Introduction
The ancient arachnid order Trigonotarbida, recognized from the late Silurian (Dunlop, Reference Dunlop1996) to early Permian (Dunlop and Rößler, Reference Dunlop and Rößler2013) (ca. 419–290 Ma), represents some of the earliest terrestrial fauna (Garwood and Dunlop, Reference Garwood and Dunlop2010). This includes the oldest known non-scorpion arachnid, a trigonotarbid, Palaeotarbus jerami (Dunlop, Reference Dunlop1996), from Ludford Lane, Shropshire, UK (Dunlop, Reference Dunlop1996; Selden, Reference Selden2016). In addition to being present in deposits interpreted as early terrestrial ecosystems, morphological features such as preoral digestion (indicated by plumose setae filters around the mouth region), book lungs (connected to the outside through spiracles), and rounded (plantigrade) leg tips indicate that trigonotarbids were fully terrestrial (Garwood and Dunlop, Reference Garwood and Dunlop2010; Garwood and Edgecombe, Reference Garwood and Edgecombe2011). Fanged chelicerae indicate that trigonotarbids were predators, likely feeding on other arthropods, presumably with either an ambush or cursorial predation style (Garwood and Dunlop, Reference Garwood and Dunlop2010).
As the sister group to the Tetrapulmonata clade (which encompasses the extinct arachnid orders Uraraneida and Haptopoda and the extant arachnid orders Araneae, Amblypygi, Schizomida and Uropygi) within Pantetrapulmonata (Dunlop, Reference Dunlop1997, Reference Dunlop2010; Fayers et al., Reference Fayers, Dunlop and Trewin2005; Shultz, Reference Shultz2007), trigonotarbids share the apomorphies of two pairs of book lungs and clasp-knife chelicerae (Dunlop et al., Reference Dunlop, Kamenz and Talarico2009). Although outwardly spider-like in appearance, trigonotarbids apparently lack venom glands and silk glands and have a distinctive segmented abdomen featuring tergites that are divided into median and lateral plates (Dunlop and Selden, Reference Dunlop and Selden2004; Fayers et al., Reference Fayers, Dunlop and Trewin2005; Selden and Nudds, Reference Selden and Nudds2008). Some trigonotarbid families are heavily ornamented with distinct tubercles (raised processes) arranged in patterns on the dorsal surface of their opisthosoma.
Although trigonotarbids have been found throughout Europe from at least 66 localities (Dunlop et al., Reference Dunlop, Wang, Selden and Krautz2014) and are the most commonly found arachnids in North American Carboniferous deposits (Wright and Selden, Reference Wright and Selden2011), they previously were reported from only 12 North American localities (Dunlop et al., Reference Dunlop, Wang, Selden and Krautz2014). Because trigonotarbids are largely underrepresented from North America, any new fossil and/or locality is a significant discovery (Dunlop et al., Reference Dunlop, Wang, Selden and Krautz2014). Here, the first trigonotarbid (an eophrynid) is described from Indiana, from the Middle Pennsylvanian (lower Desmoinesian, equivalent to the Westphalian D in terms of European chronostratigraphy: Peppers and Brady, Reference Peppers and Brady2007) shale of the Shelburn Formation near Terre Haute. In addition, the first two trigonotarbid specimens reported from Oklahoma are also described, both from the Middle Pennsylvanian (Desmoinesian) Senora Formation: an Aphantomartus from near Morris in Okmulgee County, and an Anthracomartus from Sallisaw in Sequoyah County.
Geological setting
Indiana specimen
The specimen was found in an unnamed bed of shale above Coal IV—part of a cyclothem and likely deposited during infilling of the flooded paleovalley estuaries (King, Reference King1993)—in the Shelburn Formation (McLeansboro Group), Middle Pennsylvanian: lower Desmoinesian (Westphalian D), located in Vigo County, Indiana, ~8 miles south of Terre Haute, Indiana. The Shelburn Formation (ranging from 50–250 feet in thickness) contains the Busseron Sandstone; Pirtle Coal; West Franklin Limestone members; unnamed beds of shale, siltstone, and sandstone; and thin discontinuous beds of coal, clay, and limestone (Burger and Wier, Reference Burger and Wier1970). The base of the Shelburn Formation in Indiana overlies the top of the Danville Coal Member (Coal VII) of the Dugger Formation (Tri-State Committee on Correlation of the Pennsylvanian System in the Illinois Basin, 2001) (Fig. 1).
Figure 1. Generalized stratigraphic column of the Pennsylvanian in Illinois (figure and caption modified from Jacobson, Reference Jacobson2000).
Common species of flora found in associated members of the Shelburn Formation include Pecopteris miltoni Brongniart, Reference Brongniart1828; Pecopteris unita (Brongniart, Reference Brongniart1836); and Neuropteris flexuosa Sternberg, Reference Sternberg1825 (Boneham, Reference Boneham1974), which resembles the Mazon Creek Braidwood Flora (Patrick, Reference Patrick1989). The most common animal is the horseshoe crab Euproops danae Meek and Worthen, Reference Meek and Worthen1865 (Boneham, Reference Boneham1974). As indicated by the Shelburn Formation flora and fauna (which also includes the phalangiotarbid, Triangulotarbus terrehautensis Patrick, Reference Patrick1989), the depositional setting is consistent with that of an emergent swamp/coastal freshwater environment with high turbidity and low salinity levels (Patrick, Reference Patrick1989).
Oklahoma specimens
Specimen FMNH PE 56932 was found in an unspecified shale unit of the Senora Formation (Middle Pennsylvanian, Desmoinesian) in a strip mine northwest of Morris in Okmulgee County, Oklahoma. Specimen FMNH PE 56955 was found in the shale above the Croweburg Coal (Senora Formation), Sallisaw in Sequoyah County, Oklahoma. Occurring within the Middle Pennsylvanian Liverpool cyclothem (Fig. 2) (Coveney et al., Reference Coveney, Leventhal, Glascock and Hatch1987), the depositional environment has been interpreted as an emergent swamp and brackish-water lagoons (Wright, Reference Wright1975). The Croweburg Coal (in which the Oklahoma spore succession shows a swamp flora consisting of ferns, horsetails, gymnosperms, and lycopods) grades into a unit of thin shale, which was deposited in brackish-water lagoons (Wright, Reference Wright1975).
Figure 2. Rock units in the Liverpool cyclothem (modified from Wright, Reference Wright1975).
Preservation
Indiana specimen
The specimen is preserved in a siderite concretion with part and counterpart with split/partial external and internal dorsal and ventral molds.
The part shows a concave ventral internal mold of the opisthosoma with superimposition of large dorsal tubercles onto internal ventral sternite surfaces and a partial external dorsal mold along the terminal margin of the opisthosoma (which shows some granular tuberculation). On the prosoma, there are partial external dorsal mold features (granular tubercles on a lobe towards the anterior margin) and internal ventral impressions of the coxae.
The counterpart shows a convex dorsal internal mold of the opisthosoma with superimposition of the ventral pygidium and sternites, as well as a partially dorsal internal mold of the prosoma with superimposition of the ventral coxae. The appendages are not preserved, apart from the trochanter, femur, and partial patella of the right posteriormost leg.
Oklahoma specimens
Both specimens FMNH PE 56932 and FMNH PE 56955 are preserved primarily as dorsal external molds in siderite concretions, each as part and counterpart. As is the case with other Carboniferous arachnids, both part and counterpart exhibit dorsal opisthosomal surfaces that have become concave, resulting in external molds that misleadingly appear to be a cast or internal mold (Selden and Romano, Reference Selden and Romano1983).
The part of specimen FMNH PE 56932 shows an external mold of most of the prosoma and opisthosoma and the remains of some of the appendages. Sternites 5–8 are visible on the dorsal side along the margins of the opisthosoma. Partially visible on the left side are a portion of leg I; the femur, patella, and tibia of leg II; the trochanter, femur, patella, tibia, and metatarsus of leg III; and the femur and patella of leg IV. On the right side, portions of the patella of leg III and femur of leg IV are visible.
The counterpart of specimen FMNH PE 56932 shows most of the prosoma, the far right side of the opisthosoma (corresponding to the far left side of the part) with sternites 5–7 visible on the dorsal side along the margin of the opisthosoma, and the remnants of some of the appendages. On the left side of the specimen (corresponding to the right side of the part), portions of the pedipalp as well as the trochanter and femur of leg II are visible. On the right side (corresponding to the left side of the part), portions of leg I and leg II; the trochanter, femur, patella, and tibia of leg III; and the femur and patella of leg IV are visible.
On both the part and counterpart of specimen FMNH PE 56955, the posterior area of the prosoma, anterior area of opisthosoma, and some portions of unidentified legs are visible.
Materials and methods
These specimens were examined and photographed with Canon EOS 5D Mark II digital camera attached to a Leica M205C microscope with low-angle light to emphasize the surface relief. The stacked photographs were combined using the Photomerge tool in Adobe Photoshop. Illustrations of the specimens were then created in Adobe Illustrator using the composite photographs and the specimen under microscope for reference. For the Indiana specimen (Fig. 3), using a Multistripe Laser Triangulation (MLT) scanner at the University of Kansas, a three-dimensional digital model was created to better show the relief and morphological features of the specimen (Fig. 4). MLT scanning is a low-cost method that previously has been used in the analyses of ichnofossils (see Platt et al., Reference Platt, Hasiotis and Hirmas2010 for scanning settings and methods). All measurements are in millimeters (mm), to the nearest 0.5 mm. Where total lengths or widths are visible, they are given as =; where the total cannot be assessed due to obscurity or lack of preservation, measurements are given as ≥.
Figure 3. Eophrynid trigonotarbid, FMNH PE 9940, shale above Coal IV, Shelburn Formation (McLeansboro Group), Middle Pennsylvanian, lower Desmoinesian (Westphalian D), Vigo County, 8 miles south of Terre Haute, Indiana. (1) Photograph of part; (2) explanatory drawing of part; (3) photograph of counterpart; (4) explanatory drawing of counterpart. Cx = coxa; Fe = femur; Op = opisthosoma; Pa = patella; Pr = prosoma; Py = pygidium; S = sternite; T = tergite; Tr = trochanter; Ts = terminal spine. Scale bars = 5 mm.
Figure 4. Eophrynid trigonotarbid, FMNH PE 9940, shale above Coal IV, Shelburn Formation (McLeansboro Group), Middle Pennsylvanian, lower Desmoinesian (Westphalian D), Vigo County, 8 miles south of Terre Haute, Indiana, MLT scans. (1) Part; (2) counterpart. Fe = femur; Op = opisthosoma; Pa = patella; Pr = prosoma; Py = pygidium; S = sternite; T = tergite; Tr = trochanter; Ts = terminal spine. Blue pixels in the MLT images are areas lacking data (the surface in those areas was obscured from the laser by other portions of the surface relief). Scale bar = 5 mm.
Repository and institutional abbreviation
The specimens are held in the collection of the Field Museum of Natural History with the numbers FMNH PE 9940 (Fig. 3) for the Indiana specimen and FMNH PE 56932 (Fig. 5) for the specimen found near Morris in Okmulgee County, Oklahoma, and FMNH PE 56955 (Fig. 6) for the specimen from Sallisaw in Sequoyah County, Oklahoma.
Figure 5. Aphantomartus sp. indet., FMNH PE 56932, shale unit of Senora Formation (Middle Pennsylvanian, Desmoinesian), strip mine northwest of Morris, Okmulgee County, Oklahoma. (1) Photograph of part; (2) explanatory drawing of part; (3) photograph of counterpart; (4) explanatory drawing of counterpart. I–IV leg numbers; Ca = carapace; Cl = clypeus; Fe = femur; Mt = metatarsus; Op = opisthosoma; Pa = patella; Pd pedipalp; Pr = prosoma; S = sternite; T = tergite; Ti = tibia; Tr = trochanter. Scale bars = 5 mm.
Figure 6. Anthracomartus sp. indet., FMNH PE 56955, shale above Croweburg Coal (Senora Formation), Sallisaw, Sequoyah County, Oklahoma. (1) Photograph of part; (2) explanatory drawing of part; (3) photograph of counterpart; (4) explanatory drawing of counterpart. 1–7 = tergites, Ca = carapace; L = leg (unidentified); Op = opisthosoma; Pr = prosoma. Scale bars = 5 mm.
Systematic paleontology
Class Arachnida Lamarck, Reference Lamarck1801
Order Trigonotarbida Petrunkevitch, Reference Petrunkevitch1949
Eophrynid assemblage Dunlop and Brauckmann, Reference Dunlop and Brauckmann2006
Family Eophrynidae Karsch, Reference Karsch1882
Eophrynidae gen. et sp. indet.
Figures 3, 4
Description
Large trigonotarbid, total preserved length 32.0 mm. Opisthosoma heavily ornamented, posterior margin slightly scalloped. Opisthosoma almost as wide as long (maximum width 18.0 mm, length 19.0 mm). Seven opisthosomal sternites visible (segments 4–10); widths: 4 ≥ 14.0 mm, 5 ≥ 16.0 mm, 6 = 17.0 mm, 7 = 18.0 mm, 8 = 15.0 mm, 9 = 10.0 mm, 10 = 4.0 mm. Degree of sternite curvature increases posteriorly. Two pairs of spines (length 1.0–2.0 mm) on terminal opisthosomal margin, aligned with posterior margins of sternites 8 and 9. Diameter of pygidium: 2.0 mm. Two rows of large tubercles (one located more medially and the other more laterally) superimposed onto sternites 5–9. Large tubercles rounded in shape, diameter 1.5–2.0 mm. Small (0.5–0.75 mm), granular tubercles along terminal margin of opisthosoma and on medial and anterior portions of lobed carapace. Maximum width of carapace 11.0–12.0 mm, length 13.00 mm. Length of partial leg (curved) 16.0 mm total (trochanter = 4.0 mm, femur = 8.0 mm, patella = 4.0 mm).
Materials
FMNH PE 9940, unnamed bed of shale above Coal IV, Shelburn Formation (McLeansboro Group), Middle Pennsylvanian, lower Desmoinesian (Westphalian D), Vigo County, ~8 miles south of Terre Haute, Indiana.
Remarks
The heavy ornamentation of the specimen and two pairs of terminal opisthosomal spines are diagnostic to the families Eophrynidae Karsch, Reference Karsch1882, and Kreischeriidae Haase, Reference Haase1890, in the monophyletic eophrynid assemblage (which consists of the families Eophrynidae, Kreischeriidae, and Aphantomartidae). The families in this assemblage are characterized by having heavy ornamentation (in the form of tubercles) on the dorsal surface and a deep, laterally lobed carapace with two pairs of terminal opisthosomal spines (found in kreischeriids and eophrynids) (Jones et al., Reference Jones, Dunlop, Friedman and Garwood2014). The particularly large tubercles seen in FMNH PE 9940, which also extend to the lateral tergal regions (Fig. 3), are more similar to those seen in eophrynids than the sparser tubercles confined to the axial region of kreischeriids (Garwood et al., Reference Garwood, Dunlop and Sutton2009, fig. 1d; Jones et al., Reference Jones, Dunlop, Friedman and Garwood2014, fig. 1H, I).
Family Aphantomartidae Petrunkevitch, Reference Petrunkevitch1945
Remarks
The heavily ornamented dorsal surface and lobed, subtriangular carapace of FMNH PE 56932 are characteristic of the monophyletic eophrynid assemblage, which consists of the families Eophrynidae Karsch, Reference Karsch1882, Kreischeriidae Haase, Reference Haase1890, and Aphantomartidae Petrunkevitch, Reference Petrunkevitch1945. The rounded clypeus and lack of terminal opisthosomal spines denote this specimen as belonging to the family Aphantomartidae (Rössler et al., Reference Rössler, Dunlop and Schneider2003).
Genus Aphantomartus Pocock, Reference Pocock1911
Type species
Aphantomartus areolatus Pocock, Reference Pocock1911, from the upper Carboniferous (Westphalian D) Mynyddislwyn vein, Maes-y-cwmmer, South Wales, U.K.
Aphantomartus sp. indet.
Figure 5
Description
Relatively small trigonotarbid, total preserved length 8.0 mm. Carapace subtriangular in outline, clypeus rounded; maximum width 5.0 mm, length 4.0 mm. Opisthosoma heavily tuberculated with pattern of larger tubercles along lateral edges of each median tergal area. Opisthosoma elliptical in shape, maximum width 5.0 mm (traversed sternites included 5.5 mm), length 4.0 mm. Tergites 2 and 3 fused into a diplotergite. Tergites 2/3 through 8 visible. Sternites 5–8 partially visible at left and right margins. Tergite widths: 2/3 ≥ 4.5 mm, 4 ≥ 5.0 mm, 5 ≥ 4.25 mm, 6 ≥ 4.0 mm, 7 ≥ 3.5 mm.
Materials
FMNH PE 56932, unspecified shale unit of Senora Formation (Middle Pennsylvanian, Desmoinesian), strip mine northwest of Morris, Okmulgee County, Oklahoma.
Remarks
The opisthosoma of specimen FMNH PE 56932 is more rounded in shape, indicating it be Aphantomartus sp. rather than the other genus in the family, Alkenia Størmer, Reference Størmer1970, which possesses a more elongated opisthosoma (Dunlop and Selden, Reference Dunlop and Selden2004; Poschmann and Dunlop, Reference Poschmann and Dunlop2011).
Family Anthracomartidae Haase, Reference Haase1890
Remarks
Specimen FMNH PE 56955 has opisthosomal tergites, with a granular surface texture, divided into five plates laterally, a subquadrate carapace, and tergites 2 and 3 conjoined into a diplotergite. These characteristics refer the specimen to the family Anthracomartidae Haase, Reference Haase1890 (Garwood and Dunlop, Reference Garwood and Dunlop2011; Wright and Selden, Reference Wright and Selden2011).
Genus Anthracomartus Karsch, Reference Karsch1882
Type species
Anthracomartus voelkelianus Karsch, Reference Karsch1882, from the Pennsylvanian (Langsettian) of Nowa Ruda, Poland.
Anthracomartus sp. indet.
Figure 6
Description
Total preserved length 21.0 mm. Carapace and opisthosomal tergites finely tuberculated. Carapace subquadrate in outline; maximum width 10.0 mm, length 7.0 mm. Opisthosoma divided into five plates laterally, maximum width 14.0 mm, length ≥ 14.0 mm. Locking ridge (segment 1) width ≥ 8.0 mm. Tergites 2 and 3 combined into diplotergite. Tergite widths: 2/3 ≥ 11.0 mm, 4 ≥ 12.0 mm, 5 ≥ 13.0 mm, 6 ≥ 14.0 mm, 7 ≥ 13.5 mm. Opisthosomal posterior margin (segment 7) smoothly rounded.
Materials
FMNH PE 56955, shale above Croweburg Coal (Senora Formation), Sallisaw, Sequoyah County, Oklahoma.
Remarks
Specimen FMNH PE 56955 has a smooth opisthosomal margin (as opposed to the scalloped opisthosomal margin of Brachypyge Woodward, Reference Woodward1878, and Maiocercus Pocock, Reference Pocock1911), which is diagnostic of Anthracomartus sp. (Dunlop and Rößler, Reference Dunlop and Rößler2002).
Discussion
The eophrynid assemblage is a monophyletic group composed of the families Eophrynidae, Kreischeriidae, and Aphantomartidae (Dunlop and Brauckmann, Reference Dunlop and Brauckmann2006). These three families share the synapomorphic traits of heavy ornamentation (numerous large tubercles) on the dorsal surface and deeply lobed carapace margins, often with additional lobation of the median region of the carapace (Hradská and Dunlop, Reference Hradská and Dunlop2013). Two pairs of terminal opisthosomal spines are found in Kreischeriidae and Eophrynidae (Jones et al., Reference Jones, Dunlop, Friedman and Garwood2014). Kreischeriidae and Eophrynidae share other characteristics (generally being large in size compared to other families and having a subtriangular carapace with the clypeus forming an anterior spine [Rößler and Dunlop, Reference Rößler and Dunlop1997]) and at one time were treated as synonyms (Jones et al., Reference Jones, Dunlop, Friedman and Garwood2014). The tuberculation of kreischeriids is a “more uniform, granular pattern” opposed to “discrete large tubercles” found in eophrynids (Dunlop, Reference Dunlop1998, p. 52), but the main feature distinguishing these two families is the presence of a diplotergite (formed from the fusion of tergites 2 and 3), which is found in kreischeriids but not in eophrynids (Rößler and Dunlop, Reference Rößler and Dunlop1997). Specimen FMNH PE 9940 does not show whether there is a diplotergite 2–3; however, the particularly large tubercles, which extend to the lateral regions, suggest it is more likely an eophryind than a kreischeriid, so it is referred to that family.
Specimen FMNH PE 9940 is preserved with a substantial amount of relief to its surface features. Because the stacked photographs tend to flatten and obscure this relief, Multistripe Laser Triangulation (MLT) scanning was used to create three-dimensional digital models of the specimen. The MLT scanning of FMNH PE 9940 marks the first use of this method on a fossil arachnid.
Aphantomartids (specifically Aphantomartus pustulatus [Scudder, Reference Scudder1884]) have been described from three other localities in North America (the 7-11 mine in Ohio, Mazon Creek in Illinois, and Fern Ledges in New Brunswick), but none as far west as Oklahoma (Miller and Forbes, Reference Miller and Forbes2001; Dunlop et al., Reference Dunlop, Wang, Selden and Krautz2014). Specimen FMNH PE 56932 extends the known biogeographic range of the genus, which includes multiple localities in Europe, with Aphantomartus pustulatus known from Pas-de-Calais in northern France (Pruvost, Reference Pruvost1912) and Halleschen Mulde in Germany (Simon, Reference Simon1971; Dunlop et al., Reference Dunlop, Penney and Jekel2020).
Compared with the specimen of Aphantomartus pustulatus from Fern Ledges in New Brunswick (NBMG 4594a/b) (Miller and Forbes, Reference Miller and Forbes2001), the opisthosoma of FMNH PE 56932 appears to have a more rounded morphology (but that impression may simply be the result of its preservation in which the lateral margins of the opisthosoma are lined with traversed sternites) (Rössler, Reference Rössler1998). The rounded clypeus of FMNH PE 56932 is similar to that of the specimen of Aphantomartus described from the Kent Coalfield of England (Dunlop, Reference Dunlop1999) and the specimens of Aphantomartus pustulatus described from the Variscan Foreland Basin in the British Isles (Rössler, Reference Rössler1998).
Anthracomartidae is one of the most prevalent trigonotarbid families (Garwood and Dunlop, Reference Garwood and Dunlop2011) with specimens known from many localities across Europe, including multiple Anthracomartus taxa known from the Czech Republic (Hradská and Dunlop, Reference Hradská and Dunlop2013). In addition to multiple European localities, anthracomartids are known from Lawrence, Kansas; Mazon Creek, Illinois; Fayetteville, Arkansas; and Joggins Fossil Cliffs, Nova Scotia (Wright and Selden, Reference Wright and Selden2011; Dunlop et al., Reference Dunlop, Wang, Selden and Krautz2014). Specimen FMNH PE 56955 represents the westernmost known anthracomartid, thus expanding the recognized biogeographic range of Anthracomartidae.
Anthracomartus triangularis Petrunkevitch, Reference Petrunkevitch1913, and Anthracomartus trilobitus Scudder, Reference Scudder1884, are anthracomartid taxa known only from North America. The granular tuberculation of specimen FMNH PE 56955 bears resemblance to the pustulose nature of Anthracomartus trilobitus and A. hindi Pocock, Reference Pocock1911 (see Garwood and Dunlop, Reference Garwood and Dunlop2011), but the lack of a complete opisthosoma hinders further comparison with other Anthracomartus taxa (Wright and Selden, Reference Wright and Selden2011).
Conclusions
These specimens are the first members of Trigonotarbida reported from Indiana and Oklahoma. Previous research on trigonotarbid occurrences in North America only reported localities at Gilboa, New York; Mazon Creek, Illinois; Red Hill, Pennsylvania; Lawrence, Kansas; Alleghany Tunnel, Virginia; Cotton Hill, West Virginia; Fayetteville, Arkansas; 7-11 mine, Ohio; Pawtucket, Rhode Island; Kinney Brick Quarry, New Mexico; Joggins Fossil Cliffs, Nova Scotia; and Fern Ledges, Saint John, New Brunswick (Dunlop et al., Reference Dunlop, Wang, Selden and Krautz2014). With trigonotarbids being markedly underrepresented from North America, especially when compared to the >66 localities in which they have been found across Europe, any new fossil or locality is greatly significant (Dunlop et al., Reference Dunlop, Wang, Selden and Krautz2014). In addition, with a fairly widespread biogeographic distribution across North America, but relatively few recognized localities (Dunlop and Rößler, Reference Dunlop and Rößler2013), other similarly overlooked trigonotarbid specimens from previously unreported localities are likely to exist in museum collections.
Acknowledgments
We thank P. Mayer (Field Museum) for the loan of this specimen for study. We thank the Paleontological Society for helping to support this study with the Kenneth E. & Annie Caster Award. We also thank the Paleontological Research Institution for helping to support this study with the J. Thomas Dutro Student Award. Finally, we thank R. Garwood and an anonymous referee for their helpful comments on the manuscript.
Open access
