Type species

Saffordella tennesseensis Dunbar, Reference Dunbar1920, by original designation.

Diagnosis

As for the species, by monotypy.

Occurrence

Early Devonian, Lochkovian, Rockhouse Formation, Hardin County, Tennessee, USA.

Remarks

Saffordella is unusual because it possesses a strongly developed wide pseudoselenizone directly below the true selenizone. It resembles Shwedagonia Batten (Reference Batten1956), a Permian eotomarioid genus, in having deeply incised sutures, a narrow selenizone on the upper whorl surface, and a flat to slightly convex area with strong prosocyrt growth lines below the selenizone. The Permian genus, however, has a selenizone bounded by strongly developed lirae, whereas the selenizone in Saffordella is bounded by grooves; in addition, in Shwedagonia the growth lines in the flattened area below the selenizone are prosocline rather than prosocyrt as in Saffordella in which the area between the selenizone and periphery forms the wide pseudoselenizone.

I have placed this genus in the family Gosseletinidae and, with some uncertainty, the subfamily Gosseletininae, because of the flat, narrow selenizone without bordering threads or cords. Within the family, it most closely resembles Stenoloron Oehlert and Oehlert (Reference Oehlert and Oehlert1888) in being turbiniform, but that genus lacks both a distinct subangular periphery at midwhorl and a wide pseudoselenizone. It is possible that additional, better-preserved specimens will show that Saffordella belongs in a separate subfamily, or perhaps even a distinct family.

Saffordella tennesseensis Dunbar, Reference Dunbar1920

Figure 3.1–3.6

Figure 3

Saffordella tennesseensis Dunbar, Reference Dunbar1920, from the Lower Devonian Rockhouse Formation of Hardin County, Tennessee: (1–5), photographs of the holotype, YPM 9728; (1) apertural view showing the aperture clogged by matrix and the effect of knobby silicification on preservation; (2) abapertural view showing the subdued trace of the wide pseudoselenizone above the periphery; (3) apical view showing subdued growth lines sweeping back from the suture to the selenizone on the last whorl near the aperture; (4) oblique view showing an apparent lack of detail on the upper part of the outer whorl surface; (5) close-up view of the upper part of the outer whorl surface showing growth lines forming the selenizone above the pseudoselenizone and prosocyrt lunulae in the pseudoselenizone; (6) illustration of the reconstructed Saffordella tennesseensis. Scale bars =5 mm.

1920 Saffordella tennesseensis Dunbar, p. 142, pl. 4, fig. 17.

1941 Saffordella tennesseensis; Knight, p. 304, pl. 85, fig. 2.

Holotype

YPM 9728.

Diagnosis

Moderately large, turbiniform, anomphalous or slightly phaneromphalous, whorl profile rounded to subangular, periphery at midwhorl, sutures impressed, shallow sinus in outer lip generating narrow selenizone well above midwhorl, wide band of prosocyrt lunulae just below selenizone.

Occurrence

The species is known only from the Rockhouse Formation of Hardin County, Tennessee.

Description

Moderately large, dextrally coiled, turbiniform, seemingly anomphalus, gastropods with at least 3.5 rapidly expanding whorls, last whorl more than twice as large as early ones combined, nuclear whorls unknown. Whorl profile changing with ontogeny; early whorls rounded, upper whorl profile sharply convex into adapical suture, gently rounded into abapical suture; later whorls becoming roundly subangular, with periphery at midwhorl; upper whorl profile sharply rounded from adapical suture to outer whorl face; upper half of outer whorl face flattened to gently convex; periphery rounded; lower half of outer whorl profile gently rounded, becoming slightly concave into base. Sutures deep and sharply impressed. Base rounded into aperture. Aperture oblique, subquadrate, equally long and wide, deeply notched at suture, rounded at base. Columellar lip unknown; parietal inductura unknown. Outer lip prosocline from suture to relatively shallow sinus generating selenizone above midwhorl, approximately two-thirds distance from suture to whorl periphery, becoming prosocyrt from sinus to whorl periphery, then curving forward for short distance before becoming prosocyrt to base. Selenizone narrow, flat, or slightly convex, bordered by shallow grooves; upper groove very narrow and shallow, obscure in type specimen; lower groove wider and deeper. Grooves on upper whorl surface between selenizone and whorl periphery form upper border of broad, flat to slightly convex pseudoselenizone having well-developed, prosocyrt, coarse growth lines or lunulae; upper groove forms lower boundary of selenizone; lower groove occurs approximately 0.5 mm above whorl periphery. All surface ornament coarse growth lines except for grooves.

Remarks

This is the largest gastropod in Dunbar’s collection (Table 1). Only 3.5 whorls are preserved, but what remains suggests that the spire was not particularly high. Although the specimen is silicified and much of the surface is obscured by knobby texture and beekite rings (Fig. 3.1–3.4), most of the more important features can be detected with a microscope. Unfortunately, neither Dunbar’s (Reference Dunbar1920, pl. 4, fig. 17) nor Knight’s (Reference Knight1941, pl. 86, fig. 2) single photographs of the holotype show these features or help explain the complex morphology of the outer lip, as inferred from the preserved growth lines on the body whorl.

Table 1

. Measurements (in millimeters) of eight gastropod specimens representing five species described by Dunbar (Reference Dunbar1920) from the Early Devonian, Lochkovian, Rockhouse Formation of Hardin County, Tennessee. TH =total height; HLW =height of last whorl; MW =maximum width; HS =height of spire; HA =height of aperture; WA =width of aperture; WS =width of sinus or selenizone near the aperture.

There is a change in whorl profile during ontogeny. In the three preserved early whorls, the profile rises sharply out of the adapical suture and curves tightly around to the outer whorl face, which is broadly rounded, without an obvious periphery, before falling into the abapical suture, subsumed by the next whorl. In the last whorl, the profile begins by rising sharply out of the adapical suture and curves tightly around to the upper whorl face, which is very gently convex, almost flat. The whorl periphery is bluntly rounded, occurring at midwhorl. The lower whorl profile is more convex than the upper whorl profile over most of its surface. The base is even more convex.

The base of the last whorl near the outer lip is broken and the aperture is completely filled with matrix (Fig. 3.1), but the shape of the outer lip can be inferred from preserved growth lines. The aperture appears to have been relatively broad, presumably about 17 mm, and nearly equally as long. Because the aperture is filled, the structure of the parietal inductura, if any existed, cannot be determined.

The presence of a sinus or slit in the outer lip on the upper whorl surface is deduced from the presence of a recognizable selenizone, which is preserved only on a small portion of the last whorl (Fig. 3.5, 3.6). If the sinus and selenizone existed on the earlier whorls, they are completely obscured by the knobby, silicified surface. Both Dunbar (Reference Dunbar1920) and Knight (Reference Knight1941) misinterpreted the presence of the selenizone. Dunbar, in describing what he thought was the selenizone, actually described the pseudoselenizone directly beneath the selenizone. This rather conspicuous band is bordered by shallow grooves, as stated in the preceding; the growth lines crossing the band form broad, coarse, prosocyrt lunulae. Dunbar (Reference Dunbar1920) considered this band to be a selenizone, and Knight (Reference Knight1941) seems to have simply paraphrased what Dunbar wrote. Although Knight acknowledged the shell had a broad shallow sinus and, supposedly, a broad slit of unknown depth, he suggested that it was a pseudoselenizone because the convex-forward lunulae were anomalous. Knight must have taken Dunbar’s description at face value, rather than closely examining the specimen himself because it is obvious that the broad band is about 4.5 times as wide as, and lies below, the selenizone. That Knight, of all people, could accept the coarsened prosocyrt growth lines (lunulae) within this band as evidence of a selenizone is extremely puzzling since he (Knight, Reference Knight1934) coined the term ‘selenizone’ to replace the older term ‘slit-band’ and should have been very familiar with its concept. Relatively minor magnification reveals that preserved growth lines and an obscure upper groove delineate the actual selenizone and therefore the correct configuration of the outer lip (Fig. 3.5).

The pseudoselenizone is the most peculiar aspect of this species. Without magnification, it seems to be such an obvious selenizone that it comes as a surprise to realize that it is not. The giveaway, however, is the shape of the lunulae within this zone. They are prosocyrt rather than opisthocyrt, as would be expected in the selenizone, and they occupy the full 4.5 mm width of the band between the grooves. In addition, they are substantially coarser than the rest of the growth lines on the whorl face, forming distinct opisthocyrt lunulae. Then, beginning at the lower groove, the lunulae, which at this point are directed backward (away from the aperture), lose their coarseness, bend forward again over the periphery, and curve gently forward down to the base. It appears, then, that the pseudoselenizone is merely a form of shell ornamentation that has no obvious importance in the inhalant or exhalant currents of the mantle cavity other than, perhaps, assisting with separation of currents.

Knight (Reference Knight1941) concluded that Saffordella merely mimicked a ‘pleurotomarian.’ Because of this opinion, and because of several other anomalous genera from the same locality (i.e., Aulopea and Distemnostoma—see the following), Knight rejected Saffordella as a pleurotomarian and allied it with the platyceratid gastropods. Eventually, it became regarded as a junior subjective synonym of Visitator Perner, Reference Perner1911, which was considered to be a subgenus of Platyceras Conrad, Reference Conrad1840 (Bowsher, Reference Bowsher1955; Yochelson, Reference Yochelson1956; Knight et al., Reference Knight, Cox, Keen, Batten, Yochelson and Robertson1960; Thompson, Reference Thompson1970). Wagner (Reference Wagner2013) considers Visitator to be a separate genus, an opinion with which I agree. The presence of a true selenizone, however, demonstrates that Saffordella is, indeed, an eotomarian gastropod. Figure 3.6 is offered as a probable reconstruction of Saffordella tennesseensis Dunbar, Reference Dunbar1920.

Superfamily uncertain

Remarks

One of Dunbar’s (Reference Dunbar1920) new gastropods, Distemnostoma curtum, herein renamed Omocordella curta, closely resembles several genera Knight et al. (Reference Knight, Cox, Keen, Batten, Yochelson and Robertson1960) included in the Raphistomatidae Koken, Reference Koken1896, which, as Wagner and Erwin (Reference Wagner and Erwin2006) pointed out, is an amalgam of several largely unrelated clades. Omocordella especially resembles Scalites Emmons, Reference Emmons1842 and Buechelia Schlüter, Reference Schlüter1894, which are characterized by a low spire, flat or concave upper whorl surface, sinus in the outer lip generating a selenizone at the edge of the upper whorl surface, and a basal extension suggesting the presence of an inhalant sinus. O. curta and related gastropods (see the following), however, have the selenizone perched atop an upward projecting shoulder whereas Scalites, Buechelia, and the remaining ‘raphistomatids’ have the selenizone projecting outward, forming the periphery of the whorl. As such, they all seem to be related in the biological sense of having two sinuses, an inhalant sinus at the base of the aperture and an exhalant sinus near the adapical suture. The direction of the exhalent current is significantly different, however, and suggests that O. curta and its relatives are different enough to be placed in a separate family from those genera.

Family Micromphalidae new family

Genera included

Micromphalus Knight, Reference Knight1945; Slehoferia Rohr and Frýda, Reference Rohr and Frýda2001; Omocordella n. gen.

Diagnosis

Small to moderately large, low- to high-spired gastropods with strongly developed shallow sinus situated on upwardly projecting rounded shoulder developed at outer edge of flat or concave upper whorl surface.

Remarks

Knight (Reference Knight1945) proposed the genus Micromphalus for moderately small (height=10–15 mm) trochiform gastropods having wide, low, shouldered whorls with a shallow sinus culminating on the rounded shoulder (Fig. 4.1). The type species, M. turris Knight, Reference Knight1945, from the Mississippian St. Genevieve Limestone of Kentucky, features a concave upper whorl surface, deeply impressed sutures, narrow umbilicus with a funicle reflexed around it, and lack of ornamentation other than growth lines. Knight (Reference Knight1945) thought Micromphalus resembled a shouldered Straparollus or a high-spired Euomphalus such as Straparollus (Euomphalus) springvalensis (White, Reference White1876) from the Mississippian of Iowa but without the wide-open umbilicus of those gastropods. He also compared it with Orecopia Knight, Reference Knight1945 because both genera generated a shallow sinus on the upper whorl surface. The sinus of Orecopia, an omphalotrochid, is shallower than that of Micromphalus and does not culminate on the shoulder, but rather on the concave upper whorl surface as in all omphalotrochids. Knight et al. (Reference Knight, Cox, Keen, Batten, Yochelson and Robertson1960) placed Micromphalus within the Euomphalidae with no explanation, and both the brief diagnosis of the genus and accompanying illustration (Knight et al., Reference Knight, Cox, Keen, Batten, Yochelson and Robertson1960, p. I194, fig. 108, 11) give no clue to the nature of the upper whorl surface, shoulder angulation, or sinus. In fact, the Treatise illustration has more in common with photographs of Tychobrahea Horný, Reference Horný1992, a sinistral hyperstrophic gastropod thought to be allied with the Trochonematidae, than with Knight’s (Reference Knight1945, pl. 80, fig. 3a–c) photographs of the type species, M. turris. This probably led Horný (Reference Horný1992) to suggest that Micromphalus was questionably a genus of his new family, Tychobraheidae. That family consists of genera that have prosocline growth lines with no indication of a sinus or selenizone. In addition, the two genera Horný unquestionably placed in the family are hyperstrophic sinistral, and there is no evidence that Micromphalus had a similar orientation.

Figure 4

Two genera of Micromphalidae n. fam.: (1) oblique view of the holotype of Micromphalus turris Knight, Reference Knight1945, USNM 111773, believed to be from the Upper Mississippian St. Genevieve Formation of Elizabethtown, Kentucky, showing the wide selenizone on the upward-projecting shoulder at the outer edge of the upper whorl surface. (2–5) Photographs of the holotype of Omocordella curta (Dunbar, Reference Dunbar1920), YPM 9752, from the Lower Devonian Rockhouse Formation of Hardin County, Tennessee: (2) apertural view showing silicification features and aperture clogged with matrix; (3) abapertural view showing growth lines and what appears to be silicified shell covering part of the specimen; (4) oblique view showing the wide selenizone on upwardly projecting shoulder; (5) apical view showing growth lines on upper whorl surface. (6–8) Photographs of a well-preserved specimen, CM 1920, from the Lower Devonian of Hardin County, Tennessee: (6) apertural view showing growth lines and fine spiral ornamentation; (7) abapertural view showing the growth lines curving into the selenizone on the shoulder; (8) oblique view showing the wide selenizone. (9–12) Apertural, abapertural, basal, and apical views of CM 1919, from the Lower Devonian of Hardin County, Tennessee. (13, 14) Photographs of a possible gerontic specimen, YPM 9727 (designated the lectotype of Distemnostoma princeps Dunbar, Reference Dunbar1920), from the Lower Devonian Rockhouse Formation of Hardin County, Tennessee: (13) apical view showing what appears to be a deformed selenizone; (14) basal view showing the formation of a basal sinus. (15) Illustration of the reconstructed Omocordella curta. Scale bars=5 mm.

Rohr and Frýda (Reference Rohr and Frýda2001) proposed the new genus Slehoferia based on Trochonema excavatum Barrande in Perner, Reference Perner1903, a small (height=7 mm), low-spired, trochiform gastropod having shouldered whorls, a concave upper whorl surface, and a shallow sinus culminating on the rounded shoulder. The type specimen attracted their attention because it preserved a round concentric, calcareous operculum. Rohr and Frýda (Reference Rohr and Frýda2001) placed Slehoferia excavata (Barrande in Perner, Reference Perner1903) in the Trochonematidae Zittel, Reference Zittel1895 because of the gross similarities of shell shape with other members of that family, differing from other genera of trochonematids essentially by the lack of a lower whorl angulation and a true selenizone on the shoulder. In this regard, it has a shell shape and shallow sinus culminating on the whorl angulation essentially similar to that of Trochonema (Eunema) nitidum Ulrich in Ulrich and Scofield, 1897, suggesting that the two species belong to the same genus. Slehoferia pachyta Ebbestad and Bassett in Ebbestad et al. (Reference Ebbestad, Bassett, Dastanpour and Popov2008), an Ordovician gastropod from Iran, was placed in this genus because of its general similarity in shape to S. excavata, despite its lack of a well-developed sinus culminating on the shoulder angulation. They allied the two species despite their quite significant differences in development of a sinus because of a supposed morphological trend in reduction of the sinus and sinus keel (shoulder angulation) (see Wagner, Reference Wagner1996, Reference Wagner1999). The depth and development of the sinus is not the only problem with considering S. pachyta a species of Slehoferia, however. The placement of the very shallow sinus entirely on the concave upper whorl surface, rather than on the shoulder angulation, suggests that it belongs in a separate genus, and probably in a separate family, and that the whorl shape is more indicative of morphological convergence than of phylogenetic relationship. This is consistent with the kinds of general patterns of gastropod form discussed by Wagner and Erwin (Reference Wagner and Erwin2006).

Omocordella n. gen., represented by O. curta (Dunbar, Reference Dunbar1920) also falls within the family grouping of Micromphalidae due to the concave upper whorl surface and shallow sinus culminating on the rounded angulation of the shoulder. It differs from Micromphalus in being larger and in having a much lower spire and deeper, more conical base. Slehoferia is much smaller and has a wider, more convex whorl profile, but like O. curtum it is low spired (spire angle>120°).

The micromphalids resemble large species of the Seguenzioidea, a Triassic to recent superfamily (Bandel, Reference Bandel1979, Reference Bandel1991, Reference Bandel2009) of generally very small trochiform or turbinate vetigastropods (Bandel, Reference Bandel1979; Quinn, Reference Quinn1983; Sasaki, Reference Sasaki1998; Bouchet and Rocroi, Reference Bouchet and Rocroi2005; Kano, Reference Kano2008). These tiny gastropods have nacreous inner shell layers, monopectinate ctenidia, distinct inhalant and exhalant sinuses, and other features typically found only in caenogastropods. Although the micromphalids generally resemble seguenzioideans in some shell characters, it is highly unlikely that the two groups are closely related. Still, it is tempting to consider that the two families were at least ecologically and functionally, if not genetically, related.

Genus Omocordella new genus

Type species

Distemnostoma curtum Dunbar, Reference Dunbar1920.

Diagnosis

As for the species, by monotypy.

Etymology

The generic name Omocordella is derived from ώμος, shoulder, and κορδέλα, ribbon.

Occurrence

Early Devonian, Lochkovian, Rockhouse Formation, Hardin County, Tennessee, USA.

Remarks

Omocordella differs from other known genera of the Micromphalidae by being significantly larger and having a well-developed basal sinus. The genotype is about 1.5 times as large as Micromphalus turris and more than three times as large as Slehoferia excavata. Slehoferia more closely resembles Omocordella in general shell shape than does Micromphalus, but the basal sinus of Omocordella distinguishes it from the Ordovician genus. Omocordella most closely resembles Raphistoma Hall, Reference Hall1847, especially the genotype R. striatum (Emmons, Reference Emmons1842), in shell shape, including basal sinus, but differs in having a wide selenizone directed upward on the vertical shoulder of the upper whorl surface as opposed to having a narrow selenizone directed outward on the shell periphery at the intersection of upper and outer whorl surfaces.

Omocordella curta (Dunbar, Reference Dunbar1920)

Figure 4.2–4.15

1920 Distemnostoma curtum Dunbar, p. 145, pl. 4, figs. 10–12.

1920 Distemnostoma princeps Dunbar, p. 143, pl. 4, fig. 19 (not fig. 18).

Holotype

YPM 9752.

Diagnosis

Moderate size, inverted conical, spire depressed, last whorl inflated, sutures canaliculate, upper whorl surface deeply concave, culminating in broadly rounded upwardly projecting shoulder, sinus in outer lip, selenizone on shoulder, basal lip forming basal sinus.

Description

Shell moderate size, narrowly phaneromphalus, of at least four whorls, with spire only slightly elevated above last whorl, nuclear whorls unknown, spire angle 125° to 150°. Sutures canaliculate. Whorl profile inverted conical, base narrower than outer whorl face, periphery broadly convex, situated at top of outer whorl surface; upper whorl surface acutely convex between suture and outer whorl surface, forming raised, rounded shoulder; outer whorl surface very gently convex, almost flat to base. Base rounded, becoming slightly concave, curving tightly into umbilicus, much narrower than upper or outer whorl surfaces. Aperture oblique, ovate to subrectangular, higher than wide, broader at base than at suture, deeply notched at suture, gently rounded to nearly flat at base. Columellar lip not well known, seemingly without parietal inductura, twisting at base to form sinus. Outer lip with broad, moderately deep, somewhat asymmetrical, U-shaped sinus at top of shoulder generating selenizone; strongly prosocline from suture to top of shoulder, curving strongly forward to periphery then becoming prosocyrt on outer whorl surface to base; selenizone broad, somewhat asymmetrical. Spiral ornament of numerous, closely spaced lirae covering whorl surface; collabral ornament of coarse growth lines.

Material

Holotype, YPM 9752; additional specimen, YPM 9727 designated as the lectotype of D. princeps; other material, CM 1919, CM 1920.

Occurrence

The species is known only from Hardin County, Tennessee. The holotype and paratype are from the Rockhouse Formation at Rockhouse on Horse Creek; two additional specimens are from an unknown stratum, presumably the Rockhouse.

Remarks

The diagnosis and description of this species are based largely on CM 1920 (Fig. 4.6–4.8), which is actually a much larger and better preserved specimen than the holotype (Table 1). Dunbar’s holotype (Fig. 4.2–4.5) is considerably smaller and has been greatly affected by the knobby texture and formation of beekite rings typical of the kind of silicification found in the Rockhouse Shale. As such, the spiral ornamentation is almost completely obscured. It can be detected in a swatch of highly etched shell material on the abapertural side of the holotype, but requires magnification to see the faint partial lines.

The two Carnegie Museum specimens are part of the Bayet collection (Holland, Reference Holland1904). They were originally identified as Platyostoma niagarensis Hall, subsequently renamed Diaphorostoma niagarensis (Hall) in the museum collections, and labeled as being from the upper Silurian of Hardin County, Tennessee. Since they are identical to the holotype, however, it is highly unlikely that they were found in Silurian rocks.

This species resembles several members of the Raphistomatidae (sensu Wagner, Reference Wagner2002) in overall shell shape, especially Scalites Emmons, Reference Emmons1842 and Raphistoma Hall, Reference Hall1847, because of the subconical base, but lacks the shallow sutures, sharply angular periphery, flattened, horizontal to slightly elevated upper whorl surface, and narrow sinus and selenizone of those shells. The sutures of O. curta are deeply impressed and occur at the bottom of a trough-like concave upper whorl surface formed between the previous whorl and the high shoulder (Fig. 4.5, 4.8, 4.12). The periphery is broadly convex and occurs below the outer rim of the shoulder. The selenizone is broad, relatively shallow, and lies at the top of the shoulder above the periphery. Judging from well-preserved growth lines in CM 1920, the exhalant sinus and selenizone are shallow and asymmetrical (Fig. 4.8). The growth lines in this specimen begin at the suture, sweeping upward and abaxially to the top of the shoulder for about 1 mm, then bending more sharply adaxially for about 5 mm and, just above the whorl periphery, falling in a broad prosocyrt sweep over the outer whorl surface to the base. The sinus and its selenizone are the most meaningful of all of the characteristics of the species.

There is a fair amount of variation in this species. The earlier whorls are so depressed in the holotype and in CM 1919 that the spire is almost concave, giving them the appearance of a shell with the top lopped off (Fig. 4.2, 4.9). CM 1920, by contrast, has a moderately high spire similar to the aforementioned Raphistomatidae. The upper whorl surface of the holotype is more broadly rounded than in either of the other two shells, and the selenizone is concomitantly wider, although only slightly so. There is also some variation in the selenizone, although not enough to consider the three specimens different species. In the holotype, the deepest part of the selenizone occurs at just outside the highest part of the shoulder, whereas in the two Carnegie specimens it lies more inward, toward the suture. In addition, the sinus appears to become deeper with age. The selenizone is very shallow in younger whorls, essentially a relatively minor bend in the growth lines, but in the older whorls it occurs as an unmistakable band on the shoulder.

Dunbar (Reference Dunbar1920) and Knight (Reference Knight1941) considered the paratype, YPM 9727, to be part of the type material of Distemnostoma princeps (it was designated a paralectotype of that species). By contrast, I am including it as a fourth specimen of O. curta. Whereas the lectotype of D. princeps is high spired and has a shallow sinus occurring entirely on the shallow, concave upper whorl surface, rather than on a shoulder at the edge of the upper whorl surface (see the following), the paralectotype, which is only a fragment of a shell, has an Omocordella-type selenizone on the shoulder. It is obvious that the two specimens are not related. The descriptions of D. princeps, the genotype of Distemnostoma, given by both Dunbar (Reference Dunbar1920) and Knight (Reference Knight1941) included elements of both shells and therefore need to be corrected. Knight (Reference Knight1941) used the differences in sinus position and depth in these two shells to suggest that the species was highly variable, with the shape of the outer lip induced by attachment to an irregular surface. Knight was correct in stating that the outer lip of the lectotype of D. princeps was shallow and did not generate a selenizone. He was also correct in stating that the outer lip of YPM 9727 was deep, but was incorrect in stating that it generated a pseudoselenizone rather than a true selenizone. The ‘pseudoselenizone’ caused him to pronounce that Distemnostoma was not a pleurotomarian as Dunbar supposed. Because YPM 9727 is only a fragment, it is difficult to accurately assess its relationship to the other material referred to O. curta. Among its observable characteristics is what appears to be a wide umbilicus (Fig. 4.13, 4.14) that may have resulted from the whorls becoming somewhat disjunct in older shells (although just a fragment, YPM9727 is much larger than the other specimens). Another unusual aspect of the shell is the position and shape of the selenizone. Rather than having an upward-projecting shoulder, the upper whorl surface of this specimen is flatly rounded. The selenizone is much wider than in the other specimens, and the growth lines indicate the sinus was sigmoidal near the final part of the preserved shell (Fig. 4.13). Although only a fragment, it represents a portion of an even larger specimen than CM 1920, the largest of the complete shells. It is possible that the seemingly disjunct nature of the last whorl and the configuration of the growth lines within the selenizone represent a gerontic aspect of the species. It is also possible that this specimen represents a separate species, but that designation will require additional, better-preserved material to be certain. Figure 4.15 shows a probable reconstruction of Omocordella curta (Dunbar).

Superfamily and Family uncertain

Remarks

Two of Dunbar’s (Reference Dunbar1920) new genera do not resemble anything I have been able to find reported from the Paleozoic. These gastropods, Aulopea and Distemnostoma, are characterized by a very shallow subsutural sinus and a basal sinus similar to that of Omocordella. It is possible that they also belong to the Micromphalidae, but the lack of a shouldered whorl and lack of a true selenizone make that unlikely. They actually resemble the more trochiform Omphalotrochidae, differing mostly in the lack of a flattened base and the presence of the basal sinus. Distemnostoma princeps, in particular, resembles Omphalotrochus Meek, Reference Meek1864 in having a forward-projecting prong of the outer lip near the periphery, but again, it lacks a flattened omphalotrochid base.

Yochelson (Reference Yochelson1956) pointed out that the Omphalotrochidae were bound together by the presence of the subsutural sinus, suggesting that the commonality of this feature among the included genera reflected a physiological function similar to the deeper, narrower sinus of the ‘pleurotomarians.’ Because of the position and shallowness of the sinus in the omphalotrochids, they probably had a single ctenidium, and it probably was bipectinate. The same was probably true also for the two Dunbar genera. Figure 5 is a hypothetical reconstruction of the anatomy of Aulopea nelsoni Dunbar, Reference Dunbar1920 as an example.

Figure 5

Hypothetical generalized reconstruction of the anatomy of Aulopea nelsoni Dunbar, Reference Dunbar1920: A=anus; CT=ctenidium; E=eye; EC=exhalant current; EMC=edge of mantle cavity; F=foot; IC=inhalant current; LA=left auricle; LHG=left hypobrachial gland; LN=left nephridium; M=mouth; MC=mantle cavity; O=operculum; OS=osphradium; R=rectum; RA=right auricle; RHG=right hypobrachial gland; RN=right nephridium; T=tentacle; V=ventrical.

Genus Aulopea Dunbar, Reference Dunbar1920

1956 Cowwarrella Talant and Phillip, p. 61.

Type species

Aulopea nelsoni Dunbar, Reference Dunbar1920, by original designation.

Diagnosis

Small to moderate large, phaneromphalus, bluntly turbiniform, sutures impressed, whorls subcylindrical, base somewhat extended, umbilicus deep and narrow, aperture with broad U-shaped subsutural sinus but without selenizone, inner lip reflexed about umbilicus, curling around to form basal sinus, ornamentation of fine growth lines and gently rounded varices.

Occurrence

Early Devonian, Lochkovian, Rockhouse Formation, Hardin County, Tennessee, USA.; Lochkovian or Pragian, Marble Creek Limestone, Victoria, Australia.

Remarks

The flattened to slightly concave whorl surface of Aulopea, with its bulging rounded periphery in the upper third of the shell, gives it a sort of aberrant cylindrical, box-like appearance. In this respect it is essentially identical to Cowwarrella Talent and Philip, Reference Talent and Philip1956 from the Lower Devonian Marble Creek Limestone of Victoria, Australia (Tassell, Reference Tassell1977, called this ‘Toongabbie Limestone,’ but see Talent et al., Reference Talent, Mawson, Aitchison, Becker, Bell, Bradshaw, Burrow, Cook, Dargan, Douglas, Edgecombe, Feist, Jones, Long, Phillips-Ross, Pickett, Playford, Rickards, Webby, Winchester-Seeto, Wright, Young and Zhen2000). Talent and Philip (Reference Talent and Philip1956) felt that Cowwarrella was distinctive enough to challenge comparison with previously described Paleozoic genera, but a close comparison of shell form indicates Cowwarrella and Aulopea to be similar in all essential aspects. Talent and Philip (Reference Talent and Philip1956) stated that Cowwarrella had no sinus, but in their description of the outer lip they described a sinus (without using that word) that occurred on the broad shoulder. Tassell (Reference Tassell1977) confirmed this and pointed out that a lower sinus occurred on the shell that is deeper and narrower than the upper sinus. All of these features, the shell shape, upper sinus, and lower sinus, essentially fit the diagnosis of Aulopea, which has priority. In addition, both genera appear to be restricted to the Lower Devonian, and both occur within a brachiopod-crinoid dominated fauna.

Dunbar (Reference Dunbar1920) relegated Aulopea to the Strophostylidae (=Platyceratidae) because, in general, it resembled Holopea, Diaphorostoma (=Platyostoma), and Platyceras. The Rockhouse genus is distinguishable from those genera by its subcylindrical whorl profile. The type species A. nelsoni, in fact, could be easily confused with Platyostoma quadrangulare (Dunbar, Reference Dunbar1920), also from the Rockhouse Formation, were it not for the upper and lower sinuses of the former. The lower (abapical) sinus was one of the more important factors that led Dunbar to designate a new genus.

Aulopea nelsoni Dunbar, Reference Dunbar1920

Figure 6.1–6.6

Figure 6

Aulopea nelsoni Dunbar, Reference Dunbar1920, from the Lower Devonian Rockhouse Formation of Hardin County, Tennessee. (1–5) Photographs of the holotype, YPM 9725: (1) apertural view showing the umbilicus and narrowing of the aperture at the base; (2) abapertural view showing growth lines on the outer whorl surface; (3) apical view showing growth lines on the upper whorl surface; (4) oblique view showing the trace of growth lines from the suture to the base, with a typical growth line drawn on for emphasis (black line); (5) basal view showing the umbilicus and more tightly curved growth lines of the basal sinus. (6) Illustration of the reconstructed Aulopea nelsoni. Scale bars=5 mm.

1920 Aulopea nelsoni Dunbar, p. 145, pl. 4, figs. 14–16.

1941 Aulopea nelsoni; Knight, p. 48, pl. 86, figs. 1a, 1b.

Holotype

YPM 9725.

Diagnosis

As for the genus.

Description

Shell moderately large, low-spired, turbiniform, phaneromphalus, of at least three whorls, with spire only slightly elevated above last whorl, nuclear whorls unknown, spire angle > 120°. Sutures canaliculate. Whorl profile laterally compressed, last whorl subequally high and wide, periphery broadly rounded, occupying upper one-third of whorl; upper whorl surface convex, rising out of suture and curving evenly around to periphery, forming broadly rounded shoulder; outer whorl surface gently convex in younger whorls, last whorl laterally compressed below protruding periphery, becoming flat to concave at middle one-third of whorl, then gently convex on lower one-third of whorl. Base rounded, curving into umbilicus in irregular folds; umbilicus prominent, narrow but deep. Aperture narrow close to suture, wider in upper one-third, narrowing to base, with deep, rounded channel at base. Inner lip thin, wrinkled around umbilicus, parietal inductura very thin. Outer lip with broad, shallow, U-shaped sinus between suture and periphery, occupying entire upper whorl surface. Basal lip with prominent, broad, shallow, U-shaped sinus occupying most of base, about 5–6 mm wide according to growth lines. Growth lines on outer lip where preserved fine, prosocyrt over periphery, becoming prosocline to base; gently rounded varices on last whorl, formed at ~3 mm intervals close to aperture.

Occurrence

The species is known only from the Rockhouse Formation at Rockhouse on Horse Creek, Hardin County, Tennessee.

Remarks

Neither Dunbar (Reference Dunbar1920) nor Knight (Reference Knight1941) provided much detail of this species beyond the basic descriptions. In his redescription of Aulopea, Knight (Reference Knight1941) merely referred the reader to his discussion of Distemnostoma in which he speculated that the gastropod had a highly variable form induced by a stationary habit on an irregular surface, inferred but not stated to be a crinoid calyx. This is an odd suggestion considering the obvious presence of sinuses on both ends of the aperture, sinuses so regular and U-shaped (Fig. 6.3–6.5) that it is very difficult to think that they could be confused with shell growth around substrate irregularities. It is especially confusing given Knight’s description of the basal sinus (Fig. 6.5), which is thoroughly unlike the distorted reentrants seen in many platyceratids. It could be argued that the irregular folds seen on the base where it curves into the umbilicus (Fig. 6.1) are very similar to the outer lip of certain platyceratids, but the presence of distinct sinuses should put that argument to rest.

The adapical sinus is shallow and occupies the entire upper whorl surface (Table 1), which forms a broadly rounded shoulder rising out of the deeply channeled suture. This sinus is much too shallow to form a selenizone, but apparently still acted to channel the exhalant current from the mantle cavity (Fig. 5). Its position so close to the suture indicates that the gastropod was most likely monopectinate. The presence of the narrow, almost slot-like abapical sinus at the base of the aperture suggests the gastropod had a well-developed unidirectional inhalant current, possibly, but not likely, with a siphon. This well-developed inhalant sinus suggests that this genus was more advanced than the Vetigastropoda, possibly occurring as a sister group to the Neritimorpha or even the lower Caenogastropoda, although its placement within the general gastropod classification is uncertain. Figure 6.6 shows a hypothetical generalized reconstruction of Aulopea nelsoni Dunbar.

Genus Distemnostoma Dunbar, Reference Dunbar1920

Type species

Distemnostoma princeps Dunbar, Reference Dunbar1920, by original designation.

Diagnosis

As for the species, by monotypy.

Occurrence

Early Devonian, Lochkovian, Rockhouse Formation, Hardin County, Tennessee, USA.

Remarks

Distemnostoma includes a single species, D. princeps Dunbar, Reference Dunbar1920. Dunbar had named two species of his new genus according to three specimens, the lectotype and paralectotype of D. princeps (the genotype) and the holotype of D. curtum, but as explained previously, neither D. curtum nor the paralectotype of D. princeps is associated with this genus. This leaves only the lectotype of D. princeps (and the holotype of Holopea planidorsata Dunbar, Reference Dunbar1920; see the following) to define the genus. Knight (Reference Knight1941) thought that what Dunbar referred to as the selenizone in Distemnostoma was extremely variable. Considering that both Dunbar (Reference Dunbar1920) and Knight (Reference Knight1941) were dealing with two separate, unrelated genera, it is understandable that they might think that. Knight inferred that the genus lived attached to crinoid calyces and, therefore, Yochelson (Reference Yochelson1956) and Knight et al. (Reference Knight, Cox, Keen, Batten, Yochelson and Robertson1960) placed it in the family Platyceratidae, as synonyms of Platyceras (Visitator) Perner, Reference Perner1911.

Dunbar thought Distemnostoma resembled Omospira Ulrich (in Ulrich and Scofield, Reference Ulrich and Scofield1897) because of a supposed flattened shoulder with crescentic growth lines, what he considered a selenizone high on the whorl, and the shape of the lower whorl surface, which narrows into an inverted conical base. The paralectotype of D. princeps does resemble Omospira a great deal in gross shell shape. In comparing the two, Dunbar apparently was influenced by the high-spired nature and somewhat flattened to gently concave upper whorl surface of the paralectotype of D. princeps (Fig. 7.1, 7.2), the selenizone on the shoulder of the lectotype of D. princeps (actually, Omocordella), and the somewhat conical nature of the bases in the paralectotype of D. princeps and holotype of Omocordella curta. Once O. curta is excluded from the comparison, however, there are many obvious differences between the Distemnostoma and Omospira. For example, Omospira has a sinus and true eotomarioidean selenizone at the edge of the upper whorl surface just above the periphery, whereas the sinus of Distemnostoma occupies the entire upper whorl surface (Fig. 7.3) and does not generate a true selenizone in the sense of the Eotomarioidea, Pleurotomarioidea, Murchisonioidea, and other Vetigastropoda having paired bipectinate ctenidia and other organs within the mantle cavity. Distemnostoma also has a narrow umbilicus, and the thin inner lip wraps around into it (Fig. 7.1), whereas Omospira is anomphalus. It is interesting that Dunbar (Reference Dunbar1920) stated that Distemnostoma had a slit and selenizone whereas Omospira did not, when in fact the opposite is true.

Figure 7

Distemnostoma princeps Dunbar, Reference Dunbar1920, from the Lower Devonian Rockhouse Formation of Hardin County, Tennessee. (1–4) Photographs of the designated paralectotype, YPM 30922: (1) apertural view showing the flat to slightly concave upper whorl surface and development of a basal sinus; (2) abapertural view showing growth lines; (3) oblique view showing growth lines on the upper and outer whorl surfaces; (4) basal view showing development of a basal sinus. (5–8) Photographs of a juvenile specimen, YPM 9726 (the holotype of Holopea planidorsata Dunbar, Reference Dunbar1920): (5) apertural view showing growth lines and shell thickness; (6) apical view of the same showing faint growth lines on preserved shell of the upper whorl surface; (7) abapertural view showing silicification; (8) oblique apertural view showing shell thickness and inductura on interior of columellar lip. (9) Illustration of the reconstructed Distemnostoma princeps. Scale bars =5 mm.

Distemnostoma also possesses an abapical sinus at the base of the aperture, similar to that of Aulopea nelsoni Dunbar. Although the outer and lower lips of the aperture are broken, obscuring the finer details of the base, the form of the base near the aperture shows the growth lines wrapping tightly into the umbilicus (Fig. 7.1), similar to A. nelsoni (Fig. 7.1). This suggests the gastropod had a well-developed inhalant current more advanced than the Vetigastropoda.

There are other Paleozoic gastropods, representing various families, that have broad, shallow sinuses near the adapical suture, but the flattened to gently concave upper whorl surface appears to be unique to Distemnostoma. The Permian Cyclobathmus Knight, Reference Knight1940, originally considered a genus of Trochonematidae but currently assigned to Microdomatidae (Wagner, Reference Wagner2013), is similar to Distemnostoma in having a turreted or gradate, turbiniform shell with a broad, shallow sinus on the upper whorl surface. Cyclobathmus differs from the Devonian genus, however, by having an outward-sloping upper whorl surface and numerous strong, spiral cords covering the whorls, and by lacking an abapical sinus in the basal lip. Many genera of the Omphalotrochidae, such as Omphalotrochus Meek, Reference Meek1864, and Orecopia Knight, Reference Knight1945, have broad, shallow sinuses in their outer lips close to the adapical suture, but all have wide, generally flattened bases and most have wide umbilici and no abapical sinus. Several genera of the caenogastropod Loxonematidae and heterobranch Streptacididae have broad, shallow adapical sinuses and broad, shallow abapical sinuses, but all are high-spired, many-whorled, slender shells unrelated to Distemnostoma. Havlicekiela parvulum (Horný, Reference Horný1992), from the Lower Devonian of Bohemia also has a shallow adapical sinus, but unlike Distemnostoma it has coarse, collabral ribs and a bulbous shell.

Distemnostoma princeps Dunbar, Reference Dunbar1920

Figure 7.1–7.9

1920 Distemnostoma princeps Dunbar, p. 143, pl. 4, fig. 19 (not fig. 18).

1920 Holopea planidorsata Dunbar, p. 147, pl. 4, fig. 13.

1941 Distemnostoma princeps; Knight, p. 103, pl. 86, fig. 3a, 3b (not figs. 3c, 3d).

Lectotype

YPM 30922.

Diagnosis

Moderately large, high-spired, turbiniform, turreted, phaneromphalus, upper whorl surface a flat to concave subsutural ramp with broad, shallow, U-shaped sinus, base extended abapically into broad, shallow, U-shaped sinus.

Description

Shell moderately large, turbiniform, high spired with turreted whorls, narrowly phaneromphalus, of more than four whorls, nuclear whorls unknown, spire angle 56°. Sutures impressed. Whorl profile changing with age; younger whorls rounded to subconical, with lower part of outer whorl face narrower than upper part in more subconical whorls; later whorls with flat to concave upper whorl surface, acutely rounded periphery, and flatly convex outer whorl surface. Base broadly rounded, curving tightly into umbilicus in irregular folds; narrower than outer whorl surface, forming inverted cone. Aperture oblique, subrectangular. Inner lip with parietal inductura curving around into umbilicus. Outer lip with broad, shallow, asymmetrical, U-shaped sinus occupying entire upper whorl surface; gently concave from suture to near outer edge of upper whorl surface, becoming more linear and overlapping abaxially, projecting forward to periphery and over outer whorl surface. Basal lip extending abapically, forming slot-like sinus. Ornament of prosocyrt growth lines only, some coarser than others.

Material

Paralectotype, YPM 30922; holotype of Holopea planidorsata, YPM 9726.

Occurrence

The species is known only from the Rockhouse Formation of Hardin County, Tennessee.

Remarks

Dunbar’s (Reference Dunbar1920) type specimens have created a great deal of confusion. Although he designated D. princeps as the genotype of Distemnostoma, Dunbar did not designate which of the two specimens in his collection, the relatively complete high-spired form or the fragment, was to be the holotype. In fact, his only designations were, “the most perfectly preserved specimen” and “the best preserved specimen” (Dunbar, Reference Dunbar1920, p. 144). These were reposited at the Yale Peabody Museum as syntypes. Knight (Reference Knight1941) subsequently designated Dunbar’s “best preserved specimen” as the holotype and the “most perfectly preserved specimen” as a paratype. At the time, both specimens were catalogued as YPM 9727, but at some point they were separated and the second specimen (Knight’s paratype) was renumbered as YPM 30922. Then, in keeping with the Code of Zoological Nomenclature, the holotype was redesignated as lectotype and the paratype as paralectotype. Unfortunately, this history overly complicates the problem of identity. As explained previously, the lectotype of D. princeps is actually assignable to Omocordella.

The single specimen of Holopea planidorsata Dunbar (YPM 9726), the designated holotype of that species, is so similar to the earlier whorls of D. princeps that I cannot consider them separate genera and species. This specimen is smaller than the designated paralectotype of D. princeps (Table 1), but otherwise they are very similar in shell features. Both have evenly rounded early whorls, which, in the last whorl preserved of H. planidorsata becomes flattened on the upper whorl surface adjacent to the impressed suture (Fig. 7.5–7.7). Preserved growth lines indicate the upper surface is occupied by a broad, shallow, U-shaped sinus. The shell is minutely phaneromphalus, with a thin inner lip that wraps around the umbilicus. The greatest differences are in the shape of the aperture and the presence or lack of an abapical sinus. The aperture of H. planidorsata is more ovoid than subrectangular, but it is oblique and wider than high in both shells. The base of H. planidorsata is broken, so any suggestion of a slot-like sinus as seen in the paralectotype of D. princeps is obscured (Fig. 7.5, 7.8), but growth lines on both the outer whorl surface and base near the umbilicus suggest that it had an abapical sinus. A thin wash of parietal inductura occurs on this specimen, wrapping around the columellar lip into the umbilicus (Fig. 7.5, 7.8). Figure 7.9 shows a probable reconstruction of Distemnostoma princeps Dunbar.

Order Neritimorpha Koken, Reference Koken1896

Superfamily Platyceratoidea Hall, Reference Hall1879

Family Platyceratidae Hall, Reference Hall1879

Genus Platyostoma Conrad, Reference Conrad1842

Platyostoma quadrangulare (Dunbar, Reference Dunbar1920).

Figure 8.1–8.3

Figure 8

Photographs of the holotype of Platyostoma quadrangulare (Dunbar, Reference Dunbar1920), YPM 9735, from the Lower Devonian Rockhouse Formation of Hardin County, Tennessee: (1) apertural view showing the umbilicus and shell thickness; (2, 3) abapertural and apical views showing growth lines indicating the lack of a selenizone or sinus of any kind on the upper and outer whorl surfaces. Scale bars=5 mm.

1920 Diaphorostoma quadrangulare Dunbar, p. 147, pl. 4, fig. 20, 21.

Diagnosis

Moderately large, low-spired, naticiform, phaneromphalus, sutures impressed, whorls subrectangular, box-like, aperture subovate, outer lip without slit or sinus, inner lip thick, protruding trumpet-like from umbilicus.

Description

Shell moderately large, naticiform, narrowly phaneromphalus, of greater than four whorls with spire only slightly elevated above last whorl, nuclear whorls unknown, spire angle 115°. Sutures impressed to moderately canaliculate. Whorl profile subrectangular, with periphery at interface of upper and outer whorl surfaces; upper whorl surface gently convex, subhorizontal; outer whorl surface subvertical, primarily convex but with relatively narrow concavity just below midwhorl. Base gently convex, subhorizontal, turning somewhat more sharply into umbilicus. Aperture subovate to subrectangular, higher than wide, of approximately equal width throughout. Outer lip entire, without sinus or slit. Inner lip partially detached, trumpet-like, not wrapping around umbilicus, parietal inductura seemingly thick. Ornamentation of growth lines and gently raised varices or swellings at irregular intervals; growth lines prosocline from suture to periphery, becoming prosocyrt over outer whorl surface, orthocline to slightly prosocline over base, then prosocline into umbilicus.

Occurrence

The species is known only from the Rockhouse Formation of Hardin County, Tennessee.

Remarks

This relatively large shell (Table 1) is the only actual platyceratid gastropod Dunbar (Reference Dunbar1920) described and named from the Rockhouse Formation. The shape of the whorl is unusual as, typically, species of Platyostoma have well-rounded whorls. Even the unusual array of shapes exhibited by Platyostoma in the Devonian of New York (Hall, Reference Hall1879) includes only shells with rounded whorls. P. quadrangulare, shaped like a cylindrical box with squashed-in sides, however, appears to be unique. It has a configuration similar to Aulopea nelsoni. (see the preceding), which is not related. This is probably an example of convergence of morphology. It is also possible that it represents a new genus, but that determination will require additional and better-preserved specimens.