Acanthotretella decaius, Hu & Zhang & Holmer & Skovsted, 2010

Hu, Shixue, Zhang, Zhifei, Holmer, Lars E. & Skovsted, Christian B., 2010, Soft-part preservation in a linguliform brachiopod from the lower Cambrian Wulongqing Formation (Guanshan Fauna) of Yunnan, South China, Acta Palaeontologica Polonica 55 (3), pp. 495-505 : 499-501

Acanthotretella decaius sp. nov.

Figs. 2A–F , 3A–D .

Etymology: Latinised from Chinese Decai, excellent and brilliant,also acknowledges the Decai scholarship, sponsored by Professor Shu Degan in NWU for financial support of the study to excellent undergraduate and graduate students in NWU.

Type material: Holotype GKG CA−007 ( Fig. 2A ) composed of part (marked as A) and counterpart (B).

Type locality: Gaoloufang section (N 24 ° 57’10’’; E 102 ° 47’55’’) at Guangwei Village in Kunming of Yunnan Province ( Fig. 1 ) .

Type horizon: The lower part of the Wulongqing Formation ( Series 2, Stage 4), Palaeolenus Zone, Canglangpuian Stage of South China (generally thought to correlate with the Botomian Stage of the early Cambrian in Siberia) .

Material.— Holotype plus GKG CA−001AB with marginal setae and pedicle ( Fig 2D ); GKG CA−002AB with setal remains ( Fig 2E ); GKG CA−003AB with setal fringes and pedicle ( Fig 3F ); GKG CA−004AB; GKG CA−005AB; GKG CA−006, a juvenile with some setal fringes and pedicle; GKG CA−008AB with fine preservation of lophophore and coelomic pedicle ( Fig. 2B, C ); GKG CA−009; GKG CA−010, a distorted specimen with proximal pedicle; GKG CA−011 and 012, shell valves, GKG CA−013AB with 4 individuals, of them 2 with setae and pedicles; GKG CA−014 with 2 individuals with preserved pedicle. Also, two of these individuals have a partially preserved gut, while the other shows faint lophophore imprints; GKG CA−015 with setal remains and a pedicle; GKG CA−016 with pedicle and inferred muscular scars.

Diagnosis.—Shell large with minor phosphatic mineralisation; surface with thin concentric fila; maximum width anterior of mid−valve; ventral pseudointerarea well developed, occupying about 1/3 of the sagittal valve length, probably apsacline lacking distinct flexure lines; elongate and thin pedicle, corresponding to around 15% of the maximum width of the shell; visceral areas of both valves, short, triangular, approximately extending anteriorly to 1/4 of dorsal valve length, and 1/3 of the sagittal ventral valve length.

Measurements.—See Table 1.

Description.—Shell biconvex and tear−shaped to sub−triangular in outline; the maximum sagittal length is 14 mm and the maximum width is 11.5 mm, at about the anterior third of shell length in one specimen. The ratio of shell length to width ranges from 1.22–1.67 (on average 1.38; see Table 1 for details of dimensions below). The ventral pseudointerarea is well developed, apsacline, occupying 37.7% of the sagittal length of the ventral valve, and 65.3% of shell width; the ventral pseudointerarea has a somewhat wide acute angle varying between 60–75 °. The ornamentation consists of thin concentric growth lines ( Figs. 2B 1, C 1 , 3A, B ), delineated by slender reddish fila. The concentric fila are so faint that they could be directly discerned exclusively on the anterolateral portion of some specimens ( Fig. 2B 1, C 1 ). The ventral pseudointerarea is compressed as a triangular, flattened plate with reddish−brown tints. No flexure lines and transverse striations can be observed on the surface of the ventral pseudointerarea.

The setae of Acanthotretella decaius are most frequently preserved along the anterior and anterolateral margins ( Fig. 2B–F ), and poorly exposed in the posterior parts of the valves. Setae are preserved as reddish, delicate linear fringes or tints. In one specimen, the setae seemingly emerge upright to the anterior shell margin ( Fig. 2D 1 , D 2 ), but curve posteriline (tiled arrows) and the pedicle bending steeply downward into the sediment; note the paired lophophore imprints inside shell valves (simple arrows) and a longitudinal coelomic lumen in pedicle (circled arrows); details of the pedicle lumen (simple arrows) (B 2). C. GKG CA−008B, counterpart of B, showing the paired lophophore imprints in general view (C 1); details of lophophore imprints of C 1 (C 2,). D. GKG CA−001A, an anterolaterally crushed specimen presumably in life position, with the pedicle steeply buried in sediment. An overview (D 1), boxes indicate the position of D 2 and D 3; detailed views of the marginal setae along the anterior and lateral margins (D 2, D 3). E. GKG CA−002B, posterior view of a dorso−ventrally compressed specimen; note the inferred pedicle tube (simple arrow) and some small rounded scars putatively allied with some epibionts (tailed arrows). F. GKG CA−003A, a dorso−ventrally compressed specimen in general view (F 1), note the marginal setae, sub−triangular pseudointerarea and proximal pedicle; close−up view of box marked in F 1 (F 2). See Fig. 4 for interpretations.

doi:10.4202/app.2009.1106

orly along the lateral margins ( Fig. 2D 3 ). The maximum setal length is 1.2 mm and they are about 80 µm thick.

A total of 12 individuals with the pedicle preserved were found. The pedicles vary in length and width between different specimens, but the pedicle is never preserved in its entire length. The observable maximum length of the pedicle is about 52 mm, which is more than 4 times the sagittal maximum length of the shell ( Fig. 2A 1 ). The maximum width of the pedicles ranges between 0.6–1.5 mm, corresponding to an average of 15% of the maximum width of the shell. In the type specimen ( Fig. 2A ), the pedicle surface is ornamented with pronounced annulated rings, disposed at intervals of about 0.2 mm ( Fig. 2A 3 , A 4 ). However, no surface ornament could be recognised in the other specimens, probably due to preservational factors. In the centre of the pedicle ( Fig. 2B 1 ), there is a dark longitudinal lineation, extending from the proximal to the distal part of the pedicle ( Fig. 2B 2 ). It is here interpreted as the coelom of the pedicle. A terminal pedicle bulb, like that found in Lingulelltreta malongensis (Zhang et al. 2005: fig. 3O), is lacking and there is no direct evidence for benthic shell anchorage as seen in the Chengjiang taxa Longtancunella and Xianshanella ( Zhang et al. 2006, 2007c,

growth lines lophophore pseudointerarea pedicle tube mouth body cavity proximal pedicle pseudointerarea epibionts setae

2008, 2009) which could be due to the fact that no pedicle is preserved in its full length. The pedicles invariably arise from the circular foramen at the apex of the ventral valve, with the proximal pedicle end enclosing the tip of the ventral pseudointerarea. Nevertheless, at the posterior part of specimen GKG CA−002B, there is an inferred medial lineation, which is taken to represent a possible internal pedicle tube ( Figs. 2E , 3B ). The internal pedicle tube extends through a possible narrow external tube ( Figs. 2E and 5 ), which is continuous with the coelomic lumen. In most cases, the pedicles are preserved as flattened impressions, parallel to the bedding plane or slightly bending into the sediments ( Fig. 2A 1 ). By contrast, in specimen GKG CA−001( Fig. 2D 1 ), the pedicle appear to be relatively more steeply inserted into the sediments. This type of preservation may demonstrate that the brachiopod was buried in situ.

As seen in Fig. 2B 1 , C the lophophore organisation of Acanthotretella decaius is only preserved in two slightly anterolaterally compressed specimens. It is represented by a pair of darkish imprints, extending from the antero−medial position of the visceral region, and then coils inward symmetrically about the valve midline ( Figs. 2B 1 , 2C , 4A ). The mouth is presumed to be located at the base of the lophophore ( Fig. 4A )

The visceral region forms a sub−triangular section ( Fig. 2A 1 , A 2 ), with an anterodorsal projection located in a posteromedial position of the dorsal valve. The projection does not extend to midlength of the ventral valve, and it reaches to approximately the posterior 1/3 of the dorsal valve length. There are no clearly defined muscle scars in any of the specimens and this absence has also been noted from the Chengjiang lingulids ( Jin et al. 1993). Nevertheless, the digestive tract of A. decaius could be evidently detected in two specimens ( Fig. 3C, D ), where it forms a faint U−shaped tube with some relief, approximately 1 mm beyond the hinge line ( Fig. 4C, D ); the digestive tract is closely similar to that described from the Chengjiang lingulids ( Zhang et al. 2004a, 2005, 2006, 2007a, c; Balthasar and Butterfield 2009).

Figures

Fig. 2. Linguliform brachiopod Acanthotretella decaius sp. nov. from the Wulongqing Formation (Palaeolenus Zone), lower Cambrian (recently Series 2, Stage 4), China. A. Holotype, GKG CA−007 in general view (A1); posterior details of the shell valves (A2), showing the small body cavity posteromedially placed, the elongate subtriangular pseudointerarea and the proximate pedicle; details of the gentle S−shaped portion of pedicle in A1 (A3); close−up view of the pedicle annulations in A1 (A4). B. GKG CA−008A, a flattened specimen anterolaterally compressed in general view (B1), showing the concentric growth +

Fig. 3. Linguliform brachiopod Acanthotretella decaius sp. nov. from the early Cambrian Guanshan fauna, showing the proximal pedicles (tailed arrows) and gut remains (simple arrows), Yunnan, China. A. GKG CA−014, a muddy slab with two individuals preserved together. B–D. Photographs showing the U−shaped digestive tracts preserved as tubular imprints (simple arrows) with some relief. B. GKG CA−011B. C. GKG CA−007B. D. GKG CA−013(2). General view (C1, D1), close−up (C2, D2) of tubular relief of guts. See Fig. 4 for interpretations of C1 and D1.

Fig. 1. A. Map showing the locality (star) where the brachiopod Acanthotretella decaius sp. nov. was collected. B. Stratigraphical correlation of the muddy deposits yielding the Guanshan fanua including A. decaius sp. nov. in Gaoloufang section near Guangwei village, Kunming, southern China.

Fig. 4. Sketch drawings of the linguliform brachiopod Acanthotretella decaius sp. nov. illustrated in Fig. 2 and 3. A. Sketch of Fig. 2C1 (GKG CA−008B). B. Sketch of Fig. 2 E(GKG CA−002B). C. Sketch of Fig. 3C1(GKG CA−007B). D. Sketch of Fig. 3D1(GKG CA−013(2)). Scale bars 5 mm.

Fig. 5. Reconstruction of Acanthotretella decaius sp. nov. based on GKG CA−007,GKG CA−008, GKG CA−002B, showing disposition of lophophore, small visceral cavity and location of measurements indicated in Table 1.