Similarities between Altaethyrella and other groups

Before Altaethyrella was established, species were often assigned to the rhynchonellide Rhynchotrema, which is known mainly from Laurentia. Rhynchotrema and Altaethyrella are similar in shell shape, have strong and simple ribs, and share similar distinctive strongly uniplicate anterior commissures with the dorsal fold and ventral sulcus forming a tongue. Only very minor differences, including a tendency of greater variation and asymmetry and a generally more prominent and strongly curved ventral umbo in Altaethyrella differentiate it externally from Rhynchotrema (e.g., Sproat et al., Reference Sproat, Sohrabi and Jin2014, figs. 2, 3 for Rhynchotrema).

The internal structures of the shell are also similar between these two genera, but three key differences can easily be identified in serial sections (Fig. 10). The hinge plates of Rhynchotrema merge medially to form a septalium supported by median dorsal septum, but in Altaethyrella, the hinge plates remain separated by a median gap. Altaethyrella lacks a septalium, and only sometimes possessed a low dorsal median ridge rather than a true median septum. Popov et al. (Reference Popov, Nikitin and Cocks2000) also noted that the ventral muscle scars are smaller in Altaethyrella than in rhynchotrematides such as Rhynchotrema, and that the adductor scars are only slightly shorter than the diductor scars. Ventral muscle scars are often obscured, however, because specimens are only rarely found disarticulated. The shells from Tarim clearly lack a septalium and median septum (Figs. 6, 7), and thus should be assigned to Altaethyrella.

Figure 10

Serial sections of Rhynchotrema increbescens (Hall, Reference Hall1847) and Altaethyrella tarimensis n. sp. showing differences in internal structures of the shell, despite similar external shell morphology (dorsal valve uppermost). Numbers indicate distance in millimeters from the posterior-most point of the shell. Rhynchotrema from Sproat et al. (Reference Sproat, Sohrabi and Jin2014, Fig. 7); Altaethyrella from Fig. 7. herein.

Many species that were previously assigned to Rhynchotrema from the paleoplates that make up Central and Eastern Asia need to be critically re-assessed to determine whether they are in fact Rhynchotrema or Altaethyrella. There may be no true Ordovician Rhynchotrema in China (Zhan and Li, Reference Zhan and Li1998).

Some specimens of Altaethyrella tarimensis n. sp. are similar to an atrypide from the study area to be described in a future study. The atrypides are much more abundant than Altaethyrella, and can usually be differentiated externally from Altaethyrella by their smaller size, lower dorsal convexity, and slightly more rounded outline.

However, differentiating between small Altaethyrella and atrypide shells can be difficult. Small shells of Altaethyrella are also less convex and do not have a clearly subpentagonal outline as in larger shells. The only reliable external feature to differentiate the genera at this stage is the number of ribs in the sulcus and fold—Altaethyrella generally has at least three ribs in the ventral sulcus and four ribs on the dorsal fold with a sharp boundary between the fold or sulcus and shell flanks on each side, while the atrypides often have a much weaker fold and sulcus, with only a single rib in the sulcus and two ribs on the fold. Altaethyrella also tends to have sharp, angular ribs while the atrypides feature less-prominent rounded ribs. In small and/or poorly preserved material, these characters can be difficult to determine reliably. The differences in internal shell structures are immediately obvious, because the atrypides possess larger crura that diverge toward the anterior and two mediodorsally directed spiralia—rhychonellides do not have skeletal lophophore supports amenable to preservation in the fossil record.

One of the species previously classified as Rhynchotrema in China mentioned in Zhan and Li (Reference Zhan and Li1998), R. pentagonia Fang (in Liu and Fang, Reference Liu and Fang1990) also from the Hadabulaktag Formation in Xinjiang, is likely one of these atrypide forms. Figures show only one rib within the ventral sulcus that corresponds to two ribs on the dorsal fold that are separated by a median groove and the shells have a more rounded outline in comparison to the subcircular to subpentagonal Altaethyrella.

Morphological variation within Altaethyrella tarimensis n. sp

As in other species of Altaethyrella, there is considerable intraspecific variation in A. tarimensis n. sp. in terms of: shell size, shell shape, number of ribs in the ventral sulcus and dorsal fold, and degree of asymmetry.

The collection contains shells of a range of sizes and shell shapes. Shell length, for example, varies from 7 mm to 17.9 mm, or a difference of 256% (see Tables 1–3). To illustrate the variability of shell shape, shell length/width and thickness/width were plotted as proxies for shell outline and convexity, respectively, independent of shell size (Fig. 8), producing a linear trend indicative of a single morphospecies. The r² values from linear regression in both cases are near 0.80, indicating a clear trend, but the scatter of individual datapoints reveals some variability.

Most shells have three ribs in the sulcus at the anterior commissure that correspond to four ribs on the dorsal fold. This pattern is similar to the homeomorphic Rhynchotrema. However, whereas Rhynchotrema almost always has only three ribs in its sulcus, Altaethyrella exhibits greater variation similar to the rhynchonellide genus Rostricellula. Some shells of A. tarimensis n. sp. feature as many as four, and rarely five ribs in the sulcus, with five or six corresponding ribs on the fold. These extra ribs commonly cause a deflection of the tongue formed by the prominent dorsal fold and ventral sulcus (e.g., Fig. 5.11). Similar variability was also noted by Zhan and Li (Reference Zhan and Li1998) in A. zhejiangensis.

Although these extra ribs apparently contribute to asymmetry at the anterior, there is significant degree of asymmetry in shells that only have three ribs in the sulcus as well. Such asymmetry is apparent in both small and large shells, indicating that it is independent of ontogeny, beginning early in the life of the brachiopod. Like the variation in rib number, this also sets Altaethyrella apart from Rhynchotrema in North America that are almost always symmetrical.

This relatively high degree of intraspecific variation could be caused by deformation of the shells during deposition and diagenesis. However, this may be a feature of Altaethyrella because a high degree of intraspecific variation has been noted in other species from different paleoplates. Zhan and Li (Reference Zhan and Li1998) noted significant variation in shell sizes and shape, number of ribs in the fold and sulcus, and morphology of the ventral beak, even within a single collection. Nikitin et al. (Reference Nikitin, Popov and Bassett2006) noted variation in the number of ribs between populations of Altaethyrella otarica, and their shell measurements show significant variation in shell size and shape within single populations.

Paleogeographical significance

Until Altaethyrella was differentiated from Rhynchotrema, it seemed as though Rhynchotrema had a worldwide distribution during the Late Ordovician. It is now known that Altaethyrella was confined to the plates and terranes that comprise modern day Kazakhstan and China. Conversely, Rhynchotrema was mainly limited to Laurentia during the Ordovician, where by the early Katian, it and the closely related Hiscobeccus had colonized eastern North America before extending across the continent by the late Katian (Sproat et al., Reference Sproat, Sohrabi and Jin2014; Sproat and Jin, Reference Sproat and Jin2017).

Rhynchotrema has been described from outside Laurentia. Nikitin et al. (Reference Nikitin, Popov and Bassett2003) described R. seletensis from Selety River Basin of Kazakhstan. Laurie (Reference Laurie1991) and Percival (Reference Percival1991) noted several species from Australia (from Tasmania and New South Wales, respectively). Detailed serial sections are lacking in the species from Australia, although figures and descriptions show a septalium in these species (Laurie, Reference Laurie1991, figs. 46.6, 46.26, 47.14; Percival, Reference Percival1991, figs. 18.40, 18.41 as internal molds), indicating that these species probably cannot be classified as Altaethyrella. The species from Kazakhstan also possess a septalium (Nikitin et al., Reference Nikitin, Popov and Bassett2003, fig. 6), but the septalium only completely connects the hinge plates in the extreme posterior of the shell, unlike in species from Laurentia where the septalium extends further toward the anterior (e.g., Sproat et al., Reference Sproat, Sohrabi and Jin2014, figs. 6, 7). These Central Asian and Australian species may, in fact, represent distant relatives of the species from Laurentia where Rhynchotrema is dominant.

This geographic separation can be traced back to the early evolution of these lineages. Early rhynchonellide evolution is not well known, but by the Sandbian (early Late Ordovician), early rhynchotrematoides (such as Rostricella and Rhynchotrema) and ancistrorhynchoides (of which Altaethyrella is a member) had become established on the margin of Laurentia. By the middle Katian, the ancistrorhynchoides largely went extinct in Laurentia while rhynchotrematoides dispersed throughout the expanding epicontinental seas.

On the plates that now comprise Central and Eastern Asia, however, the ancistrorhynchoides became established in the shallow carbonate platforms and survived into the Devonian in Siberia. Rhynchotrema was rare on these platforms, although the closely related Rostricellula would become established there in the Silurian.

This clear paleogeographic division between these two brachiopod lineages during the Late Ordovician has some utility in paleogeographic studies. The limited geographic range of Altaethyrella re-affirms that Tarim, North and South China, and the Kazakh terranes were located close to one another. The similarities of the specimens in this study to those of Altaethyrella otarica from the Chu-Ili terrane of Kazakhstan (see discussion above in Systematic Paleontology) matches the recent paleogeographic projection of Popov and Cocks (Reference Popov and Cocks2017), showing Tarim and the Chu-Ili terrane located close to one another during the Late Ordovician. Future additional studies of this brachiopod fauna will enable a more accurate projection of the location of the Tarim paleoplate during the Late Ordovician, once the fauna can be compared to those of other nearby plates in its entirety (see Harper et al., Reference Harper, Rasmussen, Liljeroth, Blodgett, Candela, Jin, Percival, Rong, Villas and Zhan2013; Rasmussen, Reference Rasmussen2014 for a broad overview of the global paleobiogeography of the Ordovician brachiopod fauna).

Encrusting epibionts

Two Altaethyrella shells in our collection were found to have been encrusted by bryozoan colonies (Fig. 4). One is a ramose bryozoan that has encrusted the surface of a shell, while the other is a treptostome form that has anchored to a shell.

In the first example (Fig. 4.1–4.4), ramose bryozoans have encrusted both the dorsal lateral shell flanks and within the ventral sulcus near the anterior commissure. A lattice-work of individual zooaria is visible within the sulcus of the brachiopod near its anterior commissure. At first glance, it seems that the colony was taking advantage of feeding currents created by the brachiopod, but the colony continues across the commissure onto the shell flanks, confirming post mortem encrustation (Fig. 4.4). The shell was probably encrusted in living position given the poorer preservation of the bryozoans on the dorsal and posterior portions of the shell. The bryozoan colony is dissected by the fracture, showing that the fracture must have been post-depositional.

In the second example (Fig. 4.5–4.7), a treptostome bryozoan has encrusted a smaller Altaethyrella shell. The bryozoan covers the ventral valve almost entirely, and extends across the commissure on the lateral flank on one side. Given that the bryozoan terminates very abruptly at the commissure on one side, it is possible that initial colonization of the shell occurred while the brachiopod was still alive. The overturned position with the ventral valve oriented upward would not be ideal for the brachiopod, because the feeding current (i.e., incurrent) that was thought to enter the mantle cavity at the top of the tongue at the anterior commissure (Rudwick, Reference Rudwick1970) would have been drawing water in near the sediment-water interface, increasing the probability of clogging the lophophore due to sediment being drawn into the mantle cavity. Regardless of whether the brachiopod was still alive or not during initial colonization, bryozoan growth across the commissure of the shell on the shell flank indicates that the bryozoan colony expanded across the shell after the brachiopod host had died.

Although, these are not clear examples of symbiotic relationships between the bryozoans and brachiopod, they do highlight the important role that Altaethyrella would have played as an anchor for sessile benthic filter feeders like bryozoans. As noted above, the lithology of the Hadabulaktag Formation is primarily argillaceous limestone and calcareous mudstone. Prior to lithification, the relatively soft substrate was not amenable to colonization by sessile benthic filter feeders like bryozoans. The abundant large, biconvex Altaethyrella shells would have created firm “islands” within the soft surrounding mud for these encrusting forms to adhere to—particularly the branching treptostome forms that presumably would have sunk into the mud or been overturned without a firm base. The coarse ribs and well-developed fold and sulcus might provide an ideal anchor for bryozoans and other filter feeders such as cornulitids that could use the currents created by brachiopods to enhance their feeding efficiency (as shown in A. zhejiangensis in Zhan and Vinn, Reference Zhan and Vinn2007).