Argopecten irradians amplicostatus (Dall, 1898)
DALL, W. H. 1898. Contributions to the Tertiary fauna
of Florida. Silex Beds of Tampa and the Pliocene Beds of the Caloosahatchie
River. Part IV. I. Prionodesmacea: Nucula to Julia. 2. Teleodesmacea: Teredo to
Ervilia. Transactions of the Wagner Free Institute of Science of Philadelphia,
3 (4): i-viii, 571-947, pls. 23-35. [p. 747]
1898 Pecten (Plagioctenium) gibbus var. amplicostatus Dall, 1898
Pecten (Plagioctenium) gibbus var. amplicostatus Dall; W. H. Dall, 1902, Illustrations and descriptions of new, unfigured or imperfectly known shells, plate 32, figure 9.
|
«Pliocene to recent.
This differs from the typical gibbus by its fewer and broader ribs. It is about the same size as the type, and occurs chiefly west of the Mississippi, on the Texas coast, and south to Carthagena. It is usually white or nearly white on the right valve, and grayish with mottlings of white on the left valve. Of fourteen specimens, one had twelve; two, fourteen; four, fifteen; and seven, sixteen ribs. It is quite tumid and very solid, and probably inhabits coral or rocky bottom. Of the fossils one had fourteen; ten, fifteen; and sixteen, sixteen ribs.» WILLIAM HEALEY DALL, 1898 |
«Comparison. A . irradians amplicostatus differs from A. irradians concentricus in having thicker, more convex valves with fewer plicae (text- fig. 8).
Stratigraphic range. True A. irradians amplicostatus is known only from the Holocene. Many specimens found in the Pleistocene Fort Thompson Formation of Florida and identified with A. irradians concentricus are in fact morphologically intermediate compared to living A. irradians concentricus and A. irradians amplicostatus in thickness of shell, relative convexity, and number of plicae. Usually, however, they resemble the former more than the latter. Although Pleistocene shell deposits on the coast of the Gulf of Mexico west of the Florida peninsula are rare, it seems likely that typical A. irradians amplicostatus will someday be found in these areas. Geographic distribution and ecology. That A. irradians amplicostatus ranges along the northern and western coasts of the Gulf of Mexico is well established. Less well established are its limits within this area. Present records range from Galveston, Texas, on the east (recorded by the author) to the Laguna Madre, southern Texas, on the west (Parker, 1959, p, 2129; 1960, p. 312). The subspecies has apparently never been found alive in the area east of the Mississippi Delta. Although Parker (1956, p. 329) refers to an early report by Spaulding (1906) of extensive beds of A . irradians amplicostatus in the lagoons behind the Chandeleur and Breton Islands of Louisiana, Spaulding's report (p. 29) refers to the scallops only as "Pecten irradians" and Clarke (1965, p. 179) identifies specimens from this area in the collections of the U.S . National Museum as A. irradians concentricus. Two right valves of A. irradians amplicostatus in the collections of the U. S. National Museum, labeled as having been collected from off Bears Cut, Miami, Florida, at a depth of 30 fathoms, were interpreted by Clarke (1965, p. 180) as representing a disjunct populat ion of the Texas bay scallop. However, close study of the collections has revealed that the specimens from "Bears Cut" are identical in morphology, color, and degree of wear to two valves from Matagorda Bay, Texas (USNM ( Z) 448318) and that one of the Matagorda Bay shells bears the old number, 122. Apparently, through a curatorial error , this number of the Texas specimens was construed as Eolis station number 122, which is the Bears Cut locality. The extension of the range of A. irradians amplicostatus southward from the Laguna Madre of Texas is poorly known. Parker (1960, p. 312) considers the subspecies to be an element of the "open sound or open lagoon margin assemblage," which is the dominant margin assemblage of bays along the Texas coast and southward almost to Tampico, Mexico. Clarke (1965, p. 180) records the subspecies at Tamiahua, Mexico, and also lists a single right valve from Cartagena, Colombia. The Colombian record should probably be considered doubtful until more information on scallops in the area between Cartagena and Tamiahua is acquired. Shells of A. irradians amplicostatus dated at 1,940 years have been reported by Behrens (1966, p. 642) from a terrace in the state of Tamaulipas, Mexico (locality 1). The ecology of A. irradians amplicostatus is similar to that of the other subspecies of bay scallop. Parker (1959, p. 2127) lists it as one of the 17 species of invertebrates most indicative of high-salinity bay margins in the Rockport, Texas, area. These marginal areas have bottoms of sand-silt to almost pure fine sand, in contrast to the fine clayey sediments of the bay centers. According to Parker (1960, p. 306), large areas of the lagoons and estuaries between the Texas-Louisiana border and Matagorda Bay have reduced salinities. Southward, evaporation exceeds runoff for long periods, and the lagoons may temporarily have very high salinities. In these areas the bay scallops apparently prefer the areas of more normal salinity near the inlets. Remarks on type specimen. The above designation of a lectotype is necessitated by a complicated history of statements referring to "holotypes" and "cotypes" of both Argopecten irradians amplicostatus and Argopecten anteamplicostatus. Dall (1898, p. 747), in his original description of Pecten gibbus amplicostatus, included both living and fossil material, giving the geologic range as Pliocene to Holocene, but he neither designated a type nor furnished an illustration. Later, Dall (1902, p. 507) published a drawing of a living specimen of his subspecies but still did not designate a type. However, the illustrated specimen, bearing the number USNM(Z) 106990, was placed in the collection of living mollusk types, a customary museum procedure for any specimen that has been illustrated in publication. In 1905, Dall, assisting Schuchert in the preparation of a catalogue of fossil invertebrate type specimens in the U.S. National Museum (Schuchert, 1905, p. 488), listed Pecten gibbus amplicostatus Dall and referred to a sample from the Pliocene of Florida bearing the number USNM(P) 154186 as the cotypes. Presumably, mention of living specimens was omitted because the catalogue was concerned only with fossils. Twenty years later, Mansfield (1936, p. 189) recognized that the fossil specimens from the Pliocene of Florida, which he named Pecten gibbus anteamplicostatus, could be distinguished from the living subspecies, P. gibbus amplicostatus. He incorrectly assumed that USNM(Z) 106990 was the holotype of the living subspecies and designated as syntypes of the new fossil taxon specimens that he numbered USNM(P) 373078. Unfortunately, these syntypes were removed from USNM(P) 154186, Dall's cotype lot for P. gibbus amplicostatus, and renumbered, an act that can be inferred by the fact that the older numbers can still be seen on both of the syntypes. Two years after Mansfield's paper, Rowland (1938) published a set of systematic descriptions of Atlantic and Gulf Coast Tertiary Pectinidae in which she included (p. 49, 34) descriptions of both Chlamys gibbus amplicostatus and C. gibbus anteamplicostatus. For the former she repeated Dall's geologic range of Pliocene to Holocene, thereby ignoring the refinements of Mansfield (1936, p. 185), and also repeated Dall's 1905 designation of USNM(P) 154186 as the cotypes. For the latter she merely repeated Mansfield's description and syntype designation. What is surprising, however, is that, in illustrating the two taxa, she repeated photographs of the same specimens. Her illustrations of the cotypes of Chlamys gibbus amplicostatus, appearing on her plate IV, figure 18, and plate V, figure 24, represent, respectively, the same specimens illustrated as syntypes of ChIamys gibbus anteamplicostatus on her plate III, figure 3, and plate II, figure 3b. As a result of these various references to type specimens, a question arises as to which samples should be the source of a lectotype. The present International Code of Zoological Nomenclature (1964, article 73) defines a type-series as ". . . all the specimens on which the author of a species bases the species, except any that he refers to as variants, or doubtfully associated with the nominal species, or expressly excludes from it." Cotypes are considered as synonymous with syntypes, which are all of the type series (including specimens labeled "cotype") and are of equal value in nomenclature. Only a syntype may be designated as a lectotype (article 74). Because it is clear from Dall's original description of Pecten gibbus amplicostatus that both fossil and living specimens were included in his type-series and that his concept of the species included both, his 1905 reference to only a fossil sample as the cotypes can best be considered an unnecessary subsequent restriction of the type-series and of no standing in nomenclatural deliberations. Therefore, the lectotype herein designated is a specimen of the living form. Remarks on classification. A. irradians amplicostatus has been considered to be specifically distinct from the other bay scallops by Clarke (1965, p. 179-180), whose evidence consists primarily of three points: ( 1) lack of difficulty in distinguishing A. irradians amplicostatus from the neighboring subspecies, A. irradians concentricus, due to obvious differences in numbers of plicae and "general appearance"; (2) lack of intergradation in areas of proximity, with even some suggestion of character displacement in such areas; and (3) failure of A. irradians amplicostatus and A. irradians concentricus to achieve morphologicai unity despite repeated range fluctuations as shown by the presence of the former in the Pliocene and of the latter in the Pleistocene of Florida and by the present disjunct distribution of both. The reasons for considering A. irradians amplicostatus to be merely a subspecies in this study are essentially the converse of Clarke's evidence for considering the form to be a distinct species. First, the magnitude of the differences between A. irradians amplicostatus and A. irradians concentricus is hardly greater than that of the differences between the latter and A. irradians irradians. The taxa are strikingly similar when considered in the context of variation thronghout the Argopecten gibbus stock since the late Miocene. The differences in the general appearance of A. irradians amplicostatus compared to that of the other subspecies are primarily the result of a single morphological factor, the degree of inflation (see section on comparative morphology and infraspecific variation). Secondly, there is no evidence for the existcnce of A. irradians amplicostatus in the Pliocene of Florida. There is ample evidence, however, that the Pliocene form, named A. anteamplicostatus by Mansfield (1936, p. 189), was a primitive bay scallop that gave rise to A. irradians but probably not directly to a form that would be identified with modern A. irradians amplicostutus. The bay scallops in the Pleistocene of Florida have been identified here with A. irradians concentricus, recognizing that at several localities they are closer to A. irradians amplicostatus than any known A. irradians concentricus living today. The evidence for living disjunct populations of the subspecies, as pointed out above, consists of small samples that have likely become mislabeled. If A. irradians amplicostatus is indeed specifically distinct from A. irradians concentricus, the primary evidence will consist of data showing sympatry or a lack of intergradation in areas of proximity. Clarke's contention that this evidence exists is based upon samples that are separated by about 475 miles. At present no information is available on bay scallops from the Mississippi Delta area, which may be a barrier serving to limit or prevent genetic interchange between the adjacent taxa.» WALLER, T. R. 1969. The evolution of Argopecten gibbus stock (Mollusca: Bivalvia), with emphasis on the Tertiary and Quaternary species of Eastern North America. The Paleontological Society. Memoir 3 [Journal of Paleontology, 43 (5, supplement)]: 125 pp. [p. 44-46]»
|
Argopecten irradians amplicostatus (DaIl); T. R. Waller, 1969, The evolution of Argopecten gibbus stock, plate 7, figure 13.
|
«Several kinds of evidence exist which appear to indicate that A. amplicostatus is specifically distinct. Rib number and general appearance are obvious differences between A. amplicostatus and A. i. concentricus and there is no difficulty in distinguishing them. In areas of proximity no intergradation occurs between them (or between A. amplicostatus and A. gibbus); in fact there is even a suggestion of character displacement in such areas. Although no collections of the 2 forms from closely adjacent localities are available, the populations of A. i. concentricus (M and R) which are geographically nearest to populations of A. amplicostatus are morphologically more divergent from A. amplicostatus than are the other Gulf of Mexico populations of A. i. concentricus (see Fig. 5). Furthermore, the presence of A. amplicostatus in the Pliocene and A. i. concentricus in the Pleistocene of Florida (Dall 1898, 1925), together with the disjunct distribution of both A. amplicostatus and A. i. concentricus at the present time, indicates that over the past million years both A. amplicostatus and A. irradians (sensu lato) have undergone substantial range expansions and contractions and presumably have had repeated opportunities for interbreeding and for the achievement of morphological unity. This has not occurred. The evidence, therefore, indicates that A. amplicostatus is a distinct species and it is so considered here.»
CLARKE JR., A. H. 1965. The scallop superspecies Aequipecten irradians (Lamarck). Malacologia, 2 (2): 161-188, pls. 1-4. [p. 179, 180]
|
Aequipecten amplicostatus (Dall); A. H. Clarke Jr., 1965, The scallop superspecies Aequipecten irradians, plate 4, figures 9-14.
|