958
Journal of Paleontology
(1) YN-GLF-PAL-06 (Fig. 2.4) shows a more slender body profile: the upper part is less inflated while the lower part is similar to others; (2) YN-GLF-PAL-01 (Fig. 4.1) is spindle- shaped and the maximum width lies at the center while both ends appear narrower; and (3) three specimens show that the main body is deflected from the basal point (Figs. 2.1, 2.2, 3.1). Sclerites of this new species, comprised of two lateral rays
and one ascending ray, belong to the sclerite mode of Allonnia: 3+0 (Bengtson and Collins, 2015). Rays are slender with their tips toward the apical end of the body. Usually only the lateral rays are preserved on the body surface, which thus have the shape
on the body surface is defined herein only by the two lateral rays. Measurements show that on the stalk, the average ray length is 0.8mm increasing to 1.5mm on the upper part of the body. The average length of the ascending ray preserved on the folded margin increases from 1mm on the stalk to 2mm on the upper part. The mean angle between lateral rays increases from 109° to 125°. Based on these features, the sclerite size of this new species might increase adapically, but a size plateau, as
observed in Allonnia tintinopsis (Bengtson and Collins, 2015, fig. 18), cannot be obtained. Most of the specimens show that sclerites are distributed
over the body surface with a spacing of 1–1.5mm, and only a few difer: smaller in one specimen (0.5–0.6mm in YN-GLF- PAL-06, Fig. 2.4) and wider in two specimens (1.5–1.8mm in YN-GLF-PAL-01, Fig. 4.1; 2mm in YN-GLF-PAL-07, Fig. 4.2). A remarkable feature of Allonnia tenuis n. sp. can be observed from the specimens preserved with the whole stalk and the basal point: the lowermost part of stalk is devoid of sclerites (Fig. 3).
Compared to other specimens, the sclerites of YN-GLF-
PAL-07 (Fig. 4.2) are clearly bigger both in size and angle: adapically the length of lateral ray increases from 1.5 to 2mm and the angle in between from 117° to 130°. The integument of this new species is smooth, and no
granules, such as those observed in Al. phrixothrix (Bengtson and Hou, 2001, figs. 7A, 8C), Al. tintinopsis (Bengtson and Collins, 2015, fig. 31.1–31.3), and Al. erjiensis (Yun et al., 2017, fig. 3A, 3B), can be seen over the whole body surface, but longitudinal folds are rather common, e.g. YN-GLF-PAL-06 (Fig. 2.4). As with other species of chancelloriids, openings, except for the apical orifice, cannot be observed in the integument of Allonnia tenuis n. sp. The apical tuft and orifice are tiny and inconspicuous in
Allonnia tenuis n. sp. They are difficult to discern in specimens compressed parallel to the matrix because the long, slender rays of the 3 +0 sclerites are densely distributed at the apical end and obscure the structures behind (Fig. 7.1–7.3). YN-GLF-PAL-03
of a lyre, similar to Al. tintinopsis (Bengtson and Collins, 2015, fig. 2). The fully developed ascending rays can only be observed on the folded body margin where they protrude laterally,with the tips curved adapically (Fig. 5.2, 5.3). Some specimens show that the ascending ray on the body surface leaves a tiny three- dimensionally preserved basal part or a pit beside the basal disk of the lateral rays (Fig. 5.4). Afewisolated sclerites (Fig. 6) are also preserved only with lateral rays, and longitudinal cracks along the lateral ray, thus exposing the internal cavity, which is one of the defining features of the chancelloriids. Due to the missing ascending ray, the size of the sclerites
(Fig. 7.4) is a specimen compressed top-down and clearly shows the apical structures. A discernible subcircular orifice is located at the center and is surrounded concentrically by lyre-shaped sclerites. Closer to the orifice, the sclerites become denser and smaller. At the very edge of the orifice, the sclerites are modified to be a single element.
Etymology.—From the Latin word tenuis, slender, referring to the slender stalk of this species.
Materials.—89 specimens including the holotype.
Remarks.—More than 80 nominal species and 13 genera of chancelloriids have been described in the literature, but many taxa are based on isolated sclerites (Moore et al., 2014; Bengt- son and Collins, 2015). Although having distinct features, these isolated sclerites cannot provide much information about the composition of the scleritomes. Three genera of Chancel- loriidae (Chancelloria Walcott, 1920, Allonnia Doré and Reid, 1965, and Archiasterella Sdzuy, 1969) have hitherto been found distributed worldwide and established or restudied based on whole-body preservation (Bengtson and Collins, 2015). As the most distinctive feature, the morphology of sclerites, including the number and arrangement of rays, is commonly used to dis- tinguish the genera and species of chancelloriids. Allonnia is characterized by a 3+ 0 arrangement of sclerites, long and api- cally directed rays, an apical orifice, and a tuft (Bengtson and Collins, 2015), all of which can be observed in the new species, and therefore its genus placement is confirmed. Apart from the type species, Allonnia tripodophora Doré
and Reid, 1965, ten other species and an indeterminate species have been recognized as Allonnia. Among these, only three species have been studied based on whole-body preservation: Al. phrixothrix Bengtson and Hou, 2001; Al. tintinopsis Bengtson and Collins, 2015; and Al. erjiensis Yun et al., 2017. The species described herein is different from others in that it possesses the following features: the body is baseball bat- shaped; the tuft inconspicuous; the stalk slender, and is prolonged and devoid of sclerites near the basal part. These features help distinguish Al. tenuis n. sp. from other whole-body preserved species and establish it as a new one, and are discussed in more detail below. Perfectly preserved specimens always have a slender stalk
and a narrow point located at its basal end (Figs. 2, 3), which indicates that the stalk is the fixed structure of Allonnia tenuis n. sp., instead of being a possible temporary contraction of the body, as is seen in Al. tintinopsis (Bengtson and Collins, 2015). Besides the slender profile, another distinct feature of the
stalk is that the lowermost part (~3mm above the basal point in the holotype) is devoid of sclerites (Fig. 3). Compared to the natural exposure of the upper part of the body, the lower part can only be exposed by removing the overlying sediment (i.e., when slabs bearing the specimens are split during field excavation, the upper part of this new species is always exposed so it needs no further preparation in the laboratory; this is called the “natural exposure”). These features suggest that Al. tenuis n. sp. might be a kind of sessile benthos with its lowermost part being implanted in the sediment. Although no specimens of this new species have been found anchored to shell debris nor other
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