1016


Journal of Paleontology


above in Systematic Paleontology) matches the recent paleogeo- graphic projection of Popov and Cocks (2017), showing Tarim and theChu-Ili terrane located close to one another during the Late Ordovician. Future additional studies of this brachiopod faunawill 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., 2013; Rasmussen, 2014 for a broad overview of the global paleobiogeography of the Ordovician brachiopod fauna).


Encrusting epibionts.—Two Altaethyrella shells in our collec- tion 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


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 andVinn, 2007).


Conclusions


encrusted both the dorsal lateral shell flanks andwithin 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, 1970) 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


Altaethyrella tarimensis n. sp. is the first species of the genus reported from the Tarim paleoplate, and has a similar shell morphology to most rhynchonellides. Like other members of the genus, this species can be easily differentiated from home- omorphs based on the lack of a median septumand septalium in the dorsal interior, and lack of spirlia in the mantle cavity. However, these characteristics are only visible through serial sectioning of the shells. Altaethyrella tarimensis shares a number of characteristics


with other species thus far described in the genus. Shells exhibit a high degree of variation in shell size, shape, ornamentation, and bilateral symmetry, even within a single population of a particular collection. The limited paleogeographic range of Altaethyrella indi-


would have been an important substrate for other filter feeders in the relatively soft sea floor of the Kuruktag platform. Bryozo- ans, for example, were able to use these large shells as anchors on a relatively unconsolidated substrate. It is possible that the brachiopod shells were important stabilizers on the seafloor for other filter-feeding organisms, such as crinoids and corals. Further study of the fauna will no doubt shed light on the


paleoecology of the fauna and provide details on the paleogeo- graphic relationships between Tarim and adjacent paleoplates during the Late Ordovician and patterns of faunal dispersal in the region.


Acknowledgments


Zhang Y., Wang H., and Li G. from the Nanjing Institute of Geology and Palaeontology (NIGP), Chinese Academy of Sci- ences, provided assistance in the field and Rong J.-Y. (NIGP) provided many suggestions on the early version of the manu- script. The authors thank I.G. Percival and C.M.Ø. Rasmussen for their constructive reviews and editor-in-chief B. Hunda for her editorial comments on the manuscript. Funding for this study was provided by research grants to Zhan from the National Natural Science Foundation of China (Nos. 41521061 and 41290261), the Chinese Academy of Sciences (XDPB05), and the State Key Laboratory of Palaeobiology and Stratigraphy (LPS), Nanjing Institute of Geology and Palaeontology (NIGP). This is a contribution to IGCP Project 653: the onset of the Great Ordovician Biodiversification Event.


cates that the Tarim paleoplate was located close to North and South China paleoplates and the Kazakh terranes during the late Katian, all of which were likely of low paleolatitude. The high degree of similarity between A. tarimensis n. sp. and A. otarica from the Chu-Ili terrane in Kazakhstan agrees with published paleogeographic reconstructions showing the close proximity of the Tarim paleoplate to the Chu-Ili terrane at that time (e.g., Popov and Cocks, 2017). Due to its large size and abundance, A. tarimensis n. sp.


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116  |  Page 117  |  Page 118  |  Page 119  |  Page 120  |  Page 121  |  Page 122  |  Page 123  |  Page 124  |  Page 125  |  Page 126  |  Page 127  |  Page 128  |  Page 129  |  Page 130  |  Page 131  |  Page 132  |  Page 133  |  Page 134  |  Page 135  |  Page 136  |  Page 137  |  Page 138  |  Page 139  |  Page 140  |  Page 141  |  Page 142  |  Page 143  |  Page 144  |  Page 145  |  Page 146  |  Page 147  |  Page 148  |  Page 149  |  Page 150  |  Page 151  |  Page 152  |  Page 153  |  Page 154  |  Page 155  |  Page 156  |  Page 157  |  Page 158  |  Page 159  |  Page 160  |  Page 161  |  Page 162  |  Page 163  |  Page 164  |  Page 165  |  Page 166  |  Page 167  |  Page 168  |  Page 169  |  Page 170  |  Page 171  |  Page 172  |  Page 173  |  Page 174  |  Page 175  |  Page 176  |  Page 177  |  Page 178  |  Page 179  |  Page 180  |  Page 181  |  Page 182  |  Page 183  |  Page 184  |  Page 185  |  Page 186  |  Page 187  |  Page 188  |  Page 189  |  Page 190