Eight new species of Eocene Gastropoda from Texas based on the Megahan collection

The specimens described in this paper came from two localities in the middle Claiborne Group Eocene. The locality designations used are from the registry of the Bureau of Economic Geology, University of Texas (BEG). A partial set of the BEG stratigraphic localities can be consulted in Fisher et al. (1964:96–101). The molluscan fauna of the Eocene, middle Claibornian is generally quite well known (Harris 1937, Palmer 1937), those strata having been collected since the early 19^th century in the southeastern United States and since the late 19^th century in the western Gulf region. The smaller-sized taxa are much less well known, often only represented by only one, or very few specimens, so additions like these new taxa give an increasingly complete view of the middle Eocene fauna.

This well-known locality is much visited by local collectors and is the type locality of the Stone City Member of the Cook Mountain Formation. The stratigraphy, beds, location, and bivalve fauna were comprehensively treated in Stenzel et al. (1957). Bed designations, where used in the text, are those defined in Stenzel et al. (1957:22–25). The original location was given as the right bank of the Brazos River, immediately upstream from the bridge of State Highway 21 and Southern Pacific Railroad, Burleson County, Texas. The GPS coordinates of the original site are: 30°37′40.05″N, 96°32′51.91″W. Local collectors have since exposed about an additional 600+ m of fossiliferous outcrops downstream from the original site.

In bed of Spring Branch of Two Mile Creek, west of the bridge over U.S. Highway 75, and about 3.2 km north of the Madison-Leon County line, Leon County, Texas, GPS coordinates are: 31°07′4.66″N, 95°56′40.88″W. Landrum Member, Cook Mountain Formation. The geology and stratigraphy of Leon County is discussed in Stenzel (1938).

Banks and creek bed of Little Brazos River, about 160 m upstream of Highway 21 bridge, and about 14 km WSW of Bryan, Texas. GPS coordinates are: 30°38′39.50″N, 96°31′13.48″W. Cook Mountain Formation, Wheelock Member. The locality is discussed in Scott (1963).

Unlike many in the local fossil collecting community who often only surface collect, or excavate for the larger specimens, Mr. Megahan and his family also assiduously collected bulk samples which were taken to his home for further processing. After disaggregation, the sediments were examined under a binocular microscope, the interesting specimens picked out, cleaned, and many also photographed. The author was given access to the photographic images, and those fossils that appeared to be new species were sent to the author for further examination. Those specimens that became types were generously donated to the Non-Vertebrate Paleontology Laboratory (NPL) of the University of Texas at Austin.

This work has been registered in ZooBank with the registration number: urn:lsid:zoobank.org:pub:33DE7BF6-87C1-4F25-BE1D-C56836C8D863.

Class Gastropoda Cuvier, 1795 Subclass Caenogastropoda Cox, 1960 Superfamily Epitonioidea S. S. Berry, 1910 Family Epitoniidae S. S. Berry, 1910 Genus Pliciscalade Boury, 1887 

Scalaria gouldi Deshayes, 1861, by original designation.

Pliciscala obliqua, new species Fig. 1A, B

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One specimen, holotype, NPL 93894, from locality BEG 26-T-1, bed S.

Pliciscala species, with spire angle ca. 23°, occasional prominent varices, whorls evenly rounded, surface with minutely punctate spiral grooves, umbilical chink present.

Whorls 9, protoconch missing, first 2½ teleoconch whorls decorticated; teleoconch whorls inflated, suture impressed, ca. 14 axial ribs on early whorls increasing to 20 on the body whorl, 3 ribs much larger, varix-like; whorl surface covered with extremely fine spiral grooves that are minutely punctate; basal disc poorly defined by a weak carina, somewhat nodular where crossed over by the axials that continue much diminished over the disc; aperture elliptical, holostomous, outer lip with a prominent varix, just behind the edge, set at a prosocline oblique angle to the shell axis; small umbilicus.

The name obliqua (Latin, oblique) refers to the prosocline oblique varices of this species.

The closest species to Pliciscala obliqua is the upper Eocene species Pliciscala pearlensis (Meyer, 1887), which until recently was only known from a single specimen. Dockery (1977:pl. 3, fig. 12) figured another specimen, which, however, appears very worn and has the axial folds almost completely effaced on the body whorl. On the holotype of P. pearlensis, the folds continue over the basal area, with no apparent basal disc discernable; the description also noted that the protoconch has 4 whorls. Quite similar is an undescribed specimen from the Weches Formation; this specimen has 3 smooth erect whorls (tip missing), the transition to teleoconch whorls defined by a sinuous ridge; teleoconch whorls with 20 ribs on the body whorl, and almost no discernable basal disc; this specimen is deposited in the NPL collections and is designated NPL 93901. Species both with, and without a basal disc have been placed in the genus Pliciscala. The type, P. gouldi (Deshayes, 1861) has a well-defined basal disc margined by a spiral cord, as has Pliciscala cribrum (Cooke, 1926), another Moodys Branch species. MacNeil & Dockery (1984) discussed and figured four Pliciscala species from the Oligocene strata of the Gulf Coast of which Pliciscala (Nodiscala?) n. sp. appears the closest to P. obliqua; this species also has the prosocline oblique varices and a very poorly defined basal disc. However, the author does not agree with the subgeneric assignment to Nodiscalade Boury, 1890. Examining the figures of the type species of Nodiscala, Nodiscala bicarinata (Sowerby, 1844), available at: http://www.gastropods.com/7/Shell_6397.shtml (last accessed 2 Aug 2021), the spiral ornament, is seen to be particularly prominent on the body whorl, as are the subsutural crenulations, a duplex cord margins the basal disc and the final varix is in line with the shell axis, all at variance with the characters of P. (Nodiscala?) n. sp. Assignment to Pliciscala sensu stricto would appear to be a better match.

Genus Papuliscalade Boury, 1911 

Acirsa praelongaJeffreys, 1877 by original designation.

Papuliscala keani, new species Fig. 1C, D

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One specimen, holotype, NPL 93895, from locality BEG 21-T-1, bed D.

Papuliscala species with 4 spiral cords, the interspaces larger than the cords, overrun with numerous similar sized axials ribs, prickly or foliated at the intersections with the spirals.

Shell very small, early whorls missing, remaining whorls 8; whorls moderately inflated, surface smooth, shining, with thin, foliated (crinkled?) axials, 13 on the first whorl and 19 on the body whorl, the axials straight medially, adaperturally bent subsuturally, abaperturally bent suprasuturally; whorls overrun with 4 spiral cords, nodular where crossing the axials; suture (adpressed) almost invisible, defined by a faint microscopic line; basal disc margined with a strong cord, 2 weaker cords on the disc, overrun by axial threads that are the continuation of the axials; aperture rounded-quadrangular, columella concave; outer lip missing.

This species is named in honor of Mr. Megahan's son Kean for his help in collecting.

This specimen has lost the protoconch whorls, but the weakly carinated whorls, the axial and spiral sculpture, and the smooth basal disc places it in the genus Papuliscala. Other taxa that might be confused with this are Elegantiscalade Boury, 1911, but that taxon has a weakly concave basal disc and a round aperture, and CerithiellaVerrill, 1882, which can be separated by the bent, extended canal and a weak swelling on the columella. The type species Papuliscala scalariformisde Folin, 1867, has much more subdued sculpture than Papuliscala keani, but several other species assigned to the genus, for example Papuliscala tavianiiBouchet & Warén, 1986 also possess a similar foliated, prickly, or noded intersection of the axial and spiral cords. Modern members are restricted to deep water (Bouchet & Warén 1986). The earliest reported European occurrences of Papuliscala are from the Oligocene or early Miocene of France (Lozouet 1999), whereas in the Gulf Coastal plain region the genus occurs in the lower Paleocene of Texas as Papuliscala acusGarvie, 2021, from which it can be distinguished by the more carinated whorls and the foliated axials. The two images, Fig. 1C, D, show only partially cleaned specimens as any further removal of matrix would have further damaged the crinkled foliated axials.

Superfamily Buccinoidea Rafinesque, 1815 Family Fasciolariidae Gray, 1853 Genus ConradconfususSnyder, 2002 

Buccinofusus parilisConrad, 1868, by original designation.

Conradconfusus nodulinus, new species Fig. 1E, F

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Holotype, NPL 93889, from locality BEG 143-T-83. A second specimen collected by the author from the Stone City Formation, is in the NVP collection catalogued as NPL 93902.

Small Conradconfusus species, with evenly rounded whorls, sculpture of equi-sized weakly opisthocline axial and spiral cords forming a close mesh pattern, nodular at their intersections.

Nucleus enrolled, deviated, protoconch of 2¼ smooth whorls, followed by 1/8 whorl of ribblets, smoothly transitioning to the adult sculpture; teleoconch of 5 ½ whorls, inflated, early whorls somewhat posteriorly stepped, later ones smoothly rounded; sculpture of numerous opisthocyrt axial folds, overrun by 7 spiral cords, the intersection finely beaded; suture moderately impressed, axial sculpture dying out approaching the canal; callus wash thin over parietal region; outer lip broken but showing weak spiral grooves in the interior. columella sinuous, smooth, medially straight, the edge abaperturally delineated by a fine line; canal moderate in length bent to the left.

The name nodulinus refers to the noded intersections of the spiral and axial lines.

The species might be considered for assignment in MitrellaRisso, 1826. However, the protoconch of that taxon has 1½ inflated whorls with no ribblets, an absent or very short anterior canal, and no anterior constriction of the outer lip. Assignment may also be considered in Metula, H. Adams & A. Adams, 1853, where several of those species have a similar external whorl sculpture, e.g., Metula reticulataGarvie, 2013, from the upper Paleocene Seguin Formation of Texas, and Metula gracilisJohnson, 1899, from middle Eocene, Cook Mountain Formation of Texas. Both of these species have much coarser whorl sculpture, possess varices or at least one terminal varix, and a fairly short canal so are easily separated from Conradconfusus nodulinus. Much closer is Metula (Caseyella) neptuneiformisMacNeil, 1984, from the Oligocene, Red Bluff Formation of Mississippi, which has a similar sculptural mesh-like pattern of curved axial ribblets and spiral threads, not, however, nodular at their intersections. One distinguishing characteristic of both Metula sensu stricto and Metula (Caseyella) species is a tall conical protoconch, quite different from that of taxon Conradconfusus. Wenz (1944) recorded Conradconfusus from the upper Cretaceous in Europe and West Africa to the Recent. In the U.S.A. the genus is known from Danian strata of Texas (Garvie 2021), and a very similar but undescribed species occurs in the Gosport Sand Formation in Little Stave Creek in Alabama (pers. obs.); the present species may be distinguished from those by the more regularly rounded whorls and regular fine beading. The type, Conradconfusus parilis (Conrad, 1830), which is common in the Miocene St Mary's Formation of Maryland, is a much larger species with both stronger axial and spiral ornament.

Family Buccinidae Cossmann. 1906 Genus TerebrifususConrad, 1865 

Buccinum amoenumConrad, 1833, by monotypy.

Terebrifusus megahani, new species Fig. 2A–C

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Nineteen specimens. Four specimens from locality BEG 143-T-83; holotype, NPL 93899, paratype (a juvenile), NPL 93900. An additional 15 specimens from the Cook Mountain Formation, Wheelock Member, in central Texas were also examined.

Terebrifusus species with regularly spaced distinct axials overrun by 5 or 6 rounded spiral cords, nodular at their intersections. On the body whorl the spiral cords become closer together approaching the canal. Siphonal fasciole prominent.

Protoconch smooth, of 1½ whorls, tip enrolled, followed by ½ whorl of arcuate ribblets; transition to teleoconch abrupt; teleoconch, polished, of 4 whorls, with 9–10 distinct axial ribs and 6 spiral cords, nodular at the intersections, later whorls with a smaller spiral in the interfaces; last whorl with sharp outer lip and a broad varix-like axial set back from the edge; aperture lanceolate, columella with 3 plaits in the juvenile, 5 or 6 prominent in the adult; canal notched, short, twisted, siphonal fasciole strong composed of numerous closely-set cords, one larger one defining the adapertural edge.

Honoring Mr. Donald Megahan in recognition of his work in collecting, preparing, and donating these new species to the NPL.

Palmer (1937:307–308) discussed the differences between T. amoenus (Conrad, 1833) and T. multiplicatus (H. C. Lea, 1841) and mentioned the more finely striated appearance and larger longitudinal ribs of T. multiplicatus. The types of both those taxa come from the Gosport Sand Formation at Claiborne Bluff, but both species are also found in middle Claibornian strata of the Gulf States and South Carolina. Genus Terebrifusus is now known to range back to the upper Paleocene (Garvie 2013:45). The author has collected over 165 specimens of taxon Terebrifusus from the Gulf Coast Eocene, and 4 specimens from upper Paleocene strata of Texas and Alabama, and even though there is considerable variation in characters, there are two major groupings exemplified by T. amoenus and T. multiplicatus. Terebrifusus amoenus is characterized by fairly regularly spaced, raised spiral lines, the interspaces themselves sometimes, also with weaker, but always raised spiral lines. The protoconch is small, conical, with 2–2½ smooth whorls followed by ½–¾ whorl of arcuate ribblets. The new species, T. megahani, is separated from the typical amoenus group by the much stronger primary spirals that are always nodular where they cross over the longitudinal folds. Specimens intermediate in character between the two groups have not been found supporting a new species definition. The multiplicatus group is characterized by the spiral sculpture defined by impressed, not raised, lines, giving a much smoother surface to the species, while the protoconch is larger, with 2 smooth whorls followed by ½ of ribblets and with a blunt tip. The multiplicatus group has not been observed below the Cook Mountain Formation, although a few specimens in the Weches Formation are smoother than the majority and also have a flatter protoconch and probably represent the beginning of the ancestral line. Garvie (1996) identified T. multiplicatus from the Reklaw Formation, but I now believe that was an error, as the specimen can definitely be assigned to the amoenus group as here discussed, so should be named T. amoenus. In the Gosport Sand Formation, the columellar plaits become much more numerous and finer than those from earlier formations. Terebrifusus megahani has only been found in the Cook Mountain Formation and is easily distinguished from the other Gulf Coast species by the strong nodular spiral lines; a similar species is ?Terebrifusus lepusOlsson, 1930 of Eocene age from Peru that has 5 nodular spiral lines, although weaker than in T. megahani, and an excavated subsutural area. Olsson only doubtfully assigned the species to Terebrifusus, because the holotype is much obscured by matrix, but the paratype shows the surface sculpture well and so I believe can be confidently assigned to Terebrifusus. The T. amoenus group from the Cook Mountain Formation could perhaps be separated from the similar Gosport Sand Formation group as differences do appear to be significant, but without more Gosport Sand specimens to examine that is not done here.

Family Turridae H. Adams & A. Adams, 1853 Genus OrthosurculaCasey, 1904 

Pleurotoma longiformaAldrich, 1897, by subsequent designation (Gardner, 1935)

Orthosurcula ethani, new species Fig. 2D, E

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One specimen, holotype, NPL 93890, from locality BEG 21-T-1.

Slender Orthosurcula species, spire angle of ca. 20°; proportionally large protoconch; apex of anal sinus just anterior to maximum whorl diameter.

Shell elongate, very small, of 8 whorls; protoconch of 3½ smooth inflated whorls, followed by ¾ whorl or arcuate ribblets, transition to teleoconch abrupt; suture impressed, whorls with subsutural swelling and concave shoulder slope, maximum diameter in lower third of the whorl; surface polished with almost inconspicuous spiral threads covering the whorl and 1–3 larger spiral lines anterior to carina, body whorl with spiral lines to end of the canal; aperture ovate, columella smooth concave; anterior canal unnotched, medium-sized, bent to left and a moderately twisted.

This species is named in honor of Mr. Megahan's grandson Ethan, for his help in collecting.

Orthosurcula ethani appears to be the ancestor of Orthosurcula longiforma (Aldrich, 1897) from the Oligocene Red Bluff Formation of Mississippi, the type species of Orthosurcula. Although this specimen appears to be a juvenile, it can be distinguished from O. longiforma by a protoconch of one more whorl, a swollen subsutural collar, a more bent canal, and the more anterior position of the sinus. Axial sculpture is absent, except where some of the growth lines cut into the subsutural collar forming weak nodes there. Of Paleocene age, Orthosurcula persa (Whitfield, 1865), most closely approaches this O. ethani, with the sinus in a similar position, a weakly swollen collar, and axial sculpture obsolescent on later whorls. MacNeil & Dockery (1984) and Otuka (1959) both treat Orthosurcula as a subgenus of Turricula, Schumacher, 1817, whereas Palmer & Brann (1966), Harris (1937), Powell (1966), Gardner (1935), Garvie (2013, 2021) all give Orthosurcula full generic rank. Orthosurcula, or a genus very close to it, seems to have survived until Recent times as Otuka (1959) lists four species of Turricula (Orthosurcula) from Japan, two Recent, and two from Neogene rocks, so that the distinguishing characteristics of the genus are recognizable during the entire Cenozoic, thus prompting its treatment as a full genus.

Family Cancellariidae Forbes & Hanley, 1851 Genus Unitas Palmer, 1947, in Harris & Palmer, 1946–1947

Cancellaria costulata Lamarck, 1803, by original designation.

Unitas duomillias, new species Fig. 3A, B

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Two specimens, holotype, NPL 93892, paratype, NPL 93893, both from locality BEG 143-T-83.

Elongate Unitas species, with aperture at ca. 46% of height, whorls with prominent axial folds, whorls with close-set inconspicuous spiral striae.

Shell elongate, robust; whorls 5–6, protoconch of two whorls, mammilate, smooth, the nucleus minute, below the first whorl; transition to teleoconch sculpture within 1/8 of a whorl; teleoconch whorls inflated with 9–10 strong, slightly prosocline axial folds, some almost varix-like; surface polished, covered by numerous close-set spiral lines 3 or 4 of which are slightly more prominent and cause weak nodes on the crests of the axials; aperture elongate, outer lip thickened, with 9 or 10 strong lirae, 1 parietal fold and 3 columella folds, the lowest defining the start of a short canal; siphonal fasciole present, columella callus forming a well-defined edge over the last whorl.

The name duo (Latin, two), millias (Latin, miles) refers to Two Mile Creek, where the specimens were found.

Two similar species are Unitas euaniGarvie, 2021 and Unitas anderseniSchnetler & Petit, 2006, both of Danian, early Paleocene age. Unitas duomillias is primarily distinguished from U. euani and U. anderseni by the more prominent and numerous axial folds. The genus is known from the Upper Eocene Moodys Branch Formation of Mississippi (cf. Robinson 1983, fig. 2a, b), in which the axial folds are more numerous and the spiral lines much more prominent. This is the first record of the genus in the U.S.A. lower or middle Eocene strata of the Gulf or Atlantic coastal plains. The genus is well represented in Eocene localities in the Paris Basin with a somewhat morphologically similar species, albeit with spiral lines, being Unitas parnensis (Cossmann, 1896) of middle Eocene (Lutetian) age.

Family Ringiculidae Philippi, 1853 Genus RingiculaDeshayes & Milne-Edwards, 1838 

Auricula ringensLamarck, 1804, by subsequent designation (Gray, 1847).

Ringicula taenia, new species Fig. 3C, D

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One specimen, holotype, NPL 93891, from locality BEG 143-T-83.

A Ringicula species with a stepped whorl profile, whorls with a large posteriorly smooth area, and a single parietal fold.

Protoconch 2¼ smooth inflated whorls, followed by a prominent inclined axial swelling, and within ½ whorl transitioning to the adult teleoconch sculpture; suture deeply impressed; whorl profile stepped, swollen subsutural collar, collar margined by a spiral line, 1–2 more developing with age; body whorl with smooth central band, basally with about 9 spiral lines; outer lip with 5 weak denticles; 2 almost horizontal columella plaits, parietal callus developed into an inclined blade.

The name taenia (Latin, head-band) refers to the smooth area capping the top of the body whorl.

This species is unique with respect to all other Ringicula species known from Paleogene strata of the Gulf Coast, as regards the prominent shoulder and the rectangular outline of the outer lip. A wide band devoid of spiral lines below the shoulder is also a feature of Ringicula trapaquara deusseniGardner, 1927, but that species lacks the shoulder seen on this species. There is an undescribed species (NPL 93903) from the Cane River Formation at Nacogdoches with the same wide band below the shoulder, margined by a strong spiral line, and below that a similar smooth wide band. The affinities of the Nacogdoches species appear to be with Ringicula biplicataLea, 1833, so this new species appears to be a further continuation of the line. Ringicula taenia cannot be confidently placed within any of the subgenera of Ringicula as listed by Zilch, 1959–1960. It has the spiral sculpture, the thickened, crenate labrum, and the anterior plication defining the base of the columella of Ringicula sensu stricto but not the high spire of that genus, whereas it has the weakly defined anterior notch, the anteriorly constricted, notched aperture of RingiculinaMonterosato, 1884 but is lacking the smooth labrum of that taxon. Ringicula taenia probably deserves to be placed in a new subgenus of Ringicula but with only one specimen available that is not done here.

Family Pyramidellidae Gray, 1840 Genus SyrnolaAdams, 1860 

Syrnola gracillima A. Adams, 1860 by monotypy.

Syrnola debrae, new species Fig. 3E, F

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Two specimens, holotype, NPL 93896, paratype, NPL 93897, from locality BEG 21-T-1.

Small Syrnola species with columellar plait just anterior to the parietal area, punctate spiral striae and a sutural angle with respect to the shell axis of ca. 18°.

Protoconch of 2 whorls, immersed to the level of the first whorl, set at an angle of slightly more than 90° to the shell axis; teleoconch of 7–8 whorls, whorls flat, weakly constricted toward the lower suture; suture impressed; surface smooth, shining with 4–6 widely spaced spiral striae, only visible under magnification and seen to be minutely but irregularly punctate; aperture lenticular, basally weakly extended to a point, one adapical duplex fold; outer lip smooth within.

The name debrae honors Mr. Megahani's wife, Debra Lynn Megahan.

Robba (2013) is followed for the systematics and separation of pyramidelloidean taxa, where Syrnola is given generic rank. Four species of Pyramidella (Syrnola) are listed in Palmer & Brann (1966), there treated as subgenera, but none show much similarity to this species. All are much larger, with flatter whorls, and no spiral striations. A much more similar species is Syrnola pirumGarvie, 1996 from the Reklaw Formation, but it can be separated again by the lack of spiral striae, and the columella plait is in a much more anterior position in the aperture. A Cook Mountain Formation species with similarly fine spiral striations is Pyramidella pseudopygmaeaPalmer, 1937, but in this species the striae are very close-set, are not punctate, and the species is much larger in size. Another notable characteristic of Syrnola debrae is the angle of the suture with respect to the shell axis (i.e., the line between successive whorls), which is quite oblique at about 65°, whereas in all other Claibornian pyramidelloidean taxa examined the angle is about 80°. Palmer (1937:77) noted P. pseudopygmaea occurs in the lower Eocene, Claiborne strata in Texas. All specimens from Texas examined by the author have proved to be this new species not P. pseudopygmaea whose occurrence is otherwise from the Gosport Sand Formation in Alabama.