A LATE PERMIAN CHINESE GASTROPOD SPECIES, POSSIBLY LARVAL, IN THE MIDDLE PENNSYLVANIAN OF NEW MEXICO

Journal of Paleontology, Mar 2004 by Kues, Barry S, Batten, Roger L, Erwin, Douglas H, Hua-Zhang, Pan

KUES AND Batten (2001, p. 30, fig. 6.17-6.20) described several distinctive, minute, low-spired gastropod specimens from the Desmoinesian (Middle Pennsylvanian) Flechado Formation of north-central New Mexico, assigning them questionably to Lunulazona Sadlick and Nielsen, 1963 because of the strongly developed collabral elements similar to those of that genus. These shells, consisting of three or lour inflated whorls, are at most 1 mm in height and the later whorls bear conspicuous, sharp, widely spaced collabral ribs that bend strongly across a wide, slightly flattened band interpreted as a peripheral selenizone. While recognizing these specimens as a distinct, unnamed taxon, Kues and !Batten (2001) believed that they likely represent juveniles of an as yet unrecognized larger species of gastropod with a different mature morphology.

Recently, Pan and Erwin (2002) described diverse gastropod assemblages from southern China, most of them from Late Permian (Changhsingian) strata. Most of these taxa are very small, possibly reflecting differential silicification rather than adverse or unusual ecological conditions. Among the specimens is a single, beautifully preserved shell, described as Naticasinus sinus new genus and species (p. 6, fig. 4.14-4.16), from the upper member of the Heshan Formation, of very late Permian age. This specimen is similar in size, whorl proportions, and number of whorls to the New Mexico Middle Pennsylvanian specimens, and furthermore possesses nearly identical ornamentation. Pan and Erwin interpreted a sinus and lunulae just below midheight, but lacking the parallel sides that would indicate the presence of a slit. The shell characters of the New Mexico Middle Pennsylvanian specimens arc so closely similar to those of the Chinese species that, on purely morphological grounds, they would appear to represent an early occurrence of Naticasinus sinus n. gen. and sp.

The similarity between Lunulazonal sp. and Naticasinus sinus n. gen. and sp. was noted by Bandel (2002, p. 144) in a paper published while this note was in review. Bandel (2002) and Niitzel (personal commun., 2002) noted that similar small shells are known from the Late Triassic (Bandel, 1991), and from the Upper Carboniferous of Germany (Herholz, 1990; Bandel, 1991), in both cases interpreted as larval shells. Bandel (2002) discussed the status of Naticasinus sinus n. gen. and sp. and interpreted similar shells from the Pennsylvanian of Texas as being the protoconch of the neritimorph species Naticopsis ventrica (Norwood and Pratten, 1855). However, this interpretation was based on one specimen in which the protoconch was said to be attached to the initial part of a teleoconch shell identified as N. ventrica. The figures of this specimen (Bandel, 2002, figs. 144, 145) show only a small area of the shell, a slight fraction of a whorl, beyond the limit of the sinusoidal collabral ornamentation, and the identification of this with the teleoconch of N. ventrica is, in our view, not at all certain.

While accepting that Naticasinus Pan and Erwin, 2002 may represent a protoconch morphology, we doubt that it represents the protoconch of Naticopsis M'Coy, 1846. Knight (1933) described the initial whorls (nepianic and neanic whorls of his terminology) of several North American Pennsylvanian species of Naticopsis, including N. ventricosus (now N. ventrica), as being smooth and devoid of any ornamentation except growth lines. Further, N�tzel and Mapes (2001, p. 160) described the protoconch of Naticopsis sp. from the Upper Mississippian of Arkansas as being smooth and simple, and noted that it resembles the protoconch of a fully grown Naticopsis specimen from the Pennsylvanian of Oklahoma. Numerous tiny, well-preserved specimens of Naticopsis are present in the Virgilian Red Tanks Formation in New Mexico (Kues, in press) and they likewise have smooth protoconchs. It is therefore very unlikely that tiny shells similar to Naticasinus represent the protoconch of Naticopsis, although this conclusion does not preclude the possibility that Naticasinus is the protoconch of some other, as yet unidentified, neritimorph.

At present, Naticasinus sinus n. gen. and sp. can be interpreted as an unusually small mature shell with a protoconch of "little more than one smooth whorl, separated by a strong demarcation" from the later teleoconch whorls (Pan and Erwin, 2002), or the entire shell is possibly the protoconch of a larger shell with a different teleoconch morphology (Bandel, 2002; Nutzel, personal commun., 2002). Although the slight flattening of the whorl periphery and strongly lunulate collabral ornamentation across it gives the impression of a selenizone (Kues and Batten, 2001), this area is not bounded by spiral Iirae and, as noted by Pan and Erwin (2002), is not associated with a slit, and therefore cannot be a true selenizone. In either case noted above, Naticasinus sinus n. gen. and sp. represents a rare, distinctive shell morphology that, as discussed below, has a long temporal range in the late Paleozoic and a wide geographic distribution. If it is a larval shell, future work may eventually yield specimens having the Naticasinus morphology attached to a sufficient number of teleoconch whorls to indicate affinities with taxa currently known only from teleoconch whorls. Depending on whether the mature shell has previously been named, Naticasinus sinus n. gen. and sp. would either become the name of the entire shell or fall into synonymy with a name previously established for the mature part of the shell. It is worth noting, however, that extensive sampling of the Chinese Late Permian and New Mexico Middle Pennsylvania!! faunas, as well as much recent research emphasizing the morphology of late Paleozoic gastropod protoconchs (e.g., Nutzel, 1998; Nutzel and Mapes, 2001; Bandel et al., 2002) has so far failed to yield convincing evidence of the relationship of N. sinus n. gen. and sp. to any group of larger gastropods named on the basis of mature whorls.