoli), SW of Spinnersville at Millers Mills Quadrangle (Fig. 1a), in Herkimer County of New York State by Samuel J. Ciurca Jr on 27 March 1967. The specimen was originally identiﬁed as a chasmataspidid by Erik N. Kjellesvig-Waering and the counterpart sent to Kenneth E. Caster on 2 November 1967 for description; later correspondence with Leif Størmer resulted in Ciurca identifying the specimen as a new species of Borchgrevinkium (Novojilov, 1959), which occurs in the Devonian of Siberia (this may be the basis for Kjellesvig-Waering’s (1966) record of Borch- grevinkium for the upper Silurian, although he did not list a locality). No subsequent record of the counterpart is known, and it must be considered lost since Caster’s death in 1992. The part was acquired by the Yale Peabody Museum (YPM) in 2005 along with the majority of Ciurca’s collection. The specimen is preserved in a pale cream-grey dolostone and was immersed under ethanol during drawing and photographing in order to enhance the contrast (without this treatment the details are largely obscured). Photographs were taken us- ing a Canon EOS 60D digital camera with a Canon EF-S 60 mm f/2.8 Macro USM lens. Terminology follows Tetlie & Braddy (2004), who largely adopted Tollerton’s (1989) terms for eurypterid morphology, with additions from Mar- shall et al.(2014). Prosomal appendages are labelled with Roman numerals.

3. Stratigraphy The Phelps Member is the upper unit of the Fiddlers Green Formation, part of the upper Silurian (Pridoli, c. 419 Ma) Bertie Group (Ciurca, 1973). The Bertie Group is a 15–18 m thick sequence that outcrops in western and central New Yo r k S t a t e ( Fig. 1a), and in southwestern Ontario, Canada, and is composed primarily of dolomitic limestone. The occurrence of evaporites and large halite pseudomorphs has led to suggestions that the eurypterids may have inhabited a hypersaline setting (Ciurca & Hamell, 1994), but a recent study has shown that the growth of halite structures was likely post-depositional and that the eurypterids may have lived in normal marine salinities (Vrazo, Brett & Ciurca, 2016). The Bertie Group is well known for its numerous eurypterid- bearing horizons, and the Phelps Member at Millers Mills Quadrangle boasts a particularly diverse biota (Fig. 1b): in addition to five species of eurypterids (Eurypterus re- mipes, Acutiramus macrophthalmus, Dolichopterus jewetti, Pterygotus cobbi and Buffalopterus pustulosus), the locality yields Proscorpius osborni, one of the oldest scorpions (Dun- lop, Tetlie & Prendini, 2008), and the early terrestrial plants Cooksonia and Hostinella (Edwards et al. 2004), along with the synziphosurine Pseudoniscus, phyllocarids, ostracodes, gastropods and cephalopods including Hexameroceras.The entire Bertie Group appears to be defined by a shallowing Figure 1. Geographical location and stratigraphic column of the trend, culminating in the Phelps Member (Ciurca & Hamell, Passage Gulf locality (modified from Edwards et al. 2004). (a) 1994) and the presence of desiccation cracks at the top of the Map of New York State with the extent of the Bertie Group succession (Ciurca & Honan, 1965). outcrop marked in grey. (b) Stratigraphic log of the exposure at Millers Mills Quadrangle, Passage Gulf. 4. Systematic Palaeontology CHELICERATA Heymons, 1901 bridges the stratigraphic and geographic gap between the Or- EUCHELICERATA Weygoldt & Paulus, 1979 dovician, North American Chasmataspididae and the Devo- CHASMATASPIDIDA Caster & Brooks, 1956 nian, European Diploaspididae. DIPLOASPIDIDAE Størmer, 1972 Diploaspis Størmer, 1972 2. Material and methods Type species. Diploaspis casteri Størmer, 1972. The holotype and only known specimen was collected from Included species. Diploaspis muelleri Poschmann, Anderson the Phelps Member of the Fiddlers Green Formation, Ber- & Dunlop, 2005, Diploaspis praecursor sp. nov.

Figure 2. (Colour online) Holotype of Diploaspis praecursor sp. nov., YPM IP 300790. (a) Photograph. (b) Interpretive drawing. Dashed lines indicate the outline of structures obscured by overlying features. Abbreviations: C, carapace; MR, marginal rim; S, shoulder; T1–T13, tergites 1–13; III–VI, prosomal appendages III–VI. Scale bars represent 5 mm.

Stratigraphic range and distribution. Silurian (Pridoli) Diploaspis praecursor sp. nov. to Devonian (Emsian) of New York State and Germany. Figures 2, 3

Emended diagnosis. Diploaspidid with a subrectangular Material. YPM IP 300790 (holotype and only known preabdomen approximately the same size as the prosoma; specimen). postabdomen short, accounting for c. 33 % of the total body length, often with rows of elongate tubercles; telson Horizon and locality. Phelps Member of the Fiddlers Green broad, rounded, terminating in a medially positioned spine Formation (Bertie Group; Silurian, Pridoli) of Herkimer (emended after Dunlop, Poschmann & Anderson, 2001). Country, New York State, at a road cut on Spohn Road in Passage Gulf, 1.8 km SW of Spinnersville, Millers Mills Quadrangle; 0.5 km SSE of the intersection of Spohn Road Remarks. Dunlop, Poschmann & Anderson (2001) argued that Diploaspis casteri Størmer, 1972 and Heteroaspis no- and Brewer Road (42° 57 28.1 N, 75° 03 36.0 W). vojilovi Størmer, 1972, which co-occur at Alken an der Mosel Diagnosis. Diploaspis with posterolateral margins of buck- in Germany, are preservational variants of one another and ler drawn out into angular epimera; postabdomen lacking therefore synonyms. Marshall et al.(2014), however, resur- ornamentation of elongate tubercles. rected Heteroaspis based on a Siberian species that they considered synonymous with Størmer’s H. novojilovi. While Etymology. Named praecursor, or ‘forerunner’, as this pre- Marshall et al.(2014) identified the presence of lobate epi- dates the other species in the genus by more than 10 million mera on the posterior of the Heteroaspis buckler as a clear years. morphological difference that should not be influenced by taphonomy, they also reported the presence of a number of Description. The holotype and only known specimen is pre- specimens possibly referable to Diploaspis from the same served as a dorso-ventral compression retaining some degree locality. A full statistical study of the available material is of three-dimensional relief (Fig. 2). The specimen is pre- necessary in order to determine whether or not the two gen- served at a slightly oblique angle, so that the left margin era might be taphonomic variants. This is beyond the scope of the carapace and buckler are partially folded underneath of the present study, however, and we follow Marshall et al. the specimen. Cuticle is preserved as a black carbonaceous (2014) in considering Heteroaspis as a separate genus in our film. The specimen is 22.8 mm long, and preserves the car- discussion below. apace, aspects of the prosomal appendages, and the buckler

Table 1. Average body proportions of chasmataspidid species.

Proportions (% total body length)

Taxon Carapace Buckler Postabdomen

Chasmataspis 21 24 55 Loganamaraspis 27 18 55 Dvulikiaspis 20 16 64 Achanarraspis 32 23 45 Forfarella 30 24 46 Heteroaspis 34 25 41 Octoberaspis 28 22 50 Diploaspis casteri 35 32 33 muelleri 35 38 27 praecursor 34 31 35

5. Discussion Diploaspis praecursor sp.nov.(Fig. 2, reconstructed in Fig. 3) exhibits a number of synapomorphies that align it with the genus. The preabdominal buckler is subrectangular (cf. Dunlop, Poschmann & Anderson, 2001) whereas the buckler is trapezoidal in Forfarella (Dunlop, Anderson & Braddy, 1999) and rectangular in Heteroaspis (Marshall et al. 2014). The buckler is approximately of equal size to the carapace (Poschmann, Anderson & Dunlop, 2005), and the postabdomen accounts for only c. 33 % of the total body length as opposed to approximately half in other chasmata- Figure 3. Diploaspis praecursor sp. nov. Reconstruction. The spidids. D. praecursor differs from other Diploaspis species shape and position of the lateral and median eyes, swimming in lacking rows of elongate tubercles on the postabdominal paddle and telson are inferred from the other Diploaspis species. segments. The general outline is intermediate between that of Scale bar = 5 mm. Diploaspis casteri (Dunlop, Poschmann & Anderson, 2001) and Diploaspis muelleri (Poschmann, Anderson & Dunlop, and postabdomen. The carapace is subrectangular, 8.5 mm 2005). The carapace is less rounded and exhibits a slight flare long by 12 mm wide. A marginal rim on the carapace an- in the posterior half similar to that of D. casteri.Thebuckler terior is 0.5 mm wide; it narrows along the lateral edge and of D. praecursor extends into angular posterolateral epimera, is absent in the posterior third of the carapace. Small genal and the degree of postabdominal narrowing is less extreme spines are present on the postero-lateral corners of the car- than in D. muelleri. The appendages show similarities to D. apace. Lateral and median eyes are not preserved. Portions casteri: robust limbs with fixed lateral spines formed from of prosomal appendages III–VI are preserved along the right extensions of the podomere cuticle, although these spines side of the carapace. The appendages are robust: elongate appear larger than those in D. casteri. cuticular projections extend from the distal margin of the The geographic and temporal occurrence of Diploaspis podomeres in III–V to form fixed lateral spines. Only some praecursor fills a gap in the chasmataspidid fossil re- of the more proximal podomeres of appendage VI are pre- cord, linking the Ordovician–Silurian Chasmataspis and served; they appear more robust than the podomeres of III–V Loganamaraspis in Laurentia with the Devonian Dip- and lack fixed spines, indicating that this appendage may have loaspididae in Baltica (Fig. 4). The new species shows that been modified into a paddle. The buckler is subrectangular, the typical diploaspidid morphology evolved by the Silurian 7.7 mm long, and comprises four fused tergites, the first of prior to its apparent proliferation in the Devonian. The wide which is a microtergite. The microtergite is 0.7 mm long and distribution of diploaspidids in the Devonian of Baltica may 9 mm wide, and flanked laterally by the buckler shoulders reflect a biological invasion from Laurentia after its colli- which are triangular, 3 mm long at their outer margin and sion with Baltica during the Silurian (Torsvik et al. 1996; 3 mm wide across their anterior. Tergites 2–4 exhibit strong Gee et al. 2012) and subsequent radiation, but the chasmata- curvature, resulting in the buckler having prominent angular spidid fossil record is too scattered to provide a basis for any epimera. The left lateral margin is folded under the speci- biogeographical framework. The morphology of most dip- men, resulting in a reduction of the exposed tergite widths. loaspidids is eurypterid-like, with a buckler less than 25 % of The second tergite is 2 mm long and at least 11 mm wide, the total body length and a postabdomen comprising 45–55 % the third tergite is 2.5 mm long with an observed width of (Table 1). The buckler in Diploaspis, in contrast, is almost 10 mm, and the fourth tergite is 2.5 mm with a 9 mm min- equal in size to the carapace, and the postabdomen accounts imum width. The postabdomen, consisting of segments 5–13, for close to 33 % of the total body length. This morphology is is evident in its entirety and measures 6.6 mm in length. The almost certainly the derived condition, as a long postabdomen first three segments are telescoped into the buckler. The first is present in the older more xiphosurid-like Ordovician Chas- postabdominal segment is 5 mm wide; subsequent segments mataspis (Dunlop, Anderson & Braddy, 2004). Phylogen- narrow evenly to the pretelson (the 13th segment) which is etic analyses (summarized in Fig. 4)retrievetheDiploaspis 3.6 mm wide. Segments 5–10 are each 0.6 mm long, with clade nested deep within Chasmataspidida, with the more segments 11–13 increasing to 1 mm. Epimera are absent on eurypterid-like diploaspidids Loganamaraspis, Dvulikiaspis the postabdominal segments. The telson is not preserved. The and Achanarraspis forming part of a grade between the basal postabdomen is devoid of tubercle rows. Chasmataspis and Diploaspis (Lamsdell et al. 2015; Selden,

the specimen as part of over 40 years of work on the Pal- aeozoic strata of New York State. The efforts of Susan Butts (YPM) and Jessica Bazeley-Utrup (YPM) were invaluable in tracking down the history of the specimen. J.C.L. was suppor- ted by postdoctoral funding from Yale University and from a Lerner-Gray Postdoctoral Fellowship from the American Museum of Natural History to research macroevolutionary and macroecological trends among Palaeozoic arthropods.

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