Pholadinae Lamarck, 1809

Monari, Stefano, 2009, Phylogeny and biogeography of pholadid bivalve Barnea (Anchomasa) with considerations on the phylogeny of Pholadoidea, Acta Palaeontologica Polonica 54 (2), pp. 315-335 : 316-318

Subfamily Pholadinae Lamarck, 1809 Genus Barnea Leach in Risso, 1826 Subgenus Anchomasa Leach, 1852

Type species: Anchomasa pennantiana Leach, 1852 (a junior synonym of Pholas parva Pennant, 1777 ).

Characters and variability.—According to Turner (1954, 1969), B. ( Anchomasa ) belongs to the genus Barnea in having a simple (not septate) umbonal reflection and a single, anterodorsal accessory plate represented by the lanceolate, undivided and calcareous protoplax. Moreover, Barnea s.s. and B. ( Anchomasa ) share the presence of long and thin apophyses

MONARI—PHYLOGENY AND BIOGEOGRAPHY OF PHOLADID BIVALVES 317

20 mm 5 mm 5 mm 20 mm 5 mm 20 mm 5 mm 5 mm 5 mm 20 mm 5 mm

( Pacaud 1998). B. ( Anchomasa ) differs from Barnea s.s. in having a wide pedal gape ( Turner 1954, 1969) and, consequently, well−defined prora (sensu Kelly 1988a).

The main characters varying in B. ( Anchomasa ) are the general morphology of the shell, the position of the umbo and the shape of the posterior margin ( Fig. 1). In most of the living species, the shape of the shell is elongate to tapered, with length clearly exceeding height. However, in the type species, B. (A.) parva ( Fig. 1D), and in some fossil species, such as Barnea (Anchomasa) palmula ( Dujardin, 1837) and Barnea (Anchomasa) dumortieri ( Fischer, 1866) , the shell is much shorter in the antero−posterior direction. A variably marked radial umbonal−ventral depression of the shell is often present in B. (A.) parva ( Fig. 1D 1) which is hardly visible or is absent in the other species.

The umbo is variably displaced in anterior position. The outline of posterior margin is lanceolate to subtruncate. A subtruncate posterior margin is usually associated with a wide posterior gape. In contrast, species having a lanceolate posterior part of the shell commonly develop a narrow posterior gape. The pedal gape is always wide and oval in outline. However, the concavity of its margin is slightly variable and, consequently, the prora project to a varying degree. The dorsal condyle, which is formed by the internal protrusion of the umbo below the umbonal reflection, is small to strong. The robustness of the apophyses and their inclination with respect to the direction of the dorsal margin are relatively variable as well. The ornament is composed of radial alignments of imbrications. In most species, they are well−evident in the anterior slope, where they range from few, strong and well spaced, to numerous, dense and relatively feeble. They attenuate towards the posterior part, where they rarely persist.

As described in detail below, the combination of some of these aspects of variability characterizes groups with distinct geographical distribution.

Adults of B. (A.) parva lack the chondrophore and ligament (Purchon 1955) ( Fig. 1D 3 ). On the contrary, Barnea (Anchomasa) manilensis (Philippi, 1847) shows a prominent chondrophore in the early juvenile left valve becoming a spiny vestigial process in the adult shell ( Ito 1998, 1999) ( Fig. 1A 4 ). A small left chondrophore and right resilifer occur also in adult specimens of Barnea (Anchomasa) obturamentum ( Hedley, 1893) ( Fig. 1B 2 , B 4 ), Barnea (Anchomasa) truncata (Say, 1822) ( Fig. 1C 2 , C 4 ), and Barnea (Anchomasa) similis (Gray, 1835) ( Suter 1913) . Ito (1998) maintained that the left chondrophore, right resilifer and active internal ligament are present in the early post−larval shells of many pholadoideans and these features should be considered as plesiomorphies of the superfamily inherited from bivalves having a chondrophore only in the left valve. According to that author, during the adult growth these structures become vestigial or disappear in many species, but they can also persist in independent lineages by paedomorphosis of the first fibrous ligament. The phylogenetic analysis below confirms the ontogenetic plasticity of the chondrophore, which acts as a highly homoplasic character.

Comparisons.— Barnea (Anchomasa) shows similarities with B. (Umitakea) Habe, 1952. Turner (1969) considered Habe’s subgenus to be a junior synonym of B. ( Anchomasa ). On the contrary, Vokes (1980) treated B. (Umitakea) as a distinct taxon. Evseev (1993) deeply studied the type species, Barnea (Umitakea) japonica ( Yokoyama, 1920) and compared it with B. (A.) parva , mainly based on the data published by Purchon (1955) and Turner (1954). Evseev (1993) listed some details of the anatomy of the soft body, e.g., the long caudal process of the visceral mass, the isometric and compressed ventricle and the morphology of the midgut, considered to be distinctive characters at supraspecific level. However, these characters are unknown in the other species of B. ( Anchomasa ). Thus, their supraspecific variability as yet can not be defined.

Ito (1998, 1999) underlined that B. (U.) japonica differs from B. (A.) manilensis in retaining a prominent chondrophore at the adult stage. As mentioned above, the adult chondrophore in B. ( Anchomasa ) is highly variable and in some species it is slightly less prominent than that illustrated by Evseev (1993) for B. (Umitakea). Differences based on that feature may not be valid to make distinction among these taxa.

B. (Umitakea) differs from B. ( Anchomasa ) in characters of the general shape of the shell. They are evident in comparing the respective type species, although the overall variability of B. ( Anchomasa ) makes the differences less distinct. B. (Umitakea) has a much higher shell and the length to height ratio clearly exceeds the field of variability of B. ( Anchomasa ). The anteroventral margin is slightly concave to slightly convex and, consequently, the prora project less. Moreover, B. (Umitakea) has a much wider posterior gape ( Evseev 1993).

Barnea (Taiwanobarnea) Wang, 1983 , from the late Miocene of Taiwan, shares with B. ( Anchomasa ) and B. (Umitakea) the presence of a wide pedal gape and an undivided, calcareous protoplax as a single accessory plate. The degree of the antero−posterior elongation of the shell is intermediate between B. ( Anchomasa ) and B. (Umitakea). However, the moderately prominent prora and the inflation of the anterior region of the shell would indicate closer affinities with B. (Umitakea). Furthermore, according to Wang (1983), the type species, Barnea (Taiwanobarnea) shihchoensis Wang, 1983 is very similar to B. (U.) japonica , differing in the convex outline of the anteroventral margin and in the absence of a spiny sculpture on the anterior part of the shell. However, as mentioned above, the slightly convex outline of the anteroventral margin is an aspect of the variability of B. (Umitakea) as well. Since the differences in the ornament pattern could be not sufficient alone to justify the institution of a distinct subgenus, B. (Taiwanobarnea) is strongly suspected to be a junior synonym of B. (Umitakea). These considerations and the lack of information on the internal shell characters determined the exclusion of B. (Taiwanobarnea) from the cladistic analysis below.

B. ( Anchomasa ) is also very similar to Cyrtopleuropsis Pacaud, 1998 . In fact, the ranges of variation in the shape of the shell, the outline of the anterior area and the ornament are widely continuous among the two taxa. Moreover, as discussed above, the presence of the chondrophore, which Pacaud (1998) included in the diagnostic features of Cyrtopleuropsis , is not useful for distinguishing Pacaud’s genus from B. ( Anchomasa ). Cyrtopleuropsis differs from B. ( Anchomasa ) in having wide and spoon−shaped apophyses and the posterior slope ornamented by granulations. Accessory plates of Cyrtopleuropsis are unknown.

B. (A.) parva has general similarities with Nipponopholas satoi Okamoto and Habe, 1987 , type species of Nipponopholas Okamoto and Habe, 1987 and with the species belonging to the genus Zirfaea Leach in Gray, 1842. However, in Nipponopholas and Zirfaea the single accessory plate is represented by the mesoplax. Furthermore, Zirfaea has a ventral condyle and an internal umbonal−ventral ridge corresponding to an external sulcus. However, in the fully adult shells of Zirfaea crispata (Linnaeus, 1758) these features become reduced ( Turner, 1954) and an evident internal radial ridge and ventral condyle are present in the early juvenile shells of B. (A.) manilensis , but disappear completely during adult growth ( Ito 1999).

Figures

Fig. 3. Phylogeny of Pholadoidea. Most parsimonious tree corresponding to the 50% Majority Rule consensus tree obtained by successive approximation weighting. L 96. Indices under unitary weight conditions:CI 0.54, RI 0.85, RC 0.46. Indices under reweight conditions:CI 0.78, RI 0.94, RC 0.74. Abbreviations: L, length; CI, consistency index; RI, retention index; RC, rescaled consistency index. Numbers in bold indicate the Majority Rule values and, in brackets, the bootstrap support values of the individual nodes. Only bootstrap values higher than 40% are shown. Numbers in smaller type represent the most significant state changes at the respective nodes. Exclusive synapomorphies are in bold. Unambiguous not−exclusive synapomorphies are in normal type. Abbreviations A and D mark the ambiguous changes of character states under ACCTRAN and DELTRAN optimisations, respectively. For changes of character states in the Pholadinae clade see Fig. 4. The classification of Pholadoidea in Turner (1969) is shown besides the tree by comparison.

Fig. 4. Detail of the selected cladogram showing the relationships and state changes among the genera and subgenera of the Pholadinae clade. Symbols as in Fig. 3.

Fig. 2. Phylogeny of Pholadoidea. A. Strict consensus tree obtained by heuristic search with unordered and equally weighted characters. B. Strict consensus of 173 most parsimonious cladograms retained by successive approximation weighting.