Solemya, Lamarck, 1818

Bailey, Jack Bowman, 2021, The genera that never were: The impact of Janeia and Janacekia on phyletic and taxonomic relations within the Solemyidae (Bivalvia: Protobranchia), Palaeontologia Electronica (a 12) 24 (1), pp. 1-47 : 2-12

SOLEMYA , ACHARAX , AND THE SOLEMYIDAE

Ligament and Support Structures in Solemya and Acharax

In the Solemyidae an understanding of the ligament and its supporting structures is fundamental in ascertaining the systematic relations among constituent genera. In the familiar solemyids Acharax and Solemya , the ligament is amphidetic, composed of two principal components: 1) a dorsally arched, parivincular portion posterior to the beaks (= primary ligament) consisting of an inner fibrous layer ( Figure 1 , lgf) attaching on each side at insertion grooves, bordered by narrow ridges (i.e., nymphae) ( Figure 1 , ne, ns) that function as attachment sites for an outer lamellar layer ( Figure 1 , lgl); and 2) an anterior extension (not shown) of the lamellar outer layer in front of the beaks that is often asymmetrically attached between the left and right valves (see Carter, 1990, p. 174; Bailey, 2011, p. 19).

In Acharax the primary ligament is external, attaching at external nymphae ( Figures 1 , 2 , ne) and insertion grooves ( Figure 1 , gr). However, in all subgenera of Solemya the primary ligament is sunken to a shallow internal (= submarginal) position immediately below the posterodorsal margins of the shell. There it is attached to a pair of submarginal nymphae and flanking grooves ( Figure 1 , ns, gr and Figure 2 , ns), each supported at its anterior end by attachment to a ridge-like buttress (= rib, ridge, prop, or clavicle of various authors), i.e., a narrow thickening of the valve that extends along the anterior margin of the posterior adductor muscle ( Figure 2 , btc). Submarginal nymphae in Solemya are conventionally termed chondrophores (e.g., Dall, 1908) thus obscuring their homological relation to external nymphae. Unlike Solemya , the internal buttress ( Figure 2 , bts) of Acharax is not attached to nymphae and is variable in development, sometimes prominent but often weak or obsolescent. Peripheral thickenings of adductor attachment sites in other bivalve taxa ( Bailey, 1983; Taylor et al., 2008) suggest that the buttress in Acharax functions in part for added reinforcement of the thin valve along the posterior adductor margin ( Bailey, 2016).

Because of its remarkable similarities to Solemya in gross morphology, Acharax was originally recognized as a subgenus of Solemya (see Dall, 1908, p. 2). Even the diagnostic primary ligament, aside from its external vs. submarginal location, is fundamentally similar in the two genera ( Figure 1 ; see also Bailey, 2011, text-fig. 2). The homologies seem obvious. Together, Acharax and Solemya arguably form a monophyletic group (i.e., clade) in which the external ligament and nymphae of Acharax , the probable plesiomorphic condition ( Figure 1 A-B), later achieved submarginal (apomorphic) status in Solemya through depression of the nymphae and ligament below the posterodorsal margin ( Figure 1 C-E), along with attendant overgrowth of a thin outer prismatic layer (Carter, 1990, fig. 17; Bailey, 2011; Carter, et al., 2012, fig. 216). As noted by Waller (1998, p. 19), the outer prismatic layer ( Figure 1 , opl) covering the ligament “is a secondary [apomorphic] feature that is not present in the earliest ontogeny of the dissoconch…nor in the earliest members of the Solemyoidea and Solemyidae .” Depression of the nymphae may have been facilitated by the natural tendency of the thin shells and periostracum ( Figure 1 , pe) to crack and repair in the umbonal and ligamental regions during growth (Waller, 1990; Bailey, 2011).

In Solemya , depression of the nymphae below the hinge margin has resulted in: 1) fusion of the anterior end of each nymph directly to the internal buttress such that the function of the buttress becomes altered by exaptation from an original one of reinforcing the attachment site of the posterior adductor muscle along its anterior border ( Figure 2A , bts) to one of serving as a supporting brace or prop for the submarginal nymph ( Figure 2B , btc; see also Bailey, 2016); and 2) dorsal occlusion, either by embayment, truncation, or intersection, of the posterior adductor muscle by the nymph. In the case of intersection, the posterior adductor partially continues around the obstructing nymph, passing posteriorly above it ( Figure 2B , pao). The resulting cross-cutting relationship firmly establishes the submarginal posterior ligament and nymphae in Solemya as apomorphic traits (see also pl. 4, fig. 2 of Pojeta, 1988, and figs. 3h, 3i of Hryniewicz et al., 2014).

Proxy Characters Linked to Nymphal Placement

In Acharax and Solemya , nymphal placement has a significant influence on the internal morphology of the shell. If neither nymphae nor ligament are preserved, the two genera may nevertheless be distinguished from each other in internal molds or incomplete specimens so long as traces of the buttresses and posterior adductor scars are present. Serving as proxies of nymphal placement, two functionally linked character states are here designated as follows: Where the nymphae are external (a plesiomorphy), the buttress will be simple and the posterior adductor will be entire. Where the nymphae are submarginal (an apomorphy), the buttress will be compound, and the posterior adductor will be occluded. In Acharax : 1) the buttress is simple ( Figure 2A , bts), that is, from the anterior limit of the posterior adductor scar, it extends dorsally to the underside of the umbo without any evident attachment to a nymph; and 2) in the absence of a submarginal nymph the posterior adductor muscle is entire, that is, whole and unobstructed ( Figure 2A , pae). The condition is reversed in Solemya : 1) the buttress is compound, that is, it adjoins the anterior terminus of the sub- FIGURE 3. Internymphal gaps in fossil examples of Acharax . A, Acharax doderleini (Mayer, 1861) , Pliocene of Italy; dorsal view showing external nymphae and ligament insertion grooves separated by internymphal gap that in life is covered by the ligament. In Solemya , the gap is secondarily closed off by the addition of a thin, outer prismatic shell layer. (Photo by permission, Taviani et al., 2011, fig. 5; yellow arrows with notations added here.) B-E, Solemya puzosiana de Koninck, 1842 , Lower Carboniferous of Belgium. B, right lateral view of an incomplete articulated specimen (de Koninck, 1885, pl. 23, fig. 33). C, same specimen in dorsal view (de Koninck, 1880, pl. 23, fig. 34), reversed, anterior at left. D, right lateral view of an articulated specimen (de Koninck 1842, pl. 5, fig. 2b). E, same specimen in dorsal view (de Koninck 1842, pl. 5, fig. 2a). Hind (1900, p. 439) synonymized this specimen with Solemya primaeva Phillips, 1836 , the type species of Janeia King, 1850 . Internymphal gaps shown in de Koninck’s figures are consistent with Acharax but not Solemya . Red arrows with notations added here. Abbreviations: ne - external nymph; ig - internymphal gap; gr - ligament insertion groove.

marginal nymph, the resulting configuration of the two forming a “7” shape ( Figure 2B , btc); and 2) the posterior adductor is thereby occluded, becoming dorsally truncated, embayed, or intersected by the nymph ( Figure 2B , pao).

One additional proxy needs to be discussed. Among fossil solemyids, paired external nymphae are often visibly separated by a narrow space whenever the primary ligament is not preserved. In error, Hind (1900, p. 442) referred to the space as a “slit for the ligament.” Herein termed the internymphal gap ( Figures 1B , 3A-E, ig), it is a character seen in living and fossil Acharax but not in Solemya . In his original diagnosis Dall (1908, p. 2) referred to it thusly: “Ligament opisthodetic, wholly external, visible internally only where it crosses the gap between the margins of the valves.” In Acharax , the internymphal gap is covered in life by the arch of the external ligament and is thereby visible only when the ligament is not preserved. In contrast, the internymphal gap is missing in Solemya because the nymphae are submarginal and the former gap is secondarily covered over by the aforementioned outer prismatic shell layer ( Figure 1 D-E, opl). In addition to Acharax , the internymphal gap (a probable symplesiomorphy) is observed in other fossil solemyid genera with external nymphae including both Clinopistha Meek and Worthen, 1870 , and Dystactella Hall and Whitfield, 1872 .

Solemyid Origins and Phylogeny

Parivincular ligaments with supporting nymphae are key characters used in Pojeta’s (1988) proposed phylogeny of fossil and extant Solemyidae ( Figure 4A ). Pojeta (1988) and Waller (1990, 1998) posited that these characters were acquired from nymph-bearing ctenodontid nuculoid ancestors such as Ctenodonta nasuta (Hall, 1847) and, especially, Ctenodonta tennesseensis Pojeta, 1988 , which, aside from the taxondont hinge, is strikingly similar to the early solemyid Dystactella Hall and Whitfield, 1872 . Strong phylogenetic connections of the solemyids and nuculoids are supported by numerous other studies (e.g., Carter et al., 2000; Carter, 2001; Giribet and Distel, 2003; Giribet, 2008; Bailey, 2011; Carter et al., 2011; Bieler et al., 2014). However, based on stratigraphic occurrences Cope (2002) suggested that these characters might have arisen independently in the two groups. Whereas the solemyid Ovatoconcha is dated as late Early Ordovician, ligamental nymphae do not occur in ctenodontids until the Middle Ordovician. Alternatively, Cope also posited that nymphae could be persistent characters derived from earlier, as yet undocumented ctenodontid stock. Indeed, persistent solemyid traits are fairly common among much later nuculoids, for example, in Spathelopsis oakvalensis Peck, Bailey, Heck, and Scaiff, 2009 from the Mississippian of West Virginia (see Peck et al., 2009, p. 959; Bailey, 2011, p. 14).

Pojeta’s (1988, fig. 3) phyletic scheme divided the Solemyidae into two sister subfamilies, the Clinopisthinae and the Solemyinae ( Figure 4A ). With the exception of Solemya , external placement of the nymphae and ligament is broadly shared in both subfamilies. Among the Solemyinae it occurs in Psiloconcha Ulrich, 1894 (Ordovician), Acharax (Devonian-Holocene) , and Mazonomya Bailey, 2011 (Pennsylvanian). Among the Clinopisthinae , it occurs in Clinopistha Meek and Worthen, 1870 (Devonian-Permian) and Dystactella Hall and Whitfield, 1872 (Ordovician-Permian). Thus, it is likely to represent the primitive (symplesiomorphic) condition for the Solemyidae as a whole. In contrast, the submarginal ligament structure of Solemya is unique, representing a derived (autapomorphic) condition evidently not occurring prior to the Mesozoic ( Figure 4B ). Although ligament and nymphae are as yet unknown in Ovatoconcha , the genus is a near match for its possible congener, Psiloconcha with respect to: 1) shell profile, aspect ratio and shell gapes; 2) umbonal placement and elevation; 3) small size and high placement of the posterior adductors to accommodate the underlying hypertrophied gills and hypobranchial gland; and 4) pyriform outline and elevated placement of the anterior composite (adductor/visceral retractor) scars ( Bailey, 2011, p. 12). Radial elements in the prosopon are not well developed in either genus, i.e., weak in Ovatoconcha and mostly lacking in Psiloconcha .

The Problem of Range

In order to establish a convincing phylogenetic derivation of Solemya from Acharax ancestry, the geologic timing of the split must be consistent with the foregoing character analysis.

The Y-shaped burrows of the ichnogenus Solemyatuba Seilacher, 1990 occur as early as the Ordovician (Seilacher, 1990). Although they are usually attributed to Solemya , comparable burrows are also associated with Acharax (Stanley, 1970, Campbell et al., 2006; Ros-Franch et al., 2014). Thus, their occurrence cannot be attributed to a particular genus.

Early authors, including Beushausen (1895), Zittel (1913), Quenstedt (1930), Shimer and Shrock (1944), as well as more recent authors, notably Cox (1969), gave the stratigraphic range of Solemya as Devonian-Holocene under the apparent misapprehension that the submarginal ligament and compound buttress were primitive characters, whereas the external ligament and simple buttress of Acharax were tacitly regarded as later derivations. For example, Cox (1969) regarded Acharax as a later genus with a verified range limited to Miocene-Holocene. However, Pojeta (1988) subsequently emended the range of each genus thusly: For Solemya , Upper Pennsylvanian-Holocene; and for Acharax , Lower Permian-Holocene, (with a dubious Middle Devonian occurrence). Pojeta’s basis for concluding that Solemya occurred as early as the Upper Pennsylvanian was a single specimen, “ Solemya sp. ” (USNM 415967), from the Hertha Limestone, Erie, Kansas (Pojeta, 1988, pl. 23, figs. 5-8). However, this specimen is a probable Acharax because the alleged free ends of the “chondrophores” shown in his figure 8 are probably the compressed and broken termini of external nymphae, a conclusion supported by his figure 5 of the same specimen showing: 1) simple buttresses that dorsally reveal no indication of either added reinforcement or attachment to internal nymphae; and 2) posterior adductor scars that are entire, lacking the expected dorsal occlusion caused by submarginal nymphae.

Although the study of Dickins (1963), like that of Pojeta (1988), ostensibly provided support for the late Paleozoic occurrence of Solemya, Dickins himself was tentative regarding the final generic assignment of his material. His “ Solemya” holmwoodensis Dickins (1963) from the Lower Permian of western Australia is herein assigned to Acharax inasmuch as the holotype and three paratypes show strong external nymphae as well as the anterior ligamental extension (Dickins, 1963, p. 60; pl. 7, figs. 1-9). Even a dorsally arched portion of the external parivincular ligament is preserved in the holotype (Dickins, 1963, pl. 7, fig. 2). In error, Hajkr et al. (1978, p. 14) transferred this species to the Sanguinolitidae Miller, 1877 .

Contemporary studies support the view that Acharax arose much earlier than Cox (1969) had supposed. For example, Acharax has been reported from the Early Devonian of Arctic Canada ( Bailey, 2011, 2016; Bailey and Prosh, 2016). In addition, there are Pennsylvanian occurrences in both Kentucky (Carter, 1990) and the Mazon Creek Lagerstätte of Illinois ( Bailey, 2011). Furthermore, all of the Devonian-Permian examples of alleged Solemya (and its doppelgänger, Janeia King, 1850 ) reviewed herein have external ligament and nymphae like Acharax , whereas the submarginal ligament and associated compounding of the buttress of Solemya appear to be Mesozoic modifications (see Appendix; also Bailey, 2011, 2016).

Unverified examples of Solemya have been reported from the Permian (Ciriacks, 1963; Sterren and Cisterna, 2010) and Triassic (Conrad, 1870). Ciriacks (1963, p. 42, pl. 5, figs.12, 13) tentatively designated a nearly featureless internal mold of a possible solemyid as Solemya sp. (UM 5275 = “ S. radiata ?” sensu Branson, 1930) from the Park City Formation, near Cody, Wyoming. Although Ciriacks (p. 42) described the specimen as “insufficiently preserved for specific identification”, the form superficially resembles Dystactella . Sterren and Cisterna (2010, p. vi) reported an occurrence of Solemya from the Early Permian of Argentina, but they provided neither figure nor description. Conrad (1870, p. 102) described the shell exterior and radial prosopon of “an obscure cast” he designated Solemya triasina Conrad, 1870 , from the Triassic of Perkiomen Creek, Pennsylvania. However, data on the hinge and ligament are lacking, and no figure of the specimen, now lost, was provided.

Other studies place the earliest occurrence of Solemya in either the Jurassic (Coan et al., 2000; Imhoff et al., 2003; Neulinger et al., 2006) or Cretaceous (Dechaseaux, 1952). Hryniewicz et al. (2014) reported convincing examples of Solemya from Late Jurassic-Early Cretaceous hydrocarbon seeps of Spitzbergen. The silicon rubber casts of their (fig. 3h-i) internal molds of Solemya (Petrasma) cf. woodwardiana Leckenby, 1859 clearly show the compound buttress and occluded posterior adductor scar. It should be noted, however, that alleged “ Solemya woodwardiana ” reported by Duff (1978) from the Lower Oxford Clay (Middle Jurassic of England) is a possible Acharax . Although data on the posterior adductor is lacking, a simple buttress is evident. In addition, the butterflied specimen figured by Duff (1978, pl. 1, fig. 40) shows what appear to be traces of a deltoid parivincular ligament stretching across an internymphal gap with a possible nymph and insertion groove preserved in the right valve.

The accumulation of 18S rRNA gene sequence disparities (Neulinger et al., 2006) separating extant species of Acharax and Solemya seem consistent with a Mesozoic (possibly Jurassic) split. The timing of the split is interesting. Among the Bivalvia in general, the Mesozoic is associated with episodes of rapid expansion and diversification (Ros et al., 2012).

Taxonomic Dissent

Despite extensive study, solemyoid classification remains in dispute, noticeably in regard to the systematic relations of Solemya and Acharax ( Table 1). Owing to the external ligament, Cox (1969) in Part N of the Treatise first elevated Solemya (Acharax) Dall from subgeneric to full generic status while adding, in error, the vesicomyid, Adulomya Kuroda, 1931 (i.e., see Amano and Kiel, 2011) to the Solemyidae and, unfortunately, giving full recognition to the problematic Janeia King, 1850 as a subgenus of Solemya .

Based on the ontogenetic expansion vector of the shell and underlying soft anatomy, Pojeta’s (1988) landmark study argued for two main phyletic lines ( Figure 4A ) within the Solemyidae , thereby subdividing the Solemyidae into two subfamilies: 1) the Solemyinae for anteriorly elongated solemyids with “barely discernible” beaks and umbos (i.e., Solemya , Acharax , and Psiloconcha ); and 2) the Clinopisthinae for anteroventrally elongated solemyids with more conspicuous umbos and beaks (i.e., Clinopistha and Dystactella ). This system was followed by Bailey (2011) and used herein (see Figure 4B , Table 1 and Appendix).

Scarlato and Starobogatov (1979) split the order Solemyoida into two suborders, the Nucinellina and Solemyina . Emphasizing the relative significance of the submarginal vs. external ligament, they divided the Solemyina into two taxonomic divisions: 1) the Superfamily Solemyoidea /Family Solemyidae was proposed for genera with submarginal ligaments such as Solemya ; and 2) the Superfamily Acharacoidea /Family Acharachidae was proposed for genera with an external ligament, including Acharax and, in error, the vesicomyid Adulomya (see Amano and Kiel, 2011). A similarly divided system was endorsed by Zardus (2002), Nevesskaja (2009), and Nevesskaja et al. (2013).

Maxwell (1988) likewise placed Acharax and Solemya in separate superfamilies, the Acharac-

Cox, 1969

Class BIVALVIA Linné, 1758

Subclass CRYPTODONTA Neumayr, 1884

Order SOLEMYOIDA Dall, 1889

Superfamily SOLEMYOIDEA Adams and Adams, 1857 (1840)

Family SOLEMYIDAE Adams and Adams, 1857 (1840)

(Includes Solemya and Acharax ; Clinopistha [= Dystactella ] placed in

Ctenodontidae Wöhrmann )

Scarlato and Starobogatov, 1979

Class BIVALVIA Linné, 1758

Superorder PROTOBRANCHIA Pelseneer, 1889 (= Nuculoida Dall, 1889)

Order SOLEMYIDA Newell, 1965

Suborder SOLEMYINA Newell, 1965 (= Solenomyina Dall, 1889)

Superfamily ACHARACOIDEA Scarlato et Starobogatov, 1979

Family ACHARACIDAE Scarlato et Starobogatov, 1979

( Acharax placed here)

Superfamily SOLEMYOIDEA H. Adams et A. Adams, 1857

Family SOLEMYIDAE H. Adams et A. Adams, 1857

( Solemya placed here)

Suborder NUCINELLINA Scarlato et Starobogatov, 1971

Superfamily AFGHANODESMATOIDEA Scarlato et Starobogatov, 1979

Family AFGHANODESMATIDAE Scarlato et Starobogatov, 1979

Superfamily MANZANELLOIDEA Chronic, 1952

Family MANZANELLIDAE Chronic, 1952

Family NUCINELLIDAE Vokes, 1956

Superfamily HUXLEYOIDEA Scarlato et Starobogatov, 1971

Family HUXLEYIIDAE Scarlato et Starobogatov, 1971

Allen and Hannah, 1986

Class BIVALVIA Linné, 1758

Subclass PROTOBRANCHIA Pelseneer, 1889

Order SOLEMYOIDA Dall, 1889

Family SOLEMYIDAE Gray, 1840

(Includes both Solemya and Acharax )

Family NUCINELLIDAE Vokes, 1956

Maxwell, 1988

(Class BIVALVIA)

(Subclass PROTOBRANCHIA)

Order SOLEMYOIDA Dall, 1889

Suborder SOLEMYINA Dall, 1889

Superfamily SOLEMYOIDEA Gray, 1840

Family SOLEMYIDAE Gray, 1840

( Solemya placed here)

Family ACHARACIDAE Scarlato and Starobogatov, 1979

( Acharax placed here)

Suborder NUCINELLINA Scarlato and Starobogatov, 1971

Superfamily MANZANELLOIDEA Chronic, 1952

Family MANZANELLIDAE Chronic, 1952

Cope, 1996

Class BIVALVIA Linnaeus, 1758

Subclass LIPODONTA Cope, 1995

Order SOLEMYOIDA Dall, 1889

Superfamily SOLEMYOIDEA Adams and Adams, 1857

Family SOLEMYIDAE Adams and Adams, 1857

Pojeta, 1988

Class PELECYPODA Goldfuss

Subclass PALAEOTAXODONTA Korobkov

(= Subclass PROTOBRANCHIA Pelseneer)

Superfamily SOLEMYOIDEA Adams and Adams

Family SOLEMYIDAE Adams and Adams

Subfamily SOLEMYINAE Adams and Adams

(Includes both Solemya and Acharax )

Subfamily CLINOPISTHINAE Pojeta, 1988

(includes both Clinopistha and Dystactella )

Superfamily NUCINELLOIDEA Vokes, 1956

Family NUCINELLIDAE Vokes, 1956

Family MANZANELLIDAE Chronic, 1952

Amler, 1999

(Class BIVALVIA)

Subclass LIPODONTA (Iredale, 1939) Cope, 1995

Order SOLEMYOIDA Dall, 1889

Superfamily SOLEMYOIDEA (Adams and Adams, 1857) Gray, 1840

Family SOLEMYIDAE (Adams and Adams, 1857) Gray, 1840

( Solemya placed here) Family JANACEKIIDAE Růžička and Řehoř, 1978

( Janacekia placed here)

Family ACHARACIDAE Scarlato and Starobogatov, 1979

( Acharax placed here)

Nevesskaja, 2009; Nevesskaja et al., 2013

Class BIVALVIA

Superorder PROTOBRANCHIA Pelseneer, 1889

Order SOLEMYIDA Newell, 1965

Superfamily SOLEMYOIDEA H. et A. Adams, 1857

Family SOLEMYIDAE H. et A. Adams, 1857

( Solemya placed here)

Superfamily ACHARACOIDEA Scarlato et Starobogatov, 1979

Family ACHARACIDAE Scarlato et Starobogatov, 1979

( Acharax placed here)

Superfamily MANZANELLOIDEA Chronic, 1952 Family MANZANELLIDAE Chronic, 1952

Family NUCINELLIDAE Vokes, 1956

Beiler et al., 2010

Class BIVALVIA Linnaeus, 1758

Subclass PROTOBRANCHIA Pelseneer, 1889

(= Sublcass PALAEOTAXODONTA Korobkov, 1954)

Order SOLEMYOIDA Dall, 1889

Superfamily MANZANELLOIDEA Chronic, 1952

Family MANZANELLIDAE Chronic, 1952

Superfamily SOLEMYOIDEA Gray, 1840

Family SOLEMYIDAE Gray, 1840

Subfamily SOLEMYINAE Gray, 1840

(= Family JANACEKIIDAE Růžička and Řehoř, 1978 )

(Includes both Solemya and Janacekia )

Subfamily ACHARACINAE Scarlato and Starobogatov, 1979

( Acharax placed here)

Subfamily CLINOPISTHINAE Pojeta, 1988

Carter et al., 2011

Class BIVALVIA Linnaeus, 1758

Subclass PROTOBRANCHIA Pelseneer, 1889

(= Subclass PALAEOTAXODONTA Korobkov, 1954)

Superorder NUCULIFORMII Dall, 1889

(= Superorder FOLIOBRANCHIA Ménégaux, 1889 )

Order SOLEMYIDA Dall, 1889

Superfamily SOLEMYOIDEA Gray, 1840

Family SOLEMYIDAE Gray, 1840

Subfamily SOLEMYINAE Gray, 1840

(Includes both Solemya and Acharax ) Subfamily JANACEKIINAE Růžička and Řehoř, 1978

( Janacekia placed here)

Family CLINOPISTHIDAE Pojeta, 1988

Family CTENODONTIDAE Wöhrmann, 1893

Family OVATOCONCHIDAE Carter, 2011

Bailey, 2011 (and herein)

Class BIVALVIA Linnaeus, 1758

Subclass PROTOBRANCHIA Pelseneer, 1889

(= Subclass PALAEOTAXODONTA Korobkov, 1954)

Superorder NUCULIFORMII Dall, 1889

Order SOLEMYOIDA Dall, 1889

Superfamily SOLEMYOIDEA Gray, 1840

Family SOLEMYIDAE Gray, 1840

Subfamily SOLEMYINAE Gray, 1840

(includes both Solemya and Acharax )

Subamily CLINOPISTHINAE Pojeta, 1988

(includes both Clinopistha and Dystactella )

oidea and Solemyoidea , respectively, whereas Amler (1999) recognized only one superfamily, the Solemyoidea made up of three families, the Solemyidae , the Janacekiidae , and the Acharacidae .

Bieler et al. (2010) combined and modified the classifications of both Pojeta (1988) and Scarlato and Starobogatov (1979) by dividing the Solemyidae into three subfamilies, the Clinopisthinae , the Acharacinae , and the Solemyinae , the latter considered as a synonym of the Janacekiidae Růžička and Řehoř 1978 . However, this system is problematic in that it results in the division of the Acharax- Solemya clade into two paraphyletic subgroups. Furthermore, treating the Janacekiidae as a synonym of the Solemyinae is antithetical because the namesake, Janacekia Růžička and Řehoř, 1978 , has the external ligament and nymphae like Acharax but unlike Solemya , as discussed further below.

Carter et al. (2011) proposed dividing the order Solemyida into two superfamilies, the Solemyoidea and the Manzanelloidea . In this system, the Solemyoidea comprises four families: the Solemyidae , consisting of the subfamilies Solemyinae (containing both Acharax and Solemya ), as well as the Janacekiinae , Clinopisthidae , Ctenodontidae , and Ovatoconchidae . However, because Janacekia is herein accepted as a junior synonym of Acharax , the Janacekiinae becomes superfluous. In addition, Ovatoconcha , because of its similarities to Psiloconcha , may arguably be placed in the Solemyinae .

If, indeed, as posited herein, Solemya was derived from Acharax ancestors by, perhaps, mid- Mesozoic times through depression of the primary ligament and nymphae to a submarginal position, the two genera are monophyletic, a conclusion independently corroborated by both Sharma et al. (2013) and Combosch et al. (2017) in each of their phylogenetic tests of molecular markers in the genomic DNA of Solemya and Acharax . The 18S rRNA analysis of these genera by Taylor et al. (2008) reached a similar conclusion. According to Mayr and Ashlock (1991) higher taxa should consist of monophyletic groups separated from groups of similar rank by a distinctive gap. Their understanding echoes Schenck’s (1934, p. 55) classic criterion: “One basic principle is that a family should be monophyletic.” When inherent misunderstandings associated with Janeia and Janacekia are eliminated, the morphologic and temporal gaps separating Acharax and Solemya are not only minimized, but also consistent with DNA studies. Thus, of the various systems described above, Pojeta’s (1988) taxonomic scheme combining both genera within a single subfamily ( Solemyinae ) remains a viable alternative to placement into separate (paraphyletic) superfamilies or families.

Figures

FIGURE 1. Diagrammatic transverse sections comparing the posterior portion of the ligament and nymphae of Acharax and Solemya. A, the upward-arching (parivincular) external ligament and nymphae, i.e., the plesiomorphic condition as seen in Acharax. (Drawing based on Carter, 1990, fig. 17D; Bailey, 2011, pl. 4, figs. 4-6; and Taviani et al., 2011, fig. 5). B, external nymphae of Acharax with the ligament removed. When the ligament is not preserved, the nymphae are visibly separated by an internymphal gap. C, hypothetical intermediate condition showing beginning stages of ligament depression and inward rotation of nymphae (arrows). D, submarginal ligament and nymphae - the apomorphic condition seen in Solemya. The depressed ligament is secondarily overgrown by a shelly outer prismatic layer. (After Carter, 1990, fig. 17A.) E, the submarginal nymphae of Solemya with the ligament removed. Notice the outer secondary layer that covers the former internymphal gap. Abbreviations: ne - external nymph, ns - submarginal nymph gr - ligament insertion groove, ig - internymphal gap, lgf - fibrous inner layer of ligament, lgl - lamellar outer layer of ligament, pe - periostracum, opl - outer (secondary) prismatic shell layer.

FIGURE 2. Comparative internal views of the posterior portion of the right valves of Acharax (A) and Solemya (B) showing character states of the nymphae, internal buttresses and posterior adductor scars. In Acharax the nymph (ne) is external, the condition of the internal buttress is simple (bts), and the condition of the posterior adductor muscle scar is entire (pae). In Solemya, depression of the nymph to a submarginal position (ns) results in a compound buttress (btc) and an occluded posterior adductor scar (pao).

FIGURE 4. Phyletic schemes and biostratigraphic ranges (solid bars) of Paleozoic solemyoid genera. A, phyletic scheme of Pojeta (1988, fig. 3) postulating derivation of the Clinopisthinae and Solemyinae from nuculoid ancestors (i.e., Ctenodonta). B, proposed modifications (in red) of Pojeta’s scheme based on studies herein. Abbreviations of generic names: A. = Acharax; Cl. = Clinopistha; Ct. = Ctenodonta; D. = Dystactella; ‘J.’ = Janeia; and M. = Manzanella.