Sproat and Zhan—A new paleogeographically important rhynchonellide from the Tarim plate 92(6):1005–1017 1009
Table 3. Ventral valve measurements (N=8). Abbreviations are as for Table 1. L
W Ws Mean
Min Max
Median SD
12.0 12.1 3.7 6.5
16.3
14.8 14.9 5.2 8.5
23
7.3 7.1 2.7 3
10.4
Ns 3.1
3
0.4 3 4
L/W 0.82
0.80 0.10 0.67 0.97
Serial sectioning.—Three shells were selected for serial sec- tioning to analyze their internal morphology, of which two are shown below. The shells were mounted on a manual parallel grinder and ground to produce transverse sections approxi- mately perpendicular to the commissural plane. The interval between each section varied from 0.05mm to 0.5mm, depend- ing on the distance from the posterior—many of the smaller structures such as the very small cardinal process near the pos- terior necessitate a finer interval. After each session of grinding, the polished surface was treated with dilute hydrochloric acid to etch the surface. A drop
or two of acetone was then applied to the surface, and replication film was lain on the polished surface to produce a representation of the cross-section of the shell. The GNU Image Manipulation Program (GIMP) (version
2.8.22) was used to manually trace the sections after photograph- ing each section with a digital camera mounted to a microscope. The most recent version of this free and open source image processing program is available from
https://www.gimp.org.
Repository and institutional abbreviation.—All specimens examined and figured in this study are deposited at the Nanjing Institute of Geology and Palaeontology (NIGP), Chinese Academy of Sciences.
Systematic paleontology Order Rhynchonellida Kuhn, 1949
Superfamily Ancistrorhynchoidea Cooper, 1956 ?Family Ancistrorhynchidae Cooper, 1956
Remarks.—The diagnosis in the Treatise on Invertebrate Paleontology for Ancistrorhynchoidea does allow for the pre- sence of a cardinal process (Savage, 2002b, p. 1041), but that of Ancistrorhynchidae (p. 1035) states that ancistrorhynchides do not have a cardinal process. In the past, some authors had placed Altaethyrella in the Rhynchotrematidae in part because of its small cardinal process (e.g., Zhan and Li, 1998). These families are very similar, but Altaethyrella shares
more similarities with the Ancistrorhynchidae than the Rhynch- otrematidae. Most notable are the lack of a true dorsal median septum that is replaced by a small dorsal median ridge in most Altaethyrella shells, and the lack of fused hinge plates forming a septalium that is common in some members of the Rhyncho- trematidae. These differences are elaborated on below in the discussion on the homeomorphy of these two groups. Altaethyr- ella may be a transitional form with characteristics that fall between these families. Rather than erect a separate family within the Ancistror- hynchoidea for this single genus, we tentatively agree with the
opinion of Popov et al. (2000) in assigning Altaethyrella to the Ancistrorhynchidae, with the provision that its small cardinal process is different from other ancistrorhynchides. A more extensive study including other genera may allow for emending the diagnosis of the family, but it is beyond the scope of this study.
Genus Altaethyrella Severgina, 1978
Type species.—Altaethyrella megala Severgina, 1978, from the Upper Ordovician (Ashgill) of northwestern Altai, Siberia.
Remarks.—Altaethyrella has been well described elsewhere (e.g., Zhan and Li, 1998; Popov et al., 2000), and thus a full re- description here is unnecessary. However, one notable feature that has not been addressed by other studies is a very small median ridge that extends anteriorly from the cardinal process in the specimens sectioned here (best shown in Figure 6 for the current species). This feature was not noted by other authors, but it is present in illustrated sections of A. zhejiangensis (Zhan and Li, 1998, fig. 6) and A. otarica (Nikitin et al., 2006, fig. 33), and thus is probably characteristic of the genus. This differs from the much more prominent dorsal median septum in Rhyncho- trematidae such as Rostricellula, Rhynchotrema, and Hisco- beccus that supports fused hinge plates forming a septalium (Sproat et al., 2014). Species described as Drepanorhyncha by Fu (1982) from
the Jinhe Formation in northern China may belong to Altaethyrella, but cannot be confirmed without detailed study of larger collections and additional serial sectioning. Drepa- norhyncha pentagonia Fu, 1982 is flattened in form compared to our current material. Drepanorhycha triplicata Fu, 1982 is a smaller, tranversely elongated form that is similar in form to the Tarim species. Both appear to have angular ribs, indicating that they do not belong to Drepanorhyncha, which have rounded ribs, more subtriangular outline, and weak fold and sulcus. More accurate and detailed revision of these species is awaiting additional topotype material and serial sectioning.
Altaethyrella tarimensis new species Figures 1, 4–7
Holotype.—NIGP 167281 from the upper Hadabulaktag Formation (upper Katian) in the Kuruktag region, southern Xingjiang, Northwest China. NIGP 167277, NIGP 167278, and NIGP 167282 are paratypes. Collection locality consists of a small hill of rocky outcrop located at 40°23’18”N, 88°1’25.1”E.
Diagnosis.—Shell small to medium sized, dorsibiconvex, com- monly asymmetrical, ~81% as long as wide and 65% as thick as wide; ribs simple and angular, usually five to seven on shell flank, with four in dorsal fold and three in ventral sulcus but occasionally more; cardinal process and dorsal median ridge small.
Occurrence.—Middle to upper Hadabulaktag Formation (upper Katian), Kuruktag Platform, southern Xinjiang, northwestern China.
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