Included genera

Avitelmessus Dakoticancer Rathbun, Reference Rathbun1917; Tetracarcinus Weller, Reference Weller1905a.

Diagnosis

Carapace quadrate, wider than long to longer than wide; rostrum narrow, bilobed; orbits well developed, usually rimmed; eyes sheltered by orbits when retracted; anterolateral margins entire; posterior margin nearly straight; medial part of cervical groove weakly developed; gastric regions poorly to moderately separated from cardiac and intestinal regions; branchiocardiac groove well developed; pleural sutures located on sides of carapace; fifth pereiopods very reduced; sternum broad, sternites visible to posterior of carapace, sternite 4 sometimes with ridge parallel to anterior end, sternites 5, 6, and 7 with or without granular transverse ridges; sternum of female without longitudinal grooves; lateral portion of posterior part of sternites visible; male pleon with all somites free, lateral terminations on pleonites rectangular, telson rounded triangular; female pleon wide, with long epimeres, all pleonites free; coxae of pereiopods at same level as sternum; first pereiopods isochelous (modified from Feldmann et al., Reference Feldmann, Schweitzer and Karasawa2017).

Remarks

Dakoticancridae has been well studied (Bishop et al., Reference Bishop, Feldmann and Vega1998; Kornecki et al., Reference Kornecki, Feldmann and Schweitzer2017). The family is unusual in being known from two large assemblages of several hundred specimens of two separate species, one of which is composed of Dakoticancer overanus Rathbun, Reference Rathbun1917 at Mobridge, South Dakota (Jones et al., Reference Jones, Schweitzer and Feldmann2022). The other large collection of specimens is composed of those formerly referred to Dakoticancer australis Rathbun, Reference Rathbun1935; Seorsus wadei Bishop, Reference Bishop1988; and Tetracarcinus subquadratus Weller, Reference Weller1905a, all three of which we herein refer to Tetracarcinus subquadratus Weller, Reference Weller1905a, at Blue Springs, Mississippi. Sexual dimorphism and intersex individuals are well documented from the Dakoticancer overanus population from South Dakota (Jones et al., Reference Jones, Schweitzer and Feldmann2022). Avitelmessus grapsoideus is documented from several Gulf Coastal states (Rathbun, Reference Rathbun1935). This study provided an opportunity to examine hundreds of specimens and to compare and contrast morphologies known from forms that had been referred to separate genera and species.

Dakoticancridae are podotrematous crabs, with gonopores on coxae of the pereiopods in both males and females, which is the basal condition in Brachyura and indeed Decapoda as a whole (Schram and Koenemann, Reference Schram and Koenemann2021). The family originated and flourished in the Late Cretaceous but became extinct at the end of the Cretaceous, probably due to its location primarily in the Western Interior Seaway and Gulf Coastal North America proximal to the Chicxulub impact site (Schweitzer and Feldmann, Reference Schweitzer and Feldmann2023). Dakoticancroids are superficially similar to the more derived heterotrematous crabs, which have gonopores on the sternum in females, in having a flattened carapace as wide as or wider than long with very wide sterna, which is unusual among the podotrematous crabs. The dakoticancrid morphology may have been “experimental” in becoming wider than long and acquiring a body form like heterotrematous crabs, which preferentially survived the end-Cretaceous events (Schweitzer and Feldmann, Reference Schweitzer and Feldmann2023). Callichimaera Luque et al., Reference Luque, Feldmann, Vernygora, Schweitzer and Cameron2019 and Componocancer Feldmann et al., Reference Feldmann, Schweitzer and Green2008 similarly document unique brachyuran body forms that appeared and went extinct in the Cretaceous. This pattern deserves more study.

Type and sole species

Dakoticancer overanus Rathbun, Reference Rathbun1917, by monotypy.

Diagnosis (modified from Jones et al., Reference Jones, Schweitzer and Feldmann2022)

Carapace quadrate, wider than long, length/width (L/W) about 75%; rostrum narrow, bilobed; orbits well developed, rimmed, without spines, fronto-orbital width (FOW) about 48% carapace width; eyes sheltered by orbits when retracted; anterolateral margins entire, anteriorly with tiny sharp spines; with weak row of granules extending from anterolateral margin onto hepatic region; broad, granular ridge extending from lateral margin onto epibranchial region, epibranchial region forming transverse ridge nearly perpendicular to axis; narrow, granular rim extending from lateral margin onto branchial region, branchial region with broad inflation parallel to epibranchial region; gastric regions poorly separated from cardiac and intestinal regions; pereiopods 5 reduced; sternum broad, sternites visible to posterior of carapace, sternite 4 with ridge parallel to anterior end, sternites 5, 6, 7 with granular transverse ridges; sternum of female without longitudinal grooves; lateral portion of posterior part of sternites visible; male pleon with all somites free, lateral terminations on pleonites rectangular, telson rounded triangular; female pleon wide, with long epimeres, all pleonites free; coxae of pereiopods at same level as sternum; pereiopods 1 isochelous.

Remarks

Dakoticancer overanus differs in several significant ways from specimens referred to Dakoticancer australis (Table 1). Dakoticancer overanus generally is about 75% as long as wide, whereas individuals referred to D. australis are about as wide as long, with L/W ratios ranging from about 0.98 to 1.08. This difference is statistically significant (Table 2). Dakoticancer overanus has an intestinal region reduced to a very short area between the cardiac region and the posterior rim, whereas specimens of D. australis have an intestinal region occupying about 22% the maximum length of the carapace. The long intraorbital and outer-orbital spines seen in D. australis individuals are lacking in D. overanus. The epibranchial ridge in D. overanus is transverse and more or less perpendicular to the axis, whereas that of specimens of D. australis is oblique to the axis. Dakoticancer overanus also has a transverse swelling on the branchial region that is weaker in D. australis individuals. Sternal ornamentation is well developed as rows of granules in D. overanus, whereas in D. australis, there are few or no granules on the sternum.

Table 1.

Differences between dakoticancrid species discussed here; Dakoticancer overanus data from Jones et al. (Reference Jones, Schweitzer and Feldmann2022). Taxon names in the table are those used historically, to highlight morphological differences. Tetracarcinus subquadratus, Seorsus wadei, and Dakoticancer australis are herein synonymized as T. subquadratus.

Table 2.

Statistical results for comparisons of carapace length to carapace width (L/W) ratios of specimens assigned to Dakoticancer australis, Seorsus wadei, and Tetracarcinus subquadratus, as well as Dakoticancer overanus. Significant results shaded gray. Measurements in Supplementary Data tables.

These differences in morphology are not typical of allometric growth or difference in life stage. Each of the differences is noted in both males and females of each species, and the morphology of Dakoticancer overanus is remarkably consistent across individuals of various sizes (Jones et al., Reference Jones, Schweitzer and Feldmann2022). Thus, D. overanus is retained as the only member of Dakoticancer. As will be discussed in the following, allometric changes and changes in development of ornamentation are documented for Tetracarcinus, and D. australis will be shown to represent larger individuals of Tetracarcinus.

A specimen of Dakoticancer overanus reported by Rathbun (Reference Rathbun1935) from New Jersey was transferred by Roberts (Reference Roberts and Richards1962) to Tetracarcinus. Rathbun (Reference Rathbun and Wade1926) reported a ventral specimen of D. overanus from the Coon Creek unit in Tennessee; it is probable that this specimen is actually Tetracarcinus subquadratus. Thus, Dakoticancer overanus is known primarily from South Dakota (Rathbun, Reference Rathbun1917; Bishop, Reference Bishop, Gray, Boucot and Berry1981; Jones et al., Reference Jones, Schweitzer and Feldmann2022; Klompmaker et al., Reference Klompmaker, Robins, Portell, De Angeli, de Baets and Huntley2022), with some occurrences in North Dakota (Tucker et al., Reference Tucker, Feldmann, Holland and Brinster1987), restricted to the northern Western Interior Seaway. All occurrences are Campanian–Maastrichtian.

Type and sole species

Tetracarcinus subquadratus Weller, 1905 (Weller, Reference Weller1905a), by monotypy.

Diagnosis

Carapace about as wide as long, L/W = 0.95–0.97, square; upper orbital margin with short inflection or spine medially, outer-orbital spine short or long, suborbital spine long, FOW 50–58% carapace width; lateral margins with beaded rims on hepatic, epibranchial, and branchial regions; epibranchial region oblique, composed of two distinct segments; intestinal region from 11% to 22% length of carapace, may form a flattened area, especially in larger individuals; sternite 4 weakly inflated anteriorly, sternites with sparse granules not arrayed into rows; male pleon with all somites free, lateral terminations on pleonites rectangular, telson rounded–triangular; female pleon wide, with long epimeres, all pleonites free; coxae of pereiopods at same level as sternum; pereiopods 1 isochelous or weakly heterochelous.

Remarks

Tetracarcinus is known from only one species, T. subquadratus. It was originally described from New Jersey and has been subsequently reported from many other Atlantic coastal and Gulf coastal North American locations. The age difference between the New Jersey and Delaware Atlantic coastal specimens of T. subquadratus (Santonian, Campanian) and those of the Gulf coastal Coon Creek localities (Maastrichtian) is notable. Therefore, we considered whether the specimens from each region should be referred to different species of Tetracarcinus. Specimens referred by Bishop (Reference Bishop1988) and Kornecki et al. (Reference Kornecki, Feldmann and Schweitzer2017) to Tetracarcinus subquadratus from the Coon Creek localities, as well as the small specimens herein, seem to share several similarities with the material figured by Weller (Reference Weller1905a, Reference Wellerb, Reference Weller1907), Rathbun (Reference Rathbun1935, pl. 10, fig. 16), and Roberts (Reference Roberts and Richards1962, pl. 87, fig. 6). The specimen illustrated by Rathbun and Roberts cited here is one of Weller's cotypes, NJSM 7788, and illustrated here also (Fig. 1.7–1.9). Note that the cotype of Weller is an eroded carapace, and the ridges and fine detail of the cuticle are visible only under extreme magnification of the image. The specimens of Bishop (Reference Bishop1988) and Kornecki et al. (Reference Kornecki, Feldmann and Schweitzer2017) and those we sorted into T. subquadratus have the square carapace, the wide posterolateral margin, the thin keel bounding the posterior margin of the epibranchial region, and the granular ornamentation seen in the specimens of Weller (Reference Weller1905a, Reference Wellerb, Reference Weller1907).

A collection of specimens from the C and D Canal, Malicks collection, in the USNM was measured, USNM 794463–69. This canal is in Delaware, directly across the Chesapeake Bay from New Jersey, close to the type locations for T. subquadratus, which are Santonian and Campanian (Weller, Reference Weller1905a, Reference Wellerb, Reference Weller1907). Decapod specimens collected from the C and D Canal are Campanian in age (Feldmann et al., Reference Feldmann, Schweitzer, Baltzly, Bennett, Jones, Mathias, Weaver and Yost2013) (Fig. 1.4–1.6). Comparison of the Delaware specimens and the Maastrichtian Coon Creek, Mississippi, specimens referred to the Tetracarcinus morphology studied here showed no differences in carapace morphology, nor were the L/W and FOW/W width ratios significantly different (Tables 3–5). Thus, comparison of the Coon Creek specimens with those collected very near to the type locality reveals that they are not different, supporting referral of the Coon Creek specimens to the same species of Tetracarcinus. Thus, we maintain all of the specimens within T. subquadratus.

Table 3.

Results of t-tests for carapace ratios of Tetracarcinus subquadratus from Blue Springs, Mississippi, locality and C and D Canal, Delaware, locality. Measurements in Supplementary Data tables.

Table 4.

Statistical results for comparisons of fronto-orbital width to carapace width (FOW/W) ratios of specimens assigned to Dakoticancer australis, Seorsus wadei, and Tetracarcinus subquadratus, as well as Dakoticancer overanus. Significant results shaded gray. Measurements in Supplementary Data tables.

Table 5.

Statistical results for comparisons of intestinal region length/carapace length (IL/L) and carapace width of specimens assigned to Dakoticancer australis, Seorsus wadei, and Tetracarcinus subquadratus. Significant results shaded gray. Measurements in Supplementary Data tables.

Some of the New Jersey specimens are Santonian in age, which ranges from 86.3 to 83.6 Ma, and some are Campanian, which ranges from 83.6 to 72.1 Ma (ICC 2022.02; Cohen et al., Reference Cohen, Finney, Gibbard and Fan2013) (Table 6). The Coon Creek specimens are Maastrichtian, making them between 72.1 and 66 Ma (ICC 2022.02; Cohen et al., Reference Cohen, Finney, Gibbard and Fan2013). At most, this would be a species range of 20 million years. Stringer et al. (Reference Stringer, Schwarzhans, Phillips and Lambert2020) refined the age of the Coon Creek to early Maastrichtian, which may narrow the species range even more. Reported average lifespans for species range from about 1 to 16 million years (Lamkin and Miller, Reference Lamkin and Miller2016), placing Tetracarcinus at the upper limit. For comparison, the life span of a genus in another group of podotrematous brachyurans, Raninoida Ahyong et al., Reference Ahyong, Lai, Sharkey, Colgan and Ng2007, ranges from 4 to 37 million years (Hartzell et al., Reference Hartzell, Schweitzer and Feldmann2022), with 20 million years for a species falling within this range.

Table 6.

Occurrences of Tetracarcinus subquadratus, based on revisions here.

Parapaguristes has previously been shown to be a junior synonym of D. australis, representing the claw morphology of specimens referred to that taxon (Kornecki et al., Reference Kornecki, Feldmann and Schweitzer2017). The synonymy of Dakoticancer australis, Seorsus wadei, and Tetracarcinus subquadratus is documented in the following, with T. subquadratus as the senior synonym. We propose that the form previously assigned to Tetracarcinus subquadratus represents the smaller, possibly juvenile or sexually immature, members of the species (Fig. 1.1). The Dakoticancer australis morphology represents the larger, probably adult and sexually mature forms of the species (Fig. 1.3). Those specimens assigned to Seorsus wadei seem to occupy a position of intermediate size and intraspecific variability, with carapaces that are slightly narrower but not statistically significantly different in size or shape from D. australis (Fig. 1.2; Tables 2, 4, 5).

Holotype

Syntypes are NJSM 7788 and 9532.

Diagnosis

As for genus.

Occurrences

Because of the similarities between and among the various species of Dakoticancer and Tetracarcinus, species occurrences have been reassigned over time (Table 6). Weller's (Reference Weller1905a, Reference Wellerb, Reference Weller1907) original localities were from the Cliffwood Clay (now Magothy Formation, Santonian) (USGS Magothy) and Woodbury Formation of Campanian age (USGS Woodbury). Rathbun (Reference Rathbun1935) later referred a specimen from the Campanian to Maastrichtian Lewis Shale of Wyoming (Gill et al., Reference Gill, Merewether and Cobban1970) to T. subquadratus. Her image (Rathbun, Reference Rathbun1935, pl. 10, fig. 17) looks like a member of Tetracarcinus, but the actual specimen (USNM PAL 73716) is quite fragmental. It may belong to a different taxon.

Previously reported Maastrichtian occurrences for Dakoticancer australis are in Arkansas (Schweitzer et al., Reference Schweitzer, Feldmann, Phillips and Armstrong2019), Mississippi, and Texas (Rathbun, Reference Rathbun1935), USA, and several locations in Mexico (Vega and Feldmann, Reference Vega and Feldmann1991). Seorsus is known only from the type locality in Mississippi. Thus, Tetracarcinus subquadratus as defined here ranges from Santonian to Maastrichtian and is found in Alabama, Arkansas, Delaware, Mississippi, New Jersey, Tennessee, and Texas, USA, as well as northern Mexico (Table 6).

Description (notable differences between smaller, Tetracarcinus-type, and larger, Dakoticancer australis-type, individuals in bold)

Carapace about as wide as long or slightly wider than long (smaller, usually <17 mm, L/W = 1.01, n = 14, Fig. 2.5, 2.6; larger, usually >17 mm, L/W averaging 1.04, n = 34, Figs. 2.1–2.4, 6.2), square, moderately vaulted transversely and longitudinally. Rostrum parallel-sided at base, narrowing distally, axially sulcate, with marginal beaded rim in smaller individuals. FOW about 58% carapace width in smaller individuals (Fig. 2.5), 50–52% in larger individuals (Figs. 2.1, 6.4) (statistically significant; Table 4); orbits directed forward, upper-orbital margin biconcave, inflection at boundary between orbit and augenrest in smaller individuals (Fig. 3.1), with intra-orbital spine in larger individuals (Fig. 3.2, IO) that is attenuated into long spine in large individuals (Fig. 3.2–3.4, IO); outer-orbital spine barely projected in smaller individuals (Fig. 3.1, OO), spine well developed in larger individuals (Fig. 3.2–3.4, OO); lower orbital margin with long narrow spine (Fig. 3, SO). In smaller individuals, orbital margin beaded, a second beaded keel diverging from orbit and bounding a flattened area posterior to orbital margin; in larger individuals, beaded rims absent and orbital margin with broad rim.

Figure 2.

Comparative morphology of variously sized carapaces of Tetracarcinus subquadratus from the Blue Springs locality, Mississippi, Maastrichtian Coon Creek Formation: (1) large specimen MMNS-PI-11874, dorsal view, arrows indicate deep muscle scars outlined in black; (2) oblique left lateral view of specimen MMNS-PI-11874; (3) intermediate-sized specimen MMNS-PI-11891, arrows indicate deep muscle scars outlined in black; (4) oblique left lateral view of specimen MMNS-PI-11891; (5) small specimen MMNS-PI-11911, arrows indicate deep muscle scars outlined in black; (6) oblique left lateral view of small specimen MMNS-PI-11911. R = beaded rim; K = narrow keel bounding posterior margin of epibranchial region.

Figure 3.

Comparative orbital morphology of variously sized carapaces of Tetracarcinus subquadratus from the Blue Springs locality, Mississippi, Maastrichtian Coon Creek Formation: (1) dorsal view of small specimen MMNS-PI-11875; (2) dorsal view of intermediate-sized specimen MMNS-PI-11891; (3) oblique frontal view of large specimen MMNS-PI-11908; (4) dorsal view of large specimen MMNS-PI-11874. IO = intraorbital spine; OO = outer-orbital spine; SO = suborbital spine.

Lateral margins weakly convex, with beaded rim extending from outer-orbital angle to intersection of cervical groove with margin, beaded rim connecting with a row of granules on hepatic region form a U-shaped granular structure (Fig. 2). Beginning just posterior to cervical groove intersection, a weak beaded rim arcs from paralleling the margin onto epibranchial region (Fig. 2). A narrow ridge extends from the lateral margin paralleling and bounding the posterior margin of the epibranchial region. Posterior to it, a beaded ridge extends obliquely onto the branchial region, terminating just short of the cardiac region (Fig. 2). Posterior margin with well-defined, beaded rim. In smaller individuals, a narrow ridge extends from the lateral margin paralleling and bounding the posterior margin of the epibranchial region (Fig. 2.5, K), narrow ridge sometimes weakly expressed in larger individuals (Fig. 2.3, K).

Mesogastric region with elongate anterior process widened posteriorly, posterior margin biconcave. Metagastric region widest anteriorly, narrowing posteriorly, posterior margin bounded by marked groove. Urogastric region developed only as a groove; cardiac region elongate, granular; intestinal area not well defined, intestinal region about 11% carapace length in smaller individuals and 22% carapace length in larger ones (Fig. 6.3). Very deep pits/muscle scars bounding lateral margin of metagastric region, just lateral to anterior edges of cardiac region, and at anterior corners of metagastric region.

Protogastric region long, quadrate; hepatic region long, narrow, with arcuate beaded rim. Epibranchial region flattened, composed of two segments oriented at sharp angle to one another, bounded anteriorly by cervical groove that extends in straight segment from lateral margin obliquely toward axis, then turning sharply and obliquely toward mesogastric region; bounded posteriorly by branchiocardiac groove more or less parallel to cervical groove but not as strongly angled. Remainder of branchial region not differentiated in smaller individuals; branchial region inflated transversely at about midlength, corresponding to arcuate granular ridge in larger individuals (Fig. 2.2).

Chelae with short manus, ornamented with rows of granules on outer surface; fingers thin, upper surface keeled; left and right of similar size or possible weakly heterochelous (Kornecki et al., Reference Kornecki, Feldmann and Schweitzer2017).

Only proximal portions of pereiopods 2–5 preserved; ischium and coxa (basis) fused, best and most often preserved; coxa of pereiopod 1 largest, pereiopod 2 and 3 coxae smaller, coxa 3 bearing female gonopore (Fig. 4.3), visible on some specimens; pereiopod 4 coxa only slightly smaller in length; pereiopod 5 coxa much reduced, often not visible.

Figure 4.

Comparative female ventral morphology of various sizes of Tetracarcinus subquadratus from the Blue Springs locality, Mississippi, Maastrichtian Coon Creek Formation: (1) sternum of small specimen MMNS-PI-11952 showing broad anterior rim bounding anterior edge of sternopleonal depression on sternite 4; (2) sternum of small specimen MMNS-PI-11943 showing broad anterior rim bounding anterior edge of sternopleonal cavity on sternite 4; (3) sternum of larger specimen MMNS-PI-11955 showing ridge bounding anterior edge of sternopleonal cavity on sternite 4 and gonopore (G) on coxa of third pereiopod; (4) sternum of large specimen MMNS-PI-11965 showing ridge bounding anterior edge of sternopleonal depression on sternite 4 and spermatheca at axial tip of sternal suture 7/8; (5) sternum of large specimen MMNS-PI-11951 showing ridge bounding anterior edge of sternopleonal depression on sternite 4 and partial pleon. S1–S3 = sternites 1–3; LN = lateral node.

Male sternites 1 and 2 triangular; sternite 3 very short, with lateral nodes just posterior to and on either side of triangular-shaped sternites 1 and 2 (Fig. 5.3), crescent-shaped swelling lies on sternite 3 between the nodes; sternites 1–3 directed dorsally at high angle to other sternites in smaller specimens and at a lower angle in larger specimens. Sternites 4–7 with sparse granules, posterolateral edge of sternites 4–7 with episternal projections; each sternite depressed axially to accommodate pleon, creating narrow, deep sterno-pleural cavity. Sternite 4 inflated along anterior margin, inflation stronger in larger individuals, rectangular before becoming axially depressed, longest of all sternites, with broad rim bounding anterior margin of sterno-pleonal cavity in smaller individuals, ridge in larger individuals (Fig. 5.2, 5.4); sternite 5 widest of all sternites, rectangular, directed laterally; sternite 6 same shape as sternite 5, but slightly less wide, directed slightly posterolaterally; sternite 7 directed posterolaterally, with sharp granules along sterno-pleonal depression in small individuals; sternite 8 smallest, usually fully covered by pleon; all sternal sutures axially interrupted. Male pleon elongate, narrowing distally with 7 free somites; somites 1–5 rectangular, with sparse granules; somite 2 widest; somite 6 same length as other somites; rounded telson; pleon extending about one-quarter the length of sternite 4.

Figure 5.

Comparative male ventral morphology of various sizes of Tetracarcinus subquadratus from the Blue Springs locality, Mississippi, Maastrichtian Coon Creek Formation: (1) sternum of small specimen MMNS-PI-11949; (2) sternum of small specimen MMNS-PI-11953 showing broad anterior rim bounding anterior edge of sternopleonal cavity on sternite 4; (3) sternum of larger specimen MMNS-PI-11928; (4) sternum of large specimen MMNS-PI-11894 showing sharp ridge bounding anterior edge of sternopleonal cavity on sternite 4. G = granules; S1–S3 = sternites 1–3; LN = lateral node. (2) Scale bar = 2 mm; (3, 4) scale bars = 1 cm.

Female sternites 1 and 2 triangular; sternite 3 with lateral nodes just posterior to and on either side of triangular sternites 1 and 2 (Fig. 4.4, 4.5), crescent-shaped swelling lying between nodes; sternites 1–3 directed dorsally, at moderate angle to other sternites. Sternites 4–8 gently slope axially, creating wide, shallow sterno-pleonal cavity to accommodate pleon; episternal projections at posterolateral edge of each sternite; sternites 4–7 flattened and without ornamentation. Sternite 4 rectangular, with granulated, inflated anterior margin, longest of all sternites, with broad rim bounding anterior margin of sterno-pleonal cavity in smaller individuals, ridge in larger individuals (Fig. 4.1–4.3); sternite 5 widest of all sternites, transversely rectangular, short; sternite 6 of same shape as sternite 5 only slightly less wide; sternite 7 directed posterolaterally; sternite 8 reduced; all sternal sutures incomplete. Spermathecae circular, occurring at termination of sternal suture 7/8 (Fig. 4.4). Female pleon wide, ovate, narrowing distally into broad triangular telson (Fig. 4.5); somites 1–5 convex with round margins; somite 3 laterally widest; somite 6 flat.

Material examined and occurrence

See Materials and methods and Table 6.

Remarks

Bishop (Reference Bishop1988) described three different genera and species of dakoticancrid crab from the Blue Springs locality. He noted that only two of 1,500 specimens were referrable to Seorsus (Bishop, Reference Bishop1988, p. 75), whereas he referred 28 to Tetracarcinus subquadratus and 528 to Dakoticancer australis (Bishop, Reference Bishop1983, table 1). Kornecki et al. (Reference Kornecki, Feldmann and Schweitzer2017) recognized no individuals of Seorsus when examining hundreds of specimens collected by us from the same locality. In our analysis, we isolated all of the rectangular, dakoticancroid specimens we collected from the Blue Springs locality. Then we compared the morphology of the specimens to images in Bishop (Reference Bishop1988) as well as Weller (Reference Weller1905a, Reference Wellerb, Reference Weller1907), Rathbun (Reference Rathbun1935), and Roberts (Reference Roberts and Richards1962) to qualify the characteristics they used to identify Tetracarcinus-type morphology. We determined that these characteristics included small, square carapaces (<17 mm wide) (Fig. 1.1); a short intestinal region compared with the overall length of the carapace; a thin keel bounding the posterior margin of the epibranchial region; beaded rims on the orbits and short intra- and outer-orbital spines; a long suborbital spine; very deep pits along the axial regions; and narrow ridges of granules on the hepatic, epibranchial, and branchial regions (Figs. 1.1, 2.5, 2.6, 3.1). We also noted that these specimens often retained cuticle but rarely retained the sternum.

Seorsus-type morphology was more difficult to define as it appears to be nearly identical to specimens assigned by Bishop (Reference Bishop1988) and Kornecki et al. (Reference Kornecki, Feldmann and Schweitzer2017) to Dakoticancer australis. The two specimens referred to Seorsus by Bishop (Reference Bishop1988) seem to be somewhat narrower than those assigned to D. australis and may have better-preserved cuticle. Bishop (Reference Bishop1988) noted that the main difference between Seorsus and other dakoticancrids was its trapezoidal shape; however, that seems to be illusory due to sediment and broken cuticle obscuring the posterolateral part of the carapace (Bishop, Reference Bishop1988, fig. 1A, E) (Figs. 1.2, 2.3, 2.4, 3.2). Otherwise, differences are not notable. Specimens assigned to Dakoticancer australis by Rathbun (Reference Rathbun1935), Bishop (Reference Bishop1988), and Kornecki et al. (Reference Kornecki, Feldmann and Schweitzer2017) are characterized by a large, square carapace (>17 mm) (Fig. 1.3), a long intestinal region compared with the overall length of the carapace, lack of a thin keel bounding the posterior margin of the epibranchial region, long intra- and outer-orbital spines, a suborbital spine, shallower pits along the axial regions, and broader ridges of granules on the hepatic, epibranchial, and branchial regions (Figs. 1.3, 2.1, 2.2, 3.3, 3.4). We sorted our specimens according to these criteria to the best of our ability.

To determine whether separation of the specimens into the three different genera and species noted in the preceding was supportable, we quantitatively examined 105 specimens and qualitatively examined several hundred. In terms of size only, larger specimens had historically been referred to D. australis and smaller ones to Tetracarcinus (Fig. 1). Indeed, in the average carapace width, the three taxa were significantly different from one another (Table 1). In terms of the ratio of carapace length to carapace width, however, there was no significant difference between and among specimens separated into the three taxa using the preceding criteria (Fig. 6.2; Table 2). Thus, although the carapace size was significantly different, the overall shape of the carapace was not, as the individuals maintained a square shape throughout growth. In addition, specimens plotted by carapace width show no distinct clusters, strongly suggesting that these three taxa represent a growth series, not separate species (Fig. 6.1).

Figure 6.

Carapace size and size ratios. Data in Supplementary Data tables 1–6. (1) Carapace size of approximately 100 specimens of Tetracarcinus subquadratus from the Blue Springs locality, Mississippi, Maastrichtian Coon Creek Formation. Solid black dots indicate specimens sorted into Dakoticancer australis morphology; hollow dots indicate specimens sorted into Seorsus-type morphology; gray dots indicate specimens sorted into Tetracarcinus-type morphology. (2) Carapace width plotted against carapace L/W ratio. (3) Carapace length plotted against IL/L ratio. (4) Carapace width plotted against FOW/W ratio. T = Tetracarcinus morphology; S = Seorsus morphology; D = Dakoticancer australis morphology.

The ratio of the FOW to carapace width and the length of the intestinal region as a ratio of the entire carapace length was significantly different between Tetracarcinus-type specimens and those referred to the morphologies of the other two genera but was not significantly different between D. australis and Seorsus-type morphologies (Fig. 6.3, 6.4; Tables 4, 5). The smaller, Tetracarcinus-type, and larger, D. australis-type, specimens differ in degree of development of carapace ornamentation (Table 1). Smaller specimens have smaller orbital spines, whereas larger ones of D. australis morphology have long orbital spines. In smaller specimens, the orbital margin is beaded, whereas in larger specimens, it is rimmed. Smaller specimens have a narrow keel bounding the posterior margin of the epibranchial region, which is missing in most larger specimens. Larger specimens have a more inflated branchial region than smaller specimens.

Sternal differences are evident between Tetracarcinus and D. australis-type morphologies (Figs. 4, 5). Seorsus sterna are uncommon and similar to D. australis. In smaller specimens (Tetracarcinus-type), the anterior margin of sternite 4 is broadly inflated (Figs. 4.1, 4.2, 5.2), whereas in larger specimens (D. australis-type), the margin forms a distinct ridge across the sternum, interrupted axially in males (Figs. 4.3, 5.4). Smaller males and females bear granules on the sternites, sometimes roughly arrayed in rows and sometimes present along the edge of the sterno-pleonal cavity. Larger specimens have few granules in males, and the females are smooth.

Changes in carapace morphology during ontogeny are documented for several species of brachyuran crab. Juvenile, smaller individuals of Metacarcinus magister (Dana, Reference Dana1852) (Eubrachyura, Cancridae) are different in carapace shape and FOW to carapace width compared with larger adults (Hiebert and Rasmussen, Reference Hiebert, Rasmussen, Hiebert, Butler and Shanks2015) (Fig. 7.5, 7.6). Eyestalks and eyes, and presumably the width of the orbits as well, are commonly relatively larger in juvenile, presumably smaller brachyuran individuals (Marochi et al., Reference Marochi, Costa, Leite, Cury da Cruz and Masunari2019). In thoracotreme crabs (Eubrachyura, Thoracotremata), the overall carapace shape changes as the animal grows, such that the posterior part of the carapace changes in overall proportion to the anterior portion (Marochi et al., Reference Marochi, Costa, Leite, Cury da Cruz and Masunari2019). In Callinectes danae Smith, Reference Smith1869 (Eubrachyura, Portunidae), the anterolateral and frontal spines were more strongly ornamented in smaller, younger specimens compared with older ones (Shinozaki-Mendes and Lessa, Reference Shinozaki-Mendes and Lessa2019). Analyses of the juvenile crab stage of four different species of panopeid crab (Eubrachyura, Panopeidae, about 1–2 mm width) showed that the FOW/W ratio notably decreased and the anterolateral spines became larger as the animal grew (Martin et al., Reference Martin, Felder and Truesdale1984). For comparison, adult Panopeus spp. may reach 15–20 mm in width (Williams, Reference Williams1984). Comparison of adult Menippe mercenaria (Say, Reference Say1818) (USNM 1568616) (accessed at https://collections.nmnh.si.edu/search/iz/) and juveniles (accessed at collierseagrant.blogspot.com) indicates that the fronto-orbital width is much wider as a ratio of the carapace width in juveniles (Fig. 7.3, 7.4).

Figure 7.

Differences between small and large individuals during ontogeny. Specimens not to scale. (1) Tetracarcinus subquadratus, morphology of larger individuals, stylized from specimens herein. (2) Tetracarcinus subquadratus, morphology of smaller individuals, stylized from specimens herein. (3) Menippe mercenaria, morphology of larger individuals, stylized from image of USNM 1568616. (4) Menippe mercenaria, morphology of smaller individuals, stylized from image of a juvenile at http://collierseagrant.blogspot.com/2012/11/a-florida-stone-crabs-life-cycle.html. (5) Metacarcinus magister, morphology of larger individuals, stylized from Hiebert & Rasmussen (Reference Hiebert, Rasmussen, Hiebert, Butler and Shanks2015). (6) Metacarcinus magister, morphology of smaller individuals, stylized from Hiebert & Rasmussen (Reference Hiebert, Rasmussen, Hiebert, Butler and Shanks2015). Art by J.F.

Considering all of this evidence, we propose that the differences between and among the Dakoticancer australis, Tetracarcinus subquadratus, and Seorsus wadei morphologies are of degrees and are most parsimoniously explained by changes in the carapace and sternum during growth (Fig. 7.1, 7.2). Those differences that are significantly different are the result of ontogenetic changes. The types of changes observed across Dakoticancer australis, Tetracarcinus subquadratus, and Seorsus wadei are those documented for many species of extant brachyurans as they grow, including changes in the FOW/W ratio, changes in length of some regions of the carapace, and changes in the size of spines and degree of ornamentation. Thus, these types of differences are well documented as ontogenetic. These three species, in addition to Parapaguristes whitteni, are therefore all referrable to the same species, Tetracarcinus subquadratus.

Specimens referred to Dakoticancer australis from the Maastrichtian Potrerillos Formation in northeastern Mexico have a slightly different morphology from those documented from the Blue Springs and nearby localities (Vega and Feldmann, Reference Vega and Feldmann1991; herein) (Fig. 8). They are most similar in size, and thus morphology, to the largest specimens from Blue Springs, which were originally referred to D. australis. The Mexican specimens range from slightly wider than long to slightly longer than wide, have more inflated and developed carapace ridges than the larger Blue Springs specimens, and have a more strongly depressed posterior margin than the Blue Springs specimens. The only preserved sternum of the Mexican specimens appears to have sternites 5 and 6 directed posterolaterally (Fig. 8.2); however, this could be because of shearing of the specimen, seen in the dorsal carapace of LP-108 (Fig. 8.3). Although there are some differences between these and the American specimens, we elect to retain them in Tetracarcinus subquadratus, most similar to the D. australis morphology. Their pronounced ornamentation might be peramorphic in nature.

Figure 8.

Tetracarcinus subquadratus from the La Popa Basin, Mexico, Portrerillos Formation, Maastrichtian. (1):LP130, dorsal carapace, cuticle missing. (2) KSU D 715, male sternites 4–6. (3) LP108, slightly sheared dorsal carapace. Scale bars = 1 cm.

Included genera

Ibericancer Artal et al., Reference Artal, Guinot, van Bakel and Castillo2008; Sodakus Bishop, Reference Bishop1978; Synoriacarcinus new genus; Tropidicarcinus Schweitzer et al., Reference Schweitzer, Feldmann, Phillips and Armstrong2019.

Diagnosis

As in Feldmann et al. (Reference Feldmann, Schweitzer and Karasawa2017).

Remarks

Seorsus is herein removed from Ibericancridae and placed into synonymy with Tetracarcinus as discussed in the preceding. Seorsus millerae (Bishop, Reference Bishop1992) had originally been referred to Diaulax on the basis of its elongate-rectangular carapace and inflated epibranchial regions (Fig. 9.1) and was later removed to Seorsus (Karasawa et al., Reference Karasawa, Schweitzer and Feldmann2011). A sternum associated with the carapace of S. millerae was subsequently described, demonstrating its affinities with Ibericancer in having a very narrow sternum, long sternites 3 and 4, and a deep, narrow sterno-pleonal cavity (Feldmann et al., Reference Feldmann, Schweitzer, Baltzly, Bennett, Jones, Mathias, Weaver and Yost2013) (Fig. 9.2). Thus, the bases for placement of Seorsus into Ibericancridae were the sternal features of Seorsus millerae as the type specimens of Seorsus wadei, the type species, did not have sterna. The discovery of dakoticancrid-type sterna with specimens of Seorsus wadei, as well as its synonymy with Tetracarcinus, necessitate a new genus for Seorsus millerae and Seorsus kauffmani (Feldmann et al., Reference Feldmann, Schweitzer, Baltzly, Bennett, Jones, Mathias, Weaver and Yost2013), previously referred to Seorsus but now shown to have carapace and especially sternal morphology that ally them with Ibericancridae.

Figure 9.

Species of Synoriacarcinus new genus. (1) Synoriacarcinus millerae (Bishop, Reference Bishop1991) new combination, NJSM 23326, dorsal view; (2) Synoriacarcinus millerae (Bishop, Reference Bishop1991) new combination, NJSM 23318, ventral view of sternum and male pleon. (3) Synoriacarcinus kauffmani (Feldmann et al., Reference Feldmann, Schweitzer, Baltzly, Bennett, Jones, Mathias, Weaver and Yost2013) new combination, USNM PAL 553513, dorsal carapace. Scale bars = 1 cm. All images from Feldmann et al. (Reference Feldmann, Schweitzer, Baltzly, Bennett, Jones, Mathias, Weaver and Yost2013, fig. 14).

Type species

Diaulax millerae Bishop, Reference Bishop1992 by original designation.

Other species

Synoriacarcinus kauffmani (Feldmann et al., Reference Feldmann, Schweitzer, Baltzly, Bennett, Jones, Mathias, Weaver and Yost2013) new combination; Synoriacarcinus millerae new combination.

Diagnosis

Carapace longer than wide, length about 90% width, widest at position of single, prominent anterolateral spine; orbits square, rimmed, with intraorbital spine; FOW 50–70% carapace width; branchiocardiac groove well defined, cervical groove moderately defined; metagastric region wider than mesogastric region; epibranchial region forming a transverse broad ridge; sternum and sterno-pleonal cavity narrow, sternites 1–3 fused, sternite 4 long, with concave lateral margins, sternal suture 4/5 incomplete; male pleon with all somites free; subdorsal pereiopods 4 and 5; major chela bulbous, granular.

Etymology

The genus name is formed from the Greek synoria, meaning borderland, and karkinos, crab, referring to its occurrence on the margins of Cretaceous North America.

Remarks

The two species referred to Synoriacarcinus n. gen. are very similar in terms of the dorsal carapace, with Synoriacarcinus kauffmani differing from the type species in its wider fronto-orbital width, 70% as compared with 50% in S. millerae, and straighter posterior margin. Synoriacarcinus kauffmani is Turonian in age, from New Mexico, whereas S. millerae is Campanian in age, known from New Jersey and Delaware.

Synoriacarcinus differs from Sodakus in having a narrower sternum overall, about half as wide, as compared with Sodakus tatankayotankaensis Bishop, Reference Bishop1978. Synoriacarcinus is widest at about the midlength whereas Sodakus is widest in the posterior one-third. Ibericancer is nearly as wide as long, whereas Synoriacarcinus is longer than wide, and Ibericancer lacks regional development as seen on the new genus. Tropidicarcinus is squarer and has a wide, arcuate posterior margin differentiating it from Synoriacarcinus.

Sodakus is represented by three species in North America, ranging from the Turonian of New Mexico to the Maastrichtian of Mexico and South Dakota, USA. Ibericancer is known from the Campanian of Spain. Synoriacarcinus is reported from the Turonian of New Mexico and the Campanian of New Jersey. Tropidicarcinus is reported from the Campanian and Maastrichtian of Mississippi. Thus, not unlike Dakoticancridae, Ibericancridae was well established on Gulf Coastal and Western Interior North America (Fig. 10).

Figure 10.

Geographic distribution of Dakoticancridae and Ibericancridae, plotted on a 65 Ma map (earliest Danian). Map by R. Blakey (Reference Blakey2012), reproduced under the CC Attribution 1.0 generic license. Red squares = Tetracarcinus; blue squares = Dakoticancer; green squares = Avitelmessus; red circles = Synoriacarcinus n. gen.; blue circles = Tropidicarcinus; green circles = Sodakus; yellow circle = Ibericancer (off map, locality in Spain).