Hu et al.—A possible early Cambrian pterobranch hemichordate
Description.—Maximum height of the tubarium is 6 cm. Tubarium branches are 0.4 to 0.5mm wide. Distances between bifurcations vary from 1mm to several centimeters. Remains of soft-bodied zooids are not evident. Thecal tubes mostly straight or curved, sometimes bent (Fig. 1.1). The angle of the thecal tubes to the stipe varies from 15 to 70 degrees. The width of the terminal thecal tubes varies from 250 μm at the aperture to 400 μm at the location just above the branching point. The number of terminal thecal tubes is usually four, or less commonly five (Hu, 2005, pl. 18, figs. 4, 5). The terminal thecal tubes are free with similar widths and lengths. The apertures of the thecal tubes appear to narrow slightly and are thickened (Fig. 1.6; Hu, 2005, pl. 18, fig. 5). Energy-dispersive spectro- scopic analysis reveals that the dark remains on the tubes are organic carbon (Fig. 2.9–2.15). A number of remains of tubaria show a darker internal
thread of relatively constant width, herein interpreted as remains of an original stolon system (Figs. 1.5, 2.3, 2.6). As a result of weathering and early decay, the stolon system is preserved as fragments (Figs. 1.5, 2.4), with only a few cases in which the stolon is visible as a continuous thread for a distance of more than a few millimeters (Fig. 2.6). The stolon is divided where the tubarium branches. The width of the stolon is about 10 μm. Transverse annulations of the tubaria are sometimes faintly preserved (Fig. 1.7–1.10) and may have resulted from previously existing fusellar structures. The heights of the annulations are about 20 μm. However, no unequivocal fusellar structures are presently documented. Due to fragmentary preservation of all available specimens,
no basal parts of tubaria are observed; thus, the means of colony attachment to the substrate cannot be determined.
Other material.—About 20 specimens with incomplete parts of the tubarium and a large number of incomplete thecal tubes or branches.
Remarks.—When the taxon Malongitubus kuangshanensis was erected, the phylogenetic affinity of the species was kept open, and the similarity with Dalyia racemata from the Burgess Shale was discussed briefly. At that time, the latter was considered to be an alga, but is now recognized as a possible pterobranch hemichordate (Maletz and Steiner, 2015). The possible ptero- branch affinity of M. kuangshanensis was also mentioned by Maletz and Steiner (2015) but not discussed in detail. The identification of a resistant stolon system indicates that it has a close affinity to the Pterobranchia and more generally that it is a colonial organism. A “stolon” has sometimes been reported from extant hydrozoan colonies, which are morphologically and functionally differentiated into hydrorhiza, hydrocaulus, and especially swollen hydrotheca (Brusca and Brusca, 2003; Ruppert et al., 2004). However, in colonial hydrozoans, the “stolon” represents a thin extension of the hydropolyp, which is
29
not sclerotized as in pterobranchs and is thus differentiated as coenosarc. While the chitinous perisarc and hydrothecae of colonial hydrozoans may have some fossilization potential, it is rather unlikely that the coenosarc would be preserved in the fossil record. By contrast, decay experiments on modern ptero- branchs have demonstrated that the sclerotized stolon system is one of the most durable structures of pterobranch colonies and may prevail until most of the zooids and tubarium have decayed (Beli et al., 2017). In Malongitubus, and notably also in the closely related Dalyia racemata from the Cambrian Stage 5 Burgess Shale, the putative stolon system is the most resistant structure of the fossil remains. This is also shown by the fact that sometimes only stolons remain after decay or parts of stolons extend out of decayed tubaria (Fig. 3). Dalyia was originally interpreted as a red alga (Walcott, 1919), but was recently reinterpreted as a colonial graptolite by Maletz and Steiner (2015) from the identification of a possible stolon system. Therefore, M. kuangshanensis is also interpreted as a probable pterobranch due to the existence of a sclerotized stolon system. Its placement within the Cephalodiscida can be excluded, since the latter are noncolonial and lack a stolon system. Further assignment within the Graptolithina is not attempted herein. Attempts at identifying fusellar structures by SEM-BSE
were unsuccessful in the current study. This is probably due to the loss of most of the original organic material of the tube wall as a result of early decay and weathering, leaving only weak imprints of annulations, which are likely to be difficult to detect. The loss of organic material in the main part of the tubarium is also confirmed by the energy-dispersive spectroscopic analysis (Fig. 2.9–2.15). This interpretation also can be applied to Dalyia racemata from the Burgess Shale. As indicated by Maletz and Steiner (2015), all available specimens of D. racemata are diagenetically altered and pale in color, and no original organic material is present. Taphonomic experiments on modern pterobranchs (Briggs
et al., 1995; Beli et al., 2017) show different rates of decay between zooids, tubaria, and the stolon system. The zooids decay rapidly and become unrecognizable after a few days, whereas the tubes and the interior stolon system can last for several months. This decay process of modern pterobranchs can be applied to M. kuangshanensis, which shows well-preserved tube outlines and stolon system and the absence of any zooids. Preservation of zooids is extremely rare in the fossil record of pterobranchs. The most substantial record is the putative reports by Durman and Sennikov (1993) and Sennikov (2016) of soft tissue in a middle Cambrian (Drumian) rhabdopleurid from Siberia. Remarkably, a few specimens of arthropods (e.g., Naraoia) from the same bedding plane as M. kuangshanensis show well-preserved diverticula (Hu, 2005, pl. 13, fig. 7; Zhao et al., 2012, fig. 7N), indicating favorable conditions for soft-tissue preservation. It seems likely that individuals of M. kuangshanensis were exposed on the seafloor and subjected
Figure 1. Malongitubus kuangshanensis from the upper part of the Yu’anshan Formation. (1) NIGP-165029: a colony with a bent lower portion and a branched upper portion. (2) NIGP-105030: dense tubaria overlapping each other. The framed area is enlarged in (9). (3) NIGP-105031: a branched colony with a priapulid worm, Maotianshania cylindrica, to the right. The areas indicated by the black arrows are enlarged in (7, 8, 10). (4) ELI-B CLP K007A: a radiating colony. Note the brachiopod to the bottom left. (5) ELI- B CLP K010, part: a colony with a branching tubarium and a stolon system. (6) Thecal apertures. Close-up of the area framed in (5). (7, 8) SEM photograph showing the weak imprints of annulations. White arrows indicate the annulations interpreted as traces of possible fusellar construction. (9) Close-up of the area framed in (2), showing annulations of the tubarium. Some of the annulations are indicated by white arrows. (10) SEM photograph showing the weak imprints of annulations. (1–5)Scale bars=5mm; (6, 9, 10) scale bars=0.5mm; (7)scale bar=200μm; (8) scale bar=300μm.
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
