Type species

Nerita goldfussi Keferstein, Reference Keferstein1829 (non Zekeli, Reference Zekeli1852) by original designation (Oppenheim, Reference Oppenheim1892, p. 773, pl. 36, fig. 5). Gosau Formation of Austria (Bandel and Kiel, Reference Bandel and Kiel2003).

Diagnosis

Shell smooth with collabral growth lines, becoming ornamented with spiral cords by the third whorl; inner lip with one strong, posterior denticle set off from zero to five subequal denticles; outer lip without denticles.

Occurrence

Nerita (=Neritoptyx) goldfussi Keferstein, Reference Keferstein1829, the type species of Neritoptyx was described from the Austrian, Late Cretaceous Gosau Formation. Species are recognized from France, Austria, Germany, Spain, California, the Campanian Owl Creek Formation of Mississippi, and late Campanian and early Maastrichtian Ripley Formation of Tennessee (de Montfort, Reference de Montfort1810; Keferstein, Reference Keferstein1829; Vidal, Reference Vidal1917; Cossmann, Reference Cossmann1925; Dockery, Reference Dockery1993; Kowalke and Bandel, Reference Kowalke and Bandel1996; Bandel and Kiel, Reference Bandel and Kiel2003). Late Maastrichtian N. hogansoni n. sp. is the youngest member of the genus yet recorded.

Remarks

Six genera of Neritidae are believed to be represented in rocks of Late Cretaceous age globally (Bandel and Kiel, Reference Bandel and Kiel2003). These include Mesoneritina, Schwardtina, Otostoma, Neritoplica, Neritoptyx, and Nerita. This is a reduced number because several neritid genera have been synonymized in recent years. Corsania, an Old World taxon, and its recognized subgenera C. (Corsania) and C. (Januncia) (Woods and Saul, Reference Woods and Saul1986) as well as Nerita (Bajanerita) (Squires Reference Squires1993) have been synonymized with Otostomia and Neritoplica, respectively, as discussed by Bandel and Kiel (Reference Bandel and Kiel2003) and Kiel and Aranda-Manteca (Reference Kiel and Aranda-Manteca2002).

A key to genera of the Neritimorpha accompanying the description of seven new species by Bandel and Kiel (Reference Bandel and Kiel2003) seems less able to distinguish between genera than one might wish in this complicated clade, which has evolved similar morphologies among its numerous species several times since the Jurassic (Quintero-Galvis et al., Reference Quintero-Galvis and Castro2013). Applying the key to assign the genus, four genera are eliminated based on absence of strong axial ornament, smooth inner lip, or flared, Hydrobia-like aperture, leaving assignment of the new taxon between Nerita and Neritoptyx. Although the new species has multiple teeth on the inner (columellar) lip as in Nerita, the posterior tooth is clearly the strongest and is set off from the lesser denticles by a gap indentation (for opercular muscle accommodation) of the lip as in Neritoptyx (Reference Bailey and EricksonFigs. 2, Reference Bandel3). Furthermore, the early shell is smooth; cords only develop late in the ontogeny of the new species as in Neritoptyx, to which the new species is assigned. The emended diagnosis follows the treatment of Bandel and Kiel (Reference Bandel and Kiel2003) for spirally sculptured neritid shells.

Bandel and Kiel (Reference Bandel and Kiel2003) noted that both Wenz (Reference Wenz and Schindewolf1938) and Cossmann (Reference Cossmann1925) considered the type species of Neritoptyx, Nerita goldfussi, to belong to Neritopsis, which would place the present genus in the Neritopsidae. However, Kowalke and Bandel (Reference Kowalke and Bandel1996) pointed out that Neritoptyx goldfussi dissolved the interior walls of the spire, thus placing it in the Neritidae (see also Scheltema, Reference Scheltema1971). It is noted that in computer-based taxonomic references such as the Paleobiology Database, Neritoptyx is listed in the Neritopsidae, contrary to this present morphological understanding of the genus; the same familial assignment is made in the GBIF Database although the status of the genus is given as ‘doubtful’ in this iteration.

Types

Sixteen specimens constitute the studied material (Table 1). All 13 specimens of the type series are housed in the North Dakota State Fossil Collection in the Heritage Center, Bismarck, North Dakota. The collection is curated by North Dakota Geological Survey staff. The holotype is ND 95-11.1 (Fig. 2.5–2.8). Illustrated paratypes include ND 95-13.70 (Fig. 2.1–2.4), ND 99-6.5 (Fig 2.9–2.11), ND 95-13.60 (Fig. 2.12–2.14), ND 95-13.65 (Fig. 2.15), ND 95-13.61 (Fig. 2.16), ND 94-37.1 (Fig. 2.17, 2.18), ND 13.66 (Fig. 2.19), ND 95-13.69 (Fig. 2.20). Unillustrated paratypes are ND 95-13.62, ND 95-13.63, ND 95-13.4, ND 95-13.67. Three additional unfigured specimens are listed in Table 1 but are not part of the type series.

Figure 2.

Neritoptyx hogansoni n. sp. (1) Apertural, (2) abapertural, (3) spiral, and (4) umbilical views of pigment (= color?) patterned, juvenile smooth paratype ND 95-13.70; (3, 4) showing initiation of spiral ornament. (5–8) Views of the holotype, ND 95-11.1, a repaired specimen that shows well-preserved parietal shelf with columellar denticles and strong spiral ornamentation on mature specimen; (5) apertural view showing position of large posterior denticle relative to smaller anterior denticles; (6) apertural view of holotype approximately twice natural size; (7) abapertural view of holotype illustrating spiral ornament and shape of posterior slope (= ramp) and well-preserved spire; (8) posterior view of holotype spire showing protoconch and juvenile (smooth) shell transitioning to spiral cord ornament. (9–11) Apertural, abapertural, and spiral views of mature paratype, ND 99-6.5, illustrating pigment pattern on shell with spiral cord transition completed but without development of a strong shoulder; (9) apertural view illustrating complete parietal callus (deck) and denticles; (10) abapertural view illustrating transition from zig-zag to weakly collabral color bands as cord ornament strengthens; (11) ornament and pattern transition in posterior (spiral) view. (12–14) Paratype, ND 95-13.60; (12, 13) apertural views of specimen with somewhat worn inner and outer lips and ornament of smooth, fine, collabral growth lines before transition; (14) abapertural view, posttransition, illustrating small size at which ornament of cords begins. (15) ND 95-13.65 paratype, apertural view of specimen with poorly developed denticles. (16) ND 95-13.61 apertural view of paratype and largest specimen. (17, 18) ND 94-37.1 paratype, slightly decorticated shell with mature pigment pattern of straighter bands on worn, corded shell. (19) Abapertural view of ND 95-13.66 illustrating transition from smooth to spiral ornament on specimen with strongly patterned shell. (20) Abapertural view of ND 95-13.69, paratype, a mature specimen with corded ornament and lack of pigment banding; a strong shoulder is developed on last whorl of this partly decorticated specimen. Scale bars = 5 mm. Scale bars for 1.1 and 1.3 apply also to 1.2 and 1.4, respectively.

Table 1.

Data for figured and unfigured specimens of Neritoptyx hogansoni n. sp. studied. NDGS # = North Dakota Geological Survey state fossil catalog number; bkn = broken specimen.

All specimens are from NDGS L140 (Fig. 1.2), a north-facing road cut at the center, Sec. 26, T. 134 N., R. 71 W., Logan County, North Dakota, USA, which constitutes the type locality.

Diagnosis

Neritoptyx of 3.5 whorls expanding at average rate for genus; larval shell of 1.5 nearly planispiral, unpatterned whorls; juvenile conch smooth, transitioning to spirally ornamented with approximately 27 rounded cords on an adult shell of 3.25 whorls; ramp flat, gently sloping; shoulder rounded (juvenile?) becoming subangular (mature?); parietal shelf flat, calloused, well developed; columella bearing one strong denticle (posterior-most) separated from variably four, occasionally five, evenly distributed, weak denticles by proportionately larger, excavated space (Fig. 3); outer lip smooth, subovate, nonflaring; pigment (color) pattern, when preserved, zig-zag on younger shells becoming somewhat axial as ornament transitions from smooth to spiral (Fig. 4).

Figure 3.

Characteristics of the apertural region of Neritoptyx hogansoni n. sp. (1) Index view of ND 95-13.61, the largest paratype; (2) aperture of ND 95-13.61 enlarged to illustrate five subtle denticles of inner lip marked by arrows on this gerontic specimen, posterior–most is largest, separated from rest by indentation in columella; (3) index view of holotype, ND 95-11.1, demonstrating repaired fractures; (4) arrows indicate locations of six denticles on inner lip of deck and ridge formed where apertural wall merges with callus of inner lip, which defines the margins of missing operculum; (5) paratype, ND 94-37.1, with index frame in apertural view; (6) apertural details of ND 94-37.1, an eroded specimen, with arrows indicating locations of four denticles on inner lip, a fifth, anterior-most denticle is very subdued and is not arrowed. Scale bars = 5 mm.

Figure 4.

Neritoptyx hogansoni n. sp. View of paratype, ND 95-13.66, enlarged to illustrate conditions of shell morphology and pigment pattern at the transition from smooth to corded ornament with beginning of the transition marked by arrows at collabral growth line. Cords appear first near anterior of shell and are developed toward the posterior as the conch is produced. Scale bar = 5 mm.

Occurrence

Neritoptyx hogansoni n. sp. is known from a single outcrop where it occurred with a mixed fauna of estuarine and marine fossils in the upper Timber Lake Member (Feldmann and Palubniak, Reference Feldmann, Palubniak and Caldwell1975) of the Fox Hills Formation in south-central North Dakota. The unit is late Maastrichtian in age. At the site, which is one of the eastern-most exposures of the formation, the Fox Hills is a medium to fine, friable, marine sandstone that overlies the Pierre Shale and underlies the terrestrial Hell Creek Formation, which is the youngest Cretaceous unit in the Williston Basin. Molluscan taxa occurring with it included the gastropods Neritina (= Neritoplica) loganensis Erickson, Reference Erickson1974, Euspira obliquata (Hall and Meek, Reference Hall and Meek1856), Piestochilus feldmanni Erickson, Reference Erickson1974, Pachymelania wyomingensis, (Meek, Reference Meek1873), P. insculpta, (Meek, Reference Meek1873), Rhombopsis subturritus,(Meek and Hayden, Reference Meek and Hayden1857), Acmaea sp., and Amuletum sp., and bivalves Crassostrea subtrigonalis (Evans and Shumard, Reference Evans and Shumard1857), Anomia gryphorhycha Meek, Reference Meek1873, Tancredia americana (Meek and Hayden, Reference Meek and Hayden1856b), , Panopea occidentalis, Meek and Hayden, Reference Meek and Hayden1856b, Crassatellina hollandi Feldmann and Kammer,Reference Feldmann and Kammer1976, Corbicula spp., Dosiniopsis deweyi (Meek and Hayden, Reference Meek and Hayden1856a) Cymbophora warrenana (Meek and Hayden, 1865b), and Nucula sp.

Description

These are typical neritiform gastropods of small to medium size; the largest studied specimen (Fig. 2.16) is 12.8 mm high and 12.9 mm wide. Undamaged, mature specimens normally slightly wider than high; younger specimens nearly equidimensional (Table 1). Number of cords varies during growth of the shell; young (immature?) specimens are smooth, obvious transition to spiral; corded ornament begins when shell reaches 4 to 4.5 mm width; cords appear earliest at anterior and posterior apertural margins and increase in number to cover the shell as growth continues (Fig. 2.3, 2.10, 2.18, 2.19). This ‘transition’ is recognized as an allometric change in the shell margin (see Figs. 2.4, 2.19, 4.1). As transition progresses, a shoulder develops on posterior slope of body whorl (Fig. 2.7, 2.8, 2.20). Posttransition cord count is 17 to 19, a number that increases as growth continues, reaching a maximum of 27 cords on the largest specimens seen.

Neritoptyx hogansoni n. sp. possessed a pigment (color?) pattern of strong, zig-zag bars on juvenile shells (Fig. 2.1–2.4, 2.10, 2.11, 2.18, 2.19). At the transition from smooth to corded ornament, a change began from zig-zag to somewhat more axial pigmented bars of the type seen on the holotype (Fig. 2.7) and paratype ND 94-37.1 (Fig. 2.18). It is assumed that pigmented lines represent original colored patterns on N. hogansoni. Three unillustrated paratypes (ND 95-13.62, ND 95-13.63, and ND 95-13.67) also show such patterns but have broken apertures or other damage (Table 1).

The spire is elevated and is capped by a protoconch of 1.5 whorls, which is flat. Teleoconch is produced by rapid streptospiral expansion of the aperture having posterior margin sutured to previous whorl exposing a small fraction of that whorl (Fig. 2.2, 2.7, 2.10, 2.14) but resorbing interior portion. A slightly curved posterior shoulder on whorl 2 gradually becomes a flatter ramp by whorl 3, and an angular shoulder (Fig. 2.7, 2.20) is produced after approximately 3.25 whorls as part of this allometric maturational change.

Aperture somewhat D-shaped but modified in posterior corner of the inner lip near where largest of the columellar denticles is present (Figs. 2.9, 2.12, 2.16, 3.1–3.6). Outer lip smooth, whereas the inner lip possesses five or six weak denticles of which the posterior-most is roughly twice as large as the next largest, that one being approximately central on the inner lip (Fig. 3). Inner lip (parietal) callus is flat. Anterior and posterior margins of the aperture create a rim that, in the case of the anterior margin, borders a thickened shelf extension of the inner lip (parietal) callus that housed the operculum when the animal was emerged in life (Fig. 3.1–3.4). This morphology is well defined on the holotype (Fig. 3.3, 3.4).

Etymology

The trivial name honors Dr. John W. Hoganson, State Paleontologist Emeritus of North Dakota, who has greatly advanced the public understanding of both fossils and the geologic history of life throughout the state of North Dakota.

Remarks

The Late Cretaceous was a time of global diversification of the Neritoidea. At least four invasions of brackish and freshwater habitats by neritids took place during the Mesozoic, with the marine Neritoplica giving rise to brackish Neritina during the Maastrichtian (Bandel and Kiel, Reference Bandel and Kiel2003). Neritoptyx remained a fully marine genus throughout the Cretaceous and is not known in the Paleogene. Both marine taxa occur in the Fox Hills at this locality, but a portion of the associated fauna is distinctly estuarine based on the presence of dense populations of Crassostrea subtrigonalis.

Erickson (Reference Erickson1974) described Neritina loganensis, and considered it a brackish-water species, from the same site as the present material. Bandel and Kiel (Reference Bandel and Kiel2003) reassigned Neritina loganensis to Neritoplica, to which it has some resemblance as a smooth, high-spired neritid, but teeth of N. loganensis are better developed and do not wrap around the columella (Bandel and Kiel, Reference Bandel and Kiel2003, p. 61), and its sutures are not impressed as in most Neritoplica species, so perhaps that assignment needs to be further evaluated. Until that time, however, I follow Bandel and Kiel (Reference Bandel and Kiel2003). Neritoplica loganensis (Erickson, Reference Erickson1974), is a smooth form, quite distinct from the present species.

Nerita reticulirata Dockery, Reference Dockery1993, from the Campanian Coffee Sand of Mississippi, was reassigned to Neritoptyx by Bandel and Kiel (Reference Bandel and Kiel2003) based on its single columellar tooth and the presence of two smooth teleoconch whorls before ornament transitions to spiral cords. Neritoptyx reticulirata differs from N. hogansoni by having a more rapidly expanding, flaring last whorl, smooth shoulder, and a single tooth that is more central on the columellar lip.

Nerita? minnesotensis Bergquist, Reference Bergquist1944, was described from a small group of molds collected from the matrix of a conglomerate in the Coleraine Formation of the Iron Ranges region of Minnesota. Geographically, this is the nearest occurrence of a neritid to the Fox Hills location, albeit from an older rock unit, and notwithstanding the Neritoplica loganensis from this same locality as Neritoptyx hogansoni n. sp. Although the Coleraine material is not well preserved, the mold presents sufficient information that it should not be disregarded because the neritids have been used to infer bioprovincialism (Sohl, Reference Sohl1971; Kiel, Reference Kiel and Wagreich2002). Bergquist's (Reference Bergquist1944, pl. 11, figs. 12–14, 20, 21) illustrated material shows six parietal denticles represented by grooves on the molds as described by Bergquist. The largest specimen is 2.0 cm in diameter and 1.2 cm high. These are not conspecific with Neritoptyx hogansoni because the number of parietal teeth is more numerous on N.? minnesotensis and teeth are more evenly sized and distributed; shell sizes are larger and seem to have been much wider than high and more convolute as expressed by the molds. The description is thorough enough to confirm that Bergquist's species is a neritid, and likely a Nerita, but the genus should remain questionable until reexamined. It is noteworthy that a neritid was present in this Santonian (?) formation as will be discussed in the following.

Feldmann and Palubniak (Reference Feldmann, Palubniak and Caldwell1975) illustrated a suite of fossils collected at the present locality. Their specimen identified as Oligoptycha? concinna (p. 229, pl. 1, fig. 15) is not that opisthobranch, but is a partial specimen of Neritoptyx hogansoni.

Paguroid occupation

Walker (Reference Walker1989, Reference Walker1995) described postmortem conditions of epibiont associations and of shell abrasion that demonstrate occupation of spent shells by hermit crabs in littoral conditions. Neither Neritoptyx hogansoni n. sp. nor the other gastropods present at this locality demonstrate significant epibiont activity that might be characteristic of pagurid presence, although scarce examples of both sponge and barnacle boring into Crassostrea subtrigonalis valves were noted by Feldmann and Palubniak (Reference Feldmann, Palubniak and Caldwell1975). However, Walker (Reference Walker1995) also noted that a pattern of abrasion on the adapertural side of hermit-occupied shells sometimes resulted from the occupant dragging the shell over the substrate in intertidal conditions. A very similar, consistently shaped pattern of abrasion appears on several of the specimens of Neritoptyx hogansoni present at this site (Fig. 2.5, 2.9, 2.12, 2.16, 2.17). Other surfaces (Fig. 2) show no similar abrasion, suggesting that it is confined to the adapertural side as Walker (Reference Walker1995, fig. 2) illustrated for drag marks due to abrasion from hermit crabs moving across tidal flats.

Hermit crabs (Paguroidea) may have transported shells of N. hogansoni short distances over the tidal flats and thereby separated them from their site of death. Opercula would have been left behind in this process, thus providing one possible reason for their absence from the sample site as previously noted. It seems unlikely that such transport would remove shells far enough from their life habitat to completely segregate them from opercula, yet no opercula of any type are preserved at the site.

Fossil members of the Paguroidea are rare, but they are known from rocks as old as Jurassic (Fraaije et al., Reference Fraaije, Krzemiński, van Bakel, Krzemiński and Jagt2012). Only four paguroid carapaces have been described from the Cretaceous (Fraaije et al., Reference Fraaije, Krzemiński, van Bakel, Jagt, Klompmaker and Artel2009). An investigation of all known decapod crustaceans of the Fox Hills Formation (Crawford et al., Reference Crawford, Feldmann, Waugh, Kelley and Allen2006) did not report paguroids from the formation. For that reason, it is important to recognize evidence that suggests they were once a part of the fauna.

Predation

Paratype ND 95-13.61, the largest specimen, shows that its aperture had been broken and repaired by new shell growth on two occasions prior to its eventual demise. For completeness of interpretation, it is notable that these breaks likely resulted from predation attempts by one of the decapod species such as the calappid crab Campylostoma siouxensis (Feldmann, Awotua, and Welshenbaugh, Reference Feldmann, Awotua and Welshenbaugh1976), reported as Necrocarcinus by Crawford et al. (Reference Crawford, Feldmann, Waugh, Kelley and Allen2006). Erickson and Jones (Reference Erickson and Jones.1998) demonstrated predation effects, both repair scars and fatal breaks, on numerous small naticid gastropod shells from the Fox Hills Formation, and it is reasonable to suggest that Neritoptyx hogansoni, being of similar size and shape, occasionally met, or evaded, the same fate.

Pigment patterns

The Fox Hills Formation has yielded some of the best-preserved marine Late Cretaceous material yet discovered, most of which is extracted from dense limestone and siltstone concretions, ammonites of which are world famous for their beautiful, unaltered, aragonitic nacre (Landman and Waage, Reference Landman and Waage1993). Bivalves and gastropods typically retain original aragonite as well, a preservation shown to have been the result of early sediment diagenesis (Carpenter et al., Reference Carpenter, Erickson, Lohmann and Owen1988). Some bivalves from concretions have been known to retain pigment patterns (Feldmann, Reference Feldmann1968; Carpenter et al., Reference Carpenter, Erickson, Lohmann and Owen1988), which are interpreted to reflect original color patterns on the shells.

Rather than being concretionary, Neritoptyx hogansoni n. sp. was taken from unconsolidated, friable, fine to very fine sandstone. Of the many molluscan specimens at the site, only a few of the N. hogansoni and a single acmaeid have preserved pigmented color patterns. Some specimens, although seemingly well preserved, show little or no pattern (Fig. 2.20). Multiple pattern designs on the same species are a hallmark of this group. Considered a polymorphism, they persist in modern neritids (Eichhorst, Reference Eichhorst2016) and have been present since at least the Late Cretaceous. Specimens that show signs of having spent time in the swash zone are often unpatterned (Squires, Reference Squires1993). Both patterned and unpatterned specimens are included in the figured Neritoptyx hogansoni type material (Fig. 2.1–2.20).

Fossil specimens of neritid gastropods are noted to reveal color patterns more often than many other gastropod groups (Woods and Saul, Reference Woods and Saul1986; Squires, Reference Squires1992, Reference Squires1993; Dockery, Reference Dockery1993; Squires and Saul, Reference Squires and Saul1993) without use of chemical agents or fluorescent photography to retrieve the pattern (Hendricks, Reference Hendricks2018). As Squires (Reference Squires1993, p. 1086) pointed out after a study of 114 specimens of the Late Cretaceous neritid Neritoplica californiensis, nearly half showed a color pattern. On those that were patterned, there was a color-pattern polymorphism; a faint, divaricate pattern was nearly twice as common as a zig-zag pattern. Squires (Reference Squires1993) noted that this polymorphism is frequent among neritids today as well as in the late Campanian and Maastrichtian. His specimens came from buildups of the rudist Coralliochama sp. during the Maastrichtian of southern California and Baja California, Mexico. He suggested a shallow photic zone to rocky intertidal zone habitat range for Neritoplica californiensis in tropical waters. This is the same sort of polymorphism that is demonstrated by N. hogansoni, suggesting the species occupied a similar shallow subtidal to intertidal habitat where they were similarly attached to shell debris.

Organisms evolve colorings and color patterns that serve to provide advantage of one sort or another. Mate attraction and aposematic or cryptic visibility are generally invoked as behavioral applications for coloration (Thayer, Reference Thayer1918) and pattern in invertebrates. Because most stimuli for snails were likely thermal and chemical, it is unlikely that color was a useful reproductive stimulus for Neritoptyx hogansoni. Its presence in the intertidal zone may well have made variable color patterns helpful as camouflage when avoiding predation by sighted predators (Thayer, Reference Thayer1918) such as marine birds, chimaeroid fish, marine turtles (Hoganson et al., Reference Hoganson, Erickson, Holland, Martin and Hoganson2007), decapod crustaceans, or as yet unidentified predators of the Fox Hills marine littoral zone.

Biotic provinces

Initial application of gastropod distributions to paleobiogeography of the central WIS was presented by Sohl (Reference Sohl1971), who recognized Late Cretaceous snail species whose distributions ranged northward and westward as well as members ranging southward and eastward along the Gulf Coastal Plain. A prominent part of his analysis was from snail taxa included in the Red Bird Section of the Campanian/lower Maastrichtian Pierre Shale of Wyoming (Sohl Reference Sohl1967a, Reference Sohl, Kauffman and Kentb), which were compared with faunas of the Coon Creek Tongue of the Ripley Formation of Tennessee and Mississippi (Sohl Reference Sohl1960). He concluded that the seaway contained temperate as well as subtropical or tropical water masses.

Subsequently, Erickson (Reference Erickson1973) applied Sohl's (Reference Sohl1971) analysis to the Maastrichtian Fox Hills snail fauna in North Dakota to explain its origins. He presented two hypotheses: (1) Although most Fox Hills snails were related to taxa from the Mississippi Embayment and Gulf Coast, at least three species entered the WIS from the Arctic connection, which, he believed, was a temperate water source; and (2) tropical/subtropical taxa in the Fox Hills fauna showed subdued shell ornament compared with their more highly ornamented Coon Creek and Ripley Formation tropical relatives, an indication that marine temperatures were qualitatively cooler (Brenchley and Harper, Reference Brenchley and Harper1998) in the north-central WIS. Petersen et al. (Reference Petersen, Tabor, Lohmann, Poulsen, Meyer, Carpenter, Erickson, Matsunaga, Smith and Sheldon2016) have verified that Maastrichtian marine temperatures in the Fox Hills Sea were 5 °C to 10 °C cooler than Mississippi Embayment/Gulf Coast waters according to clumped data from isotopic studies of marine bivalves that included Coon Creek fossils.

Kauffman (Reference Kauffman and Westermann1984, p. 299) synthesized biogeographic data from mollusks throughout the Cretaceous WIS, including earlier works by Jeletzky (Reference Jeletzky1971) and Sohl (Reference Sohl1971), to produce a model of temperate and subtropical paleobiogeographic ‘units’ during the last incursion of warm, normal-marine waters into the central WIS in the late Campanian. The eastward-younging Fox Hills Sandstone in South Dakota resulted from the withdrawal of the sea from that highstand. As a result of deltaic progradation, the most rapid withdrawal took place from west to east across southern North Dakota and northern South Dakota as the seaway was divided by the Sheridan Delta (Gill and Cobban, Reference Gill and Cobban1973). These workers regarded tropical conditions within the Tethyan Realm to have been demarcated by the presence of the Gulf-marginal, rudist reef line. Subtropical conditions in the WIS lay north of the reef line in waters that supported solitary or clustered rudists. Faunal assemblages, primarily of Foraminiferida, ammonites, bivalves, and gastropods (Kauffman, Reference Kauffman and Westermann1984) defined the Tropical Caribbean Province, Subtropical Gulf and Atlantic Subprovince, Warm Temperate Southern Interior Subprovince, Mild Temperate Central Interior Subprovince, and Cool Temperate Northern Interior Subprovince. Kauffman (Reference Kauffman and Westermann1984) pointed out that boundaries of this system fluctuated with the dynamism of Late Cretaceous sea level cyclicity. During the early Maastrichtian, brief transgression returned warm Gulf Coast waters to the region of the North Dakota–South Dakota border based on the presence of the rudist ‘Ichthyosarcolites’ in the base of the Fox Hills Formation (Speden, Reference Speden1970; Erickson, Reference Erickson1973; Kauffman, Reference Kauffman and Westermann1984). At the time that research was done, these warm currents were termed ‘Tethyan’ in as much as the Caribbean and Gulf of Mexico are considered to have originated as part of the ancestral Tethys Seaway. Tethys was defined biogeographically by the presence of rudist bivalves and by those gastropods, including Neritidae, often found associated with them. The Neritidae remained tropical throughout the Cenozoic (Costa et al., Reference Costa, Nehm and Hickman2001; Eichhorst, Reference Eichhorst2016).

Kiel (Reference Kiel and Wagreich2002) reevaluated the relationship between rudist distributions and tropical marine temperatures. He suggested that water chemical factors, including salinity, had more impact than temperature. This decoupled rudist distributions from those of tropical gastropods, which were more likely to have been thermally determined. Temperate gastropods were noted to have overlapped the Tethyan Realm as defined by rudists. He also postulated that a cool-water, or ‘Boreal,’ marine condition was absent from the Arctic (Kiel, Reference Kiel and Wagreich2002, p. 116) although Arctic Late Cretaceous gastropod faunas are as yet too poorly known to be certain. Further, he believed that a temperate gastropod fauna did not exist in the North Atlantic during the Cretaceous.

These conditions led Kiel (Reference Kiel and Wagreich2002, p. 116, fig. 2) to present a new, tropical ‘Atlanto-Indian Province,’ which included the entire North Atlantic from the American Gulf Coastal Plain to the European Atlantic Coast and ranged eastward through the central Tethys to Oman and India and southward through the South Atlantic to the southwest Indian Ocean. Temperate thermal conditions bordered this region both north and south according to this model. He speculated that the distribution of tropical gastropods was likely controlled by the 20 °C isotherm. Campanian/Maastrichtian distributions of three families, Neritidae, Strombidae, and Xenophoridae, were plotted and used to model the limits of the Atlanto-Indian Province of tropical marine waters (Kiel, Reference Kiel and Wagreich2002, fig. 1). The northward limit of tropical waters in the WIS was defined by the occurrence of Neritidae and Xenophoridae in the Ripley Formation of Mississippi and Tennessee. Precise location of the province boundary in the late Maastrichtian WIS was not a feature of the Kiel (Reference Kiel and Wagreich2002, fig. 2) diagram.

Discovery of Neritoptyx hogansoni n. sp. doubles the number of neritid species in the Fox Hills fauna and moves the tropical limit (Atlanto-Indian Province) several hundred kilometers northward in the WIS during the late Maastrichtian to a paleolatitude that Sohl (Reference Sohl1971) considered subtropical and Kauffman (Reference Kauffman and Westermann1984) considered at the subtropical–temperate boundary. Positions and movements of warm, temperate, and cool marine currents within the WIS are of immediate interest because of their potential influence on the central WIS as a center of endemism for Late Cretaceous mollusks (Jeletzky, Reference Jeletzky1971; Sohl, Reference Sohl1971, Reference Sohl1987; Kauffman, Reference Kauffman and Westermann1984). Therefore, the present work adds precision to biogeographic boundaries as well as marine thermal boundaries in the WIS.

Paleoceanographic implications of the Dakota Isthmus

Early studies of WIS paleoceanographic conditions used isotopic data from pristine carbonates of Pierre Shale and Fox Hills mollusk shells of South Dakota to interpret paleotemperatures (Tourtelot and Rye, Reference Tourtelot and Rye1969). Applications of isotope geochemistry to explain seaway thermal and chemical environments have become numerous, and results are often broadly applied geographically and chronostratigraphically (Tourtelot and Rye, Reference Tourtelot and Rye1969; Wright, Reference Wright1987; Carpenter et al., Reference Carpenter, Erickson, Lohmann and Owen1988; Tsujita and Westermann, Reference Tsujita and Westermann1998; Barrera and Savin, Reference Barrera, Savin, Barrera and Johnson1999; Cochran et al., Reference Cochran, Landman, Turekian, Michard and Schrag2003; Dennis et al., Reference Dennis, Cochran, Landman and Schrag2013; Niezgodzki et al., Reference Niezgodzki, Knorr, Lohmann, Tyszka and Markwick2017). The more refined the chronologic span of data becomes (Barrera and Savin, Reference Barrera, Savin, Barrera and Johnson1999), the more important a clear understanding of both marine and terrestrial paleogeography is for correct interpretation of current flows, plankton drifts, animal migration routes, freshwater sources, and proper understanding of endemism, organismal paleobiology, and evolution in the WIS (Carpenter et al., Reference Carpenter, Erickson and Hoganson2002, Reference Carpenter, Erickson and Holland2003; Petersen et al., Reference Petersen, Tabor, Lohmann, Poulsen, Meyer, Carpenter, Erickson, Matsunaga, Smith and Sheldon2016; Landman et al., Reference Landman, Grier, Cockran, Grier, Petersen and Towbin2018, Slatterly et al., Reference Slatterly, Harries and Sandness2018).

At the late Maastrichtian time of deposition, the Logan County, North Dakota, site was an extensive tidal flat and the WIS was severely restricted by encroachment of the Hell Creek-Sheridan Delta onto a basement sill represented by islands of the Sioux Arch. The Dakota Isthmus was closing (Erickson, Reference Erickson and Hartman1999), supported by the presence of littoral Neritoptyx hogansoni. When that deposystem became a complete barrier to deepwater currents, oceanic current exchange between Arctic and Gulf Coast water masses ceased.

As a result of closure, local climates became more continental. Marine influence on temperatures became much less homogeneous, with potentially as much as a 10 °C difference in submarine water masses between the north and south sides of the isthmus. Terrestrial floras began to demonstrate this influence by presence of northern (Arctic) plant assemblages such as the Nilssonia or Nilssoniocladus species (Peppe et al., Reference Peppe, Erickson and Hickey2007) assemblage (Spicer and Herman, Reference Spicer and Herman2010). This relationship was demonstrated using clumped isotope data from Campanian and Maastrichtian specimens including Fox Hills marine and estuarine fossils (Petersen et al., Reference Petersen, Tabor, Lohmann, Poulsen, Meyer, Carpenter, Erickson, Matsunaga, Smith and Sheldon2016, fig. 3). Whether considered subtropical (Kauffman, Reference Kauffman and Westermann1984) or tropical (Kiel, Reference Kiel and Wagreich2002), Neritoptyx hogansoni was part of the last Maastrichtian fauna to have a warm-water current connection to the North Dakota portion of the WIS.

Whether N. hogansoni represents a species that originated within the Western Interior Endemic Center (Kauffman, Reference Kauffman and Westermann1984), or one with zoogeographic origins elsewhere, remains unknown. The nearest Neritoptyx was N. reticulirata (Dockery, Reference Dockery1993) from the late Campanian Coffee Sandstone of Mississippi, but that species is quite distinct from N. hogansoni. Likewise, Neritoplica dockeryi Bandel and Kiel, Reference Bandel and Kiel2003, from the late Campanian or early Maastrichtian Coon Creek Tongue of the Ripley Formation of Mississippi, was a brackish-water neritid related to Neritoplica loganensis. This connection with southern gastropod faunas is the biogeographic linkage with the tropics that Erickson (Reference Erickson1974) noted as ‘Tethyan’ and presently is considered part of the larger Atlanto-Indian Province defined by Kiel (Reference Kiel and Wagreich2002). The neritids demonstrate that the Dakota Isthmus marked the northern limit of that province in the WIS by the early late Maastrichtian in central North Dakota.