Leptomitid sponges from the Cambrian (Stage 4) Balang Fauna of Guizhou, China

Journal Pre-proof Leptomitid sponges from the Cambrian (Stage 4) Balang Fauna of Guizhou, China Qiu-Jun Wang Jin Peng Rong-Qin Wen Guang-Ying Du Hui Zhang De-Zhi Wang Yi-Fan Wang

PII:

S0016-6995(19)30122-6

DOI:

https://doi.org/doi:10.1016/j.geobios.2019.10.005

Reference:

GEOBIO 889

To appear in:

Geobios

Received Date:

3 October 2018

Accepted Date:

2 October 2019

Please cite this article as: Wang, Q.-J., Peng, J., Wen, R.-Q., Du, G.-Y., Zhang, H., Wang, D.-Z., Wang, Y.-F.,Leptomitid sponges from the Cambrian (Stage 4) Balang Fauna of Guizhou, China, Geobios (2019), doi: https://doi.org/10.1016/j.geobios.2019.10.005

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Leptomitid sponges from the Cambrian (Stage 4) Balang Fauna of Guizhou, China  Qiu-Jun Wang, Jin Peng *, Rong-Qin Wen, Guang-Ying Du, Hui Zhang, De-Zhi Wang, YiFan Wang Guizhou Research Centre for Palaeontology & College of Resources and Environmental Engineering, Guizhou University, Guiyang 550000, China

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* Corresponding author. E-mail address: [email protected] (J. Peng). Corresponding editor: Xingliang Zhang.

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Abstract

New Cambrian sponge fossils can generate alternative interpretations of the phylogenetic and

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biogeographical history of early sponges. Recently, many sponge fossils have been discovered from the Stage 4 Balang Fauna of Guizhou, China; these sponges are dominated

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by protomonaxonids, some of which can provide reliable evidence for interpreting the abovementioned history. The sponge fossils, having the basic skeletal architecture of leptomitids,

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are recognized here as Jianhella obconica nov. gen., nov. sp., and Leptomitus teretiusculus Chen, Hou et Lu, 1989. The new taxon has a large obconical shape; it is characterized by a dominantly longitudinal skeleton consisting of isolated longitudinal and oblique monaxons, and apparently lacks transverse spicules. The obconical body form and unique skeleton show that Jianhella nov. gen. clearly differs from previously described leptomitids and suggest that it may represent an intermediate morphology between leptomitids and basal protomonaxonids. The occurrence of Jianhella nov. gen. from a single deposit and its absence elsewhere may indicate that it was a monospecific, endemic genus, but may also suggest that it was a rare genus exemplified by its occurrence from a single site with very few specimens. Additional specimens with an elongated tubular shape and a skeleton consisting of bundled longitudinal monaxons and isolated transverse spicules closely fit the description and illustrations of Leptomitus teretiusculus from the Cambrian biotas of South China. According

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to the spicule morphology, the degree of spicule bundling and the skeletal architecture, there likely exists a deep node separating Jianhella nov. gen. from the earlier ancestral members of Leptomitus. Fossil evidence, together with previous studies, confirms that Leptomitus generally had a relatively long stratigraphic range (Cambrian Stage 3 to Drumian) and had an effectively cosmopolitan distribution during the early and middle Cambrian. Keywords: Jianhella nov. gen. Leptomitus

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Phylogeny Biogeography

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Balang Formation

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South China 1. Introduction

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Leptomitids have been initially defined as a family based on possession of monaxons only (de Laubenfels, 1955), and were considered to be protomonaxonids by Finks and Rigby

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(2004). Fossil evidence suggests that the family Leptomitidae de Laubenfels, 1955 currently includes at least four Cambrian genera (Walcott, 1886, Rigby, 1986; Chen et al., 1989), one Ordovician genus (Botting and Zhang, 2013), one Silurian genus (Rigby and Harris, 1979) and one Triassic genus (Botting et al., 2019), i.e., Leptomitus Walcott, 1886, Leptomitella Rigby, 1986, Paraleptomitella Chen, Hou et Lu, 1989, Wapkia Walcott, 1886, Heteractenigma Botting et Zhang, 2013, Wareiella Rigby et Harris, 1979, and Pseudoleptomitus Botting, 2019. Leptomitids have been discovered so far from Cambrian Stage 3 to Wuliuan and the Lower Ordovician of South China (Chen et al., 1989; Rigby and Hou, 1995; Yang and Zhao, 2000; Hu et al., 2010; Zhao et al., 2011; Botting and Zhang, 2013; Hou et al., 2017; Wang et al., 2017, 2018); from Cambrian Stage 3 to Wuliuan, the lower Silurian and the Lower Triassic of North America (Walcott, 1886, 1920; Rigby and Harris, 1979; Rigby, 1983, 1986, 1987; Rigby et al., 1998; Rigby and Collins, 2004; Brayard et al., 2017; Botting et al., 2019); from the Cambrian Stage 4 Sinsk Biota of Siberia (Ivantsov

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et al., 2005); and from the Wuliuan−Drumian Murero Fm. of Spain (García-Bellido, 2003). In addition, some undescribed specimens in accordance with leptomitids have been discovered from the Cambrian Stage 4 Emu Bay Fauna of Australia (Paterson et al., 2016), the Early Ordovician Fezouata Biota of Morocco (Botting, 2016: p. 77, figs. 3(C), 4(A−C)) and the Darriwilian sponge faunas of Builth Inlier, Wales, UK (Muir and Botting, 2015). A series of studies suggest that leptomitids were the major components of Cambrian sponge communities (García-Bellido, 2003; Hu et al., 2010; Paterson et al., 2016; Wang et al., 2017, 2018; Botting and Muir, 2019) and had a wide palaeogeographic distribution (García-Bellido et al., 2007;

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Botting and Muir, 2019). These sponges have simple morphologies and are axially symmetrical; however, their morphologies show a wide range in expansion rate, from conicalcylindrical to rounder, goblet-shaped (Walcott, 1886, 1920; Rigby and Harris, 1979; Rigby,

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1986; Chen et al., 1989; Rigby and Collins, 2004; Botting and Zhang, 2013; Botting et al., 2013). Leptomitids are generally characterized by a skeleton consisting of large longitudinal

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monaxons and fine sub-transverse monaxons (Botting et al., 2013). Although previously

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described taxa of this family generally fall within the diagnosis summarized by Botting et al. (2013), specimens from the Niutitang Sponge Fauna of Zunyi, China (i.e., Paraleptomitella? sp.; Zhao et al., 1999), the Early Ordovician of China (i.e., Heteractenigma yui Botting et

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Zhang, 2013; Botting and Zhang, 2013), the Silurian of British Columbia (i.e., Wareiella typicala Rigby et Harris, 1979; Rigby and Harris, 1979; Rigby et al., 1998) and the Early Triassic Paris Biota (i.e., Pseudoleptomitus advenus Botting, 2019; Brayard et al., 2017; Botting et al., 2019), have increased the diversity of their skeletal architectures. Paraleptomitella? sp. possesses spicules that are largely parallel to each other and are oblique to the sponge axis (Zhao et al., 1999: fig. 1(7); Botting et al., 2013). Interestingly, W. typicala possesses a thatched skeleton principally composed of longitudinal monaxons (Rigby and Harris, 1979: p. 978−979, fig. 2(9, 10); Rigby et al., 1998: p. 210, fig. 6(5−7)) but also has horizontal to diagonal monaxons (Rigby and von Bitter, 2005). Although these sponge specimens have oblique spicules (Rigby and Harris, 1979; Rigby et al., 1998; Zhao et al., 1999), relating them to leptomitid lineages is preferable because the skeletal architecture

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allows a clear separation of these sponges from other protomonaxonid groups (Botting et al., 2013). Leptomitids are widespread through the early and middle Cambrian (Walcott, 1886, 1920; García-Bellido, 2003; Rigby and Collins, 2004; Ivantsov et al., 2005; Yang et al., 2005; Hou et al., 2017; Wang et al., 2018); however, large gaps remain in our knowledge of their phylogenetic relationships to each other and to other sponge groups, even at high taxonomic levels (Botting and Zhang, 2013; Botting and Muir, 2018). Since leptomitids may have

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different origins (Botting et al., 2013; Botting and Muir, 2018), there have been many discussions about their nature (Reitner and Mehl, 1995; Rigby and Collins, 2004; Botting and Zhang, 2013; Botting et al., 2013). The absence of hexactines implies that leptomitids are secondarily derived from the basal protomonaxonids that possess a skeleton consisting of sub-

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longitudinal or sub-helical monaxons, combined with irregularly arranged hexactine-based

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spicules (Botting et al., 2013). Alternatively, H. yui from the Early Ordovician Ningkuo Fm. suggests that leptomitids are probably derived from a related, but slightly separated branch of

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hexactine-bearing sponges that possess both longitudinal and transverse monaxons as well as heteractine (pentaradiate and triradiate) spicules (Botting and Zhang, 2013). However,

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leptomitids possess a discrete transverse (strictly, low-angle helical in some taxa) skeletal complement, and in most cases, the skeleton is combined with the bundling of longitudinal monaxons (such as Leptomitus lineatus Walcott, 1920; Botting et al., 2013: p. 7, fig. 2(1, 3)). These regular aspects of the skeletal architecture are likely to be synapomorphies of this family and indicate that these leptomitids probably evolved from a basal protomonaxonid with a more ordered architecture (Botting et al., 2013). The precise origin of leptomitids therefore remains uncertain, and they may originate from a deep division within the protomonaxonids or from a separate region of either the poriferan or a class-level stem group (Botting et al., 2013). An increasing range of skeletal architectures and spicule forms in Cambrian leptomitids can provide possible alternative interpretations of their phylogenetics. Recently, a sponge assemblage with articulated and disarticulated spiculate sponge remains was discovered from

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the Stage 4 Balang Fauna of Guizhou, China (Wang et al., 2018). Some sponge fossils having the basic skeletal architecture of leptomitids were recognized from this fauna. A new taxon and a known species are described here, including Jianhella obconica nov. gen., nov. sp. and Leptomitus teretiusculus Chen, Hou et Lu, 1989. Together with previous studies, the phylogenetic and biogeographical implications of Jianhella nov. gen. and Leptomitus are discussed. 2. Geological and chronological settings

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The Balang Fm. outcrops in eastern Guizhou and western Hunan, China (Zhou et al., 1979; Bureau of Geology and Mineral Resources of Guizhou, 1987; Peng and Babcock, 2001; Liu and Lei, 2013) represent a part of the Jiangnan Slope between the Yangtze Platform and the Jiangnan Basin (Yin, 1996; Peng and Babcock, 2001) (Fig. 1(A)). The lower part of the

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Balang Fm. is composed of grey-greenish to yellow-greenish clayey shale, while the upper

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part consists of light grey calcareous shale and silty shale with intercalations of thin argillaceous carbonates (Zhou et al., 1980; Bureau of Geology and Mineral Resources of

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Guizhou, 1987; Peng et al., 2005, 2012a, b; Peng, 2009; Liu et al., 2017, Fig. 1(B)). Generally, the Balang Fm. represents a shallowing-upwards sequence (Peng, 2009; Da et al.,

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2011; Yan et al., 2014; Liu et al., 2017; Liu, 2018). The overlying Tsinghsutung Fm. is dominated by green-grey thin limestones, while the underlying Bianmachong Fm. is mainly composed of grey-black mudstone (Bureau of Geology and Mineral Resources of Guizhou, 1987; Liu, 2018; Fig. 1(B)).

Zhou et al. (1980) first provided a biostratigraphic framework for the Balang Fm.; subsequently, the biostratigraphy has been revised on several occasions (Zhou and Yuang, 1980; Yuan et al., 2001, 2006; Peng, 2009; Qin et al., 2010). Yan et al. (2014) included the entire Balang Fm. in the Arthricocephalus chauveaui Zone based on trilobite biostratigraphy. According to this correlation of the Cambrian strata between South China and other continents (Qin et al., 2010; Yuan et al., 2011; Yan et al., 2014; Yuan and Ng, 2014; Shen et al., 2016), the Balang Fm. is now regarded as Stage 4. 3. Material and methods

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Many fossil assemblages have been discovered from the middle to upper parts of the Balang Fm. at different localities (Peng et al., 2005, 2012a, b; Peng, 2009; Ma et al., 2011; Liu, 2018; Liu et al., 2018a); the Balang Fauna comprises at least 80 species of 63 genera (Liu, 2018; Liu et al., 2018b), including chancelloriids, cnidarians, sponges, worms, brachiopods, echinoderms, molluscs, hyoliths, arthropods, algae and many trace fossils (Peng et al., 2006, 2007, 2010a, b, 2012c, 2016; Fu et al., 2010; Qin et al., 2010; Liu, 2013a, b, 2018; Zhao et al., 2015; Sun et al., 2015, 2016, 2017; Shen et al., 2016; Liang et al., 2017; Wen et al., 2017, in press; Liu et al., 2018a; Wang et al., 2018). There are currently 163

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sponge specimens (including the 36 specimens collected in 2018) collected from the very fossiliferous Lazizhai section at Lazizhai Village, Jianhe County, Kaili City, Guizhou, China (Fig. 1(A)).

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The fossils include both fully articulated skeletons and partly and completely

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disarticulated remains (Wang et al., 2018, in press). All sponge specimens are deposited at Guizhou Research Centre for Palaeontology, Guizhou University, and are indicated by their

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accession numbers. Jianhella obconica nov. gen., nov. sp. is known from three specimens (JLSHM17009, 17010, 18116) and their counterparts, while L. teretiusculus is known from

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five specimens (JLSHM17104, 17114, 17247, 17435, 2625) and their counterparts. Sponge fossils having the basic skeletal architecture of leptomitids are described based on their body forms and skeletal characters.

Photographs were taken using a VHX-100K three-dimensional microscope and a Leica M205C stereo-microscope. The gross morphology and skeletal architecture are described based on standard terminology (Zhang, 1991; Finks and Rigby, 2004; Botting and Zhang, 2013; Botting et al., 2013).

4. Systematic palaeontology Phylum Porifera Grant, 1836 Class uncertain Order? “Protomonaxonida” Finks et Rigby, 2004 sensu Botting et al., 2013

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Family Leptomitidae de Laubenfels, 1955 Genus Jianhella Wang et Peng, nov. gen. Derivation of the name: The generic name is derived from Jianhe County, in which the fossils were discovered. Type and only known species: Jianhella obconica Wang et Peng, nov. gen., nov. sp. Occurrence: Balang Formation, Lazizhai Village, Jianhe County, Guizhou, China.

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Diagnosis: Obconical sponge with a narrow base. Skeleton appearing as a longitudinal architecture and consisting of three main elements: fine longitudinal monaxons; coarse, long, (sub-)longitudinal monaxons; and coarse, long, oblique monaxons. No transverse spicules are

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present, and no spicules are clustered into bundles. Fine longitudinal monaxons, as the dominant elements, are evenly spaced and generally straight. Coarse (sub-)longitudinal

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monaxons are also major elements and are moderately widely spaced, forming slightly

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overlapping, well-defined rods.

Remarks: This new genus is characterized by a dominantly longitudinal skeleton of monaxons and thus can be placed into the Group 1 protomonaxonids of Botting et al. (2013).

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The monaxon organization, in contrast, accord with that of the basic skeletal architecture of leptomitids. However, the combination of the body form and skeletal architecture of the present specimens is currently entirely unique and is unlike that of all previously described leptomitid taxa, suggesting that these specimens should represent a previously unknown leptomitid taxon. The possible relationships of this new genus with other leptomitid protomonaxonids are discussed below, based on the phylogenetic framework proposed by Botting et al. (2013).

Jianhella obconica nov. gen., nov. sp. Figs. 2−5 2018. Leptomitus cf. L. conicus García-Bellido, Gozalo, Chirivella Martorell et Liñán, 2007 Wang et al., p. 314, text-fig. 2(h, j).

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Derivation of the name: The specific epithet is derived from the distinctive obconical body form. Holotype: JLSHM17009A, B (Fig. 2(C, D)). Paratypes: JLSHM17010A, B (Fig. 2(A, B)), JLSHM18116A, B (Fig. 5(A, B)). Material: Three specimens and their counterparts: JLSHM17009 is a large specimen with an articulated skeleton and well-preserved sponge margins; JLSHM17010 is a small specimen with longitudinal and oblique monaxons; JLSHM18116 is a large specimen with a similar

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skeletal architecture and is probably referable to this species.

Type locality and horizon: Lazizhai Village, Jianhe County, Kaili City, Guizhou, China;

Diagnosis: As for the genus, by monotypy.

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Balang Formation, Cambrian Stage 4.

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Description: The holotype (JLSHM17009A, B; Fig. 2(C, D)) is a partial specimen of a tall obconical sponge, more than 46.2 mm wide and more than 119.0 mm tall. The skeleton (Figs.

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2−4) is dominantly longitudinal and is composed of three main elements: fine longitudinal monaxons; coarse, long (often several centimetres long), sub-longitudinal monaxons; and

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coarse, long, oblique monaxons. One partial specimen (JLSHM17010A, B) is partially disarticulated and shows the longitudinal and oblique monaxons (Fig. 2(A, B)). Although the orientations of some spicules appear to be 0° in specimen JLSHM18116 (Fig. 5(A, B)), by comparison with the other specimens, these spicules are more likely to be oblique with a low angle rather than horizontal. Thus, the skeleton probably lacks transverse spicules (Fig. 4). No spicules are clustered into bundles.

The fine longitudinal monaxons are the dominant elements and are evenly spaced and generally straight (Figs. 2, 3). These spicules are 0.02−0.05 mm in width and 1.26−1.74 mm in length. The coarse sub-longitudinal monaxons are the most obvious elements and are wallparallel and moderately widely spaced, forming slightly overlapping, well-defined rods (Figs.

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2−4). The intervals between adjacent rods increase slightly from the lower to the upper parts of the sponge, with rods more densely packed at the lower end (Fig. 2(C, D)). Rods are 0.03−0.10 mm wide and up to 20.7−57.1 mm long (often several centimeters long), with the longest instance running for over half of the length of the specimens (Fig. 2(C, D)). The terminations of coarse monaxons are visible, and their distal tips are sharply pointed (Figs. 2, 3(A, B)). The coarse, non-longitudinal monaxons are generally oblique rather than perpendicular to

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the sponge axis, generally at 8−64° from the horizontal (Figs. 2, 3(A−D), 4(A−C, E)). Oblique spicules are usually present in two opposing sets at the upper regions of the holotype (Fig. 2(C, D)), and they are frequently encountered in specimen JLSHM18116 (Fig. 5(A, B)). These oblique monaxons are 0.05−0.10 mm wide and up to 16.3−28.5 mm long, with the

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longest instance obliquely running through the width of the specimens (Fig. 2(C, D)). Some

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oblique monaxons project from the margin in the upper part of the specimen (Figs. 2, 3(A, B), 4(B, C)); this appearance is the result of compression rather than originally projecting

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spicules.

A third specimen (JLSHM18116A, B; Fig. 5(A, B)), possessing a similar skeletal

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architecture and probably referable to this species, is a partially preserved articulated fragment, 9.7 mm tall and 4.3 mm wide at its maximum width. The original size of the sponge body is unknown because the lateral body margins are not preserved. The fine longitudinal monaxons are 0.03−0.05 mm in width and 0.81−2.67 mm in length, while the coarse vertical monaxons are 0.05−0.09 mm wide and 27.7−48.3 mm long. However, most vertical monaxons are sub-longitudinal rather than strictly longitudinal (Fig. 5(A, B, D)). Moreover, more oblique spicules, usually in two opposing sets, are present in this specimen (Fig. 5). These oblique spicules are also oblique to the sponge axis and are generally 6−50° from the horizontal. These coarse monaxons are 0.05−0.09 mm wide and up to at least 13.7−49.4 mm long, with the longest obliquely running through the width of the specimen (Fig. 5(A, B)). Remarks: Protomonaxonids were the major components of Cambrian sponge communities (Botting and Muir, 2019) and can fall into two unrelated but coherent groups (Botting et al.,

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2013): one primitive sponge lineage consisting of taxa with long, mostly sub-longitudinal spicules, and the other being a basal demosponge group including spiculate taxa with complex arrays composed of tracts of minute (millimeter-scale) monaxons. According to Botting et al. (2013), the current specimens are included in the first group of protomonaxonids because they are characterized by a dominantly longitudinal skeleton of monaxons that are often several centimeters in length (at least some spicules that are a minimum of 10 mm long). At least four subgroups can be recognized in the first group based on the body forms and skeletal characters (Botting et al., 2013), including basal protomonaxonids, leptomitids, hamptoniid

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and ‘choiid’ sponges, and halichondritid-piraniid sponges.

Fossil evidence suggests that the size, morphology and local arrangement of spicules, as well as the skeletal architecture, are the most reliable features for interpreting the

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protomonaxonid classification (Botting et al., 2013). Although some specimens of Choiaella,

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halichondritid and piraniid sponges may be more or less similar to the current specimens in body form and spicule arrangement, the differences are detailed below. Choiaella sponges are

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also conical; however, they are characterized by a skeleton of radiating thatch of small longitudinal monaxons of one size (Rigby and Hou, 1995; Finks and Rigby, 2004). The

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family Halichondritidae Rigby, 1986 includes conico-tubular to steeply obconical sponges with many prominent, coarse marginalia and prostalia (Rigby, 1986; Finks and Rigby, 2004). Moreover, in halichondritid sponges, the principal skeleton is made of long, upwardly plumescent, monaxial spicules, and the main endosomal net is a coarse thatch of generally vertically oriented oxeas (Rigby, 1986; Finks and Rigby, 2004). The family Piraniidae de Laubenfels, 1955 includes subcylindrical to obconical branching sponges with the wall pierced by circular canals (Rigby, 1986; Finks and Rigby, 2004). Besides, in piraniid sponges, the marginalia consist principally of tylostyles with points directed upwardly and outwardly, and the principal skeleton is composed of upwardly and outwardly radiating, subparallel tufts of oxeas (Rigby, 1986; Finks and Rigby, 2004). So, according to Finks and Rigby (2004) and Botting et al. (2013), the current fossils cannot be assigned to the other three subgroups mentioned above, while the presence of ‘rods’ composed of overlapping sub-longitudinal monaxons as well as oblique spicules is a critical feature for interpreting them as a leptomitid.

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Leptomitids currently include at least seven genera (Finks and Rigby, 2004; Botting et al., 2013; Botting and Muir, 2018; Botting et al., 2019): Leptomitus, Leptomitella, Paraleptomitella, Wapkia, Heteractenigma, Wareiella and Pseudoleptomitus. In general, leptomitids are characterized by a skeleton consisting of large longitudinal monaxons and fine, sub-transverse monaxons (Botting et al., 2013); however, the specimens from the Niutitang Sponge Fauna (Zhao et al., 1999) and the Silurian of British Columbia (Rigby and Harris, 1979; Rigby et al., 1998), suggest that some leptomitids have a skeleton composed principally of longitudinal monaxons but also possess oblique spicules. Thus, although the

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current specimens lack transverse spicules, relating them to leptomitids is preferred because the skeleton consists of dominantly longitudinal monaxons as well as oblique ones. Because the obconical body form and the unique skeleton allow a clear separation of the present

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specimens from the seven previously described leptomitid genera, they are considered to

Genus Leptomitus Walcott, 1886

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represent a new taxon of previously unrecorded leptomitids.

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Figs. 6, 7

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Leptomitus teretiusculus Chen, Hou et Lu, 1989

2018. Leptomitus teretiusculus Chen, Hou et Lu, 1989 - Wang et al., p. 314, text-fig. 2(a). Referred specimens: JLSHM17435A, B (Fig. 6(A, B)); JLSHM2625A, B (Fig. 6(C, D)); JLSHM17104A, B (Fig. 6(E, I)); JLSHM17114A, B (Fig. 6(F, G)); JLSHM17247A, B (Fig. 6(H, J)).

Material: Five partially preserved specimens and their counterparts. Locality: Lazizhai Village, Jianhe County, Kaili City, Guizhou, China. Horizon: Balang Formation, Cambrian Stage 4. Description: Fossil specimens are very elongated tubular in shape (Fig. 6) and include a bestpreserved articulated skeleton. All specimens are compressed to flattened. Specimens range from 13.5 to 45.5 mm in height and are about 2.9 to 14.4 mm in width (Fig. 6), without

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pronounced differences in morphology. Fossil specimens suggest that the sponge has a very thin-walled body with a double-layered skeleton, with single-axis (monaxon) spicules (Fig. 6). Horizontal spicules are not clustered into bundles. Walls lack parietal gaps and major canals. The outer skeletal layer consists of a vertical thatch of large oxeas and inserted fine monaxons. Large oxeas are dominant elements and arranged vertically along the length of the sponge body (Figs. 6, 7(A, B)). The spacing between large oxeas is consistent from the lower to the upper parts of the sponge fossils (Fig. 6) and is 0.20−0.35 mm in width. Large oxeas

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appear to overlap up to one-half of the total length (Fig. 6(A−D)); they are 3.2−7.5 mm in length and are 0.06−0.12 mm in width. There is a vertical thatch of small monaxons between adjacent large oxeas (Figs. 6, 7). These small monaxons are generally 0.01−0.05 mm wide

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and 1.6−2.9 mm long and appear to be arranged tip to tip with a slight overlap (Fig. 7(E)).

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The inner skeletal layer is composed of tiny, poorly defined, horizontal monaxial spicules

0.28−0.84 mm long.

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(Fig. 7(A−C)). These horizontal monaxons are generally 0.01−0.03 mm wide and

Remarks: When compared with other leptomitid sponges, these sponge fossils do not

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present a bundled appearance of small spicules in the inner skeletal layer (Fig. 7(A−C)), nor do present the diagonal net-like pattern of coarse oxeas in the outer skeletal layer (Figs. 6, 7). Thus, the combined presence of spicule characteristics assigns these sponge fossils to the genus Leptomitus. These fossils (Figs. 6, 7) are characterized by the elongated tubular shape and by the inner layer of unbundled monaxons, which closely fits the description and illustrations of L. teretiusculus provided by Chen et al. (1989). Leptomitus zitteli Walcott, 1886, from the Cambrian (Stage 3−4) Parker Slate Fm. in Vermont, is elongated tubular to obconical in shape (Rigby, 1987; García-Bellido et al., 2007) and clearly differs in its morphology. Although these fossils bear a close resemblance in morphology to L. lineatus from the Stage 5 of Canada, important differences are evident. In L. lineatus, the outer thatch of vertical spicules is distinctly striped (Walcott, 1920), with a much more regular-appearing skeletal net than that in L. teretiusculus (Rigby and Hou, 1995).

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Additionally, L. lineatus is a considerably larger sponge (Walcott, 1920; Rigby and Hou, 1995). Leptomitus undulatus Rigby et Collins, 2004, from the middle Cambrian of Walcott Quarry, has a rounder goblet-shaped body (Rigby and Collins, 2004) and is markedly different in body form. ‘Leptomitus’ conicus García-Bellido, Gozalo, Chirivella Martorell et Liñán, 2007 was described by García-Bellido et al. (2007) based on 45 specimens from the middle Cambrian Murero Fm. of Spain. This generic attribution may be problematic because its linear

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structures are not consistent with sponge spicules. One alternative interpretation is that ‘L.’ conicus is some sort of ridged conical shell, most likely a hyolithid. However, Leptomitus cf. L. lineatus Walcott, 1920 from this formation is an unambiguous fossil sponge and shows a close similarity in skeletal architecture to L. lineatus (García-Bellido, 2003) rather than to L.

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teretiusculus.

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The specimens from the Drumian Marjum Fm. of Utah were previously described as Leptomitus metta Rigby, 1983 (Rigby, 1983) but have been assigned to the genus

architecture.

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5. Discussion

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Leptomitella (Rigby, 1986; Finks and Rigby, 2004). They differ clearly in their skeletal

5.1. Phylogenetic implication

Several new, structurally different taxa of leptomitids have been discovered since 1886 (Walcott, 1886, 1920; Rigby and Harris, 1979; Rigby, 1983, 1986; Chen et al., 1989; GarcíaBellido, 2003; Rigby and Collins, 2004); however, determining the accurate phylogenetic position of early leptomitids is much more complex than previously appreciated (Botting et al., 2013). The difficulties in interpretation may be resolved by identifying intermediate forms between one group and other, more derived groups (Botting et al., 2013). According to Botting et al. (2013), the absence of hexactines implies that leptomitids are secondarily derived from basal protomonaxonids. However, it is important to note that leptomitids may have an independent origin from a different group of sponges than other protomonaxonids, as

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revealed by the Ordovician Heteractenigma fossils, which contain monaxons as well as stauractine (or hexactine) and heteractine spicules (Botting and Zhang, 2013; Botting and Muir, 2018). Although the relationships of leptomitids with other early sponge groups remain unclear (Botting and Muir, 2018) and the precise form of any intermediates between leptomitids and basal protomonaxonids remains unknown (Botting et al., 2013), the presence of oblique spicules in Paraleptomitella? sp. (Zhao et al., 1999) suggests that this sponge may represent an intermediate form between leptomitids and basal protomonaxonids (Botting et

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al., 2013). In the phylogenetic framework proposed by Botting et al. (2013) (Fig. 8), the absence of hexactine-based spicules (character 2) is an important difference between Jianhella nov. gen. and basal protomonaxonids; however, the lack of transverse spicules (character 5) is also a

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key characteristic separating Jianhella nov. gen. from the later-branching leptomitids. Based

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on this scenario, the unique skeleton (without transverse spicules or clustered spicules) suggests that Jianhella nov. gen. was probably derived from a deep division within earlier

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leptomitid sponges in which the transverse spicules had not yet developed and all spicules were not yet bundled. Jianhella nov. gen. is also somewhat intermediate in form between

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leptomitids and basal protomonaxonids (Fig. 8). According to García-Bellido et al. (2007) and Botting et al. (2013), the degree of spicule bundling is assumed to be a key feature for interpreting the internal relationships of leptomitids. Because the skeleton of Leptomitus is characterized by bundled longitudinal monaxons and isolated transverse monaxons (such as L. lineatus) (Walcott, 1886, 1920; Chen et al., 1989; García-Bellido, 2003; Finks and Rigby, 2004; Rigby and Collins, 2004; Botting et al., 2013), a deep node possibly exists that separates Jianhella nov. gen. from the earlier ancestral members of Leptomitus (Fig. 8). In the phylogenetic tree given by Botting et al. (2013), Leptomitus is presumed to have evolved from a related, but slightly separated branch of leptomitid sponges that possess bundled longitudinal and isolated transverse monaxons (Fig. 8). 5.2. Palaeobiogeography

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The longer the time that a fauna is studied, the greater the number of rare species that are encountered (Botting and Muir, 2019). Even if very rare taxa are present in very low numbers, these taxa will likely be discovered when the corresponding biotas have been sufficiently sampled. Rare species usually appear as endemic and are often represented by a single monospecific occurrence from a single deposit. For example, Guizhoueocrinus Zhao, Parsley et Peng, 2007 and Protogloboeorinus Zhao et al., 2015, having only been reported from the Balang Fm., are monospecific genera (Zhao et al., 2007, 2015) and are probably endemic. Jianhella nov. gen. may also be a monospecific, endemic genus, but its occurrence from a

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single deposit and its absence elsewhere may also be explained by life rarity. This phenomenon can also be exemplified by the monospecific sponge genera known only from the Burgess Shale (Raymond, 1931; Rigby, 1986; Rigby and Collins, 2004; Botting and Muir,

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2019), e.g., Petaloptyon Raymond, 1931, Stephenospongia Rigby, 1986, Eiffelospongia Rigby et Collins, 2004, Hamptoniella Rigby et Collins, 2004, Hapalospongia Rigby et

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Collins, 2004, and Ulospongiella Rigby et Collins, 2004, all known from very few specimens.

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None of these sponges have been reported so far from other deposits, and they all appear to have been endemic taxa, but this may change following intensive collecting of other deposits.

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Leptomitus has been found from the Cambrian strata of South China (Chen et al., 1989; Rigby and Hou, 1995; Yang and Zhao, 2000; Yang et al., 2005, 2010; Zhao et al., 2011; Hou et al., 2017; Wang et al., 2017, 2018), North America (Walcott, 1886, 1920; Rigby, 1983, 1986; Rigby and Collins, 2004) and Spain (García-Bellido, 2003). Leptomitus sponges exhibit large temporal and geographic ranges (Finks and Rigby, 2004; García-Bellido et al., 2007; Botting and Muir, 2019), ranging from the Stage 3 to the Drumian and occurring across at least three continents (i.e., South China, Laurentia and peri-Gondwana). Thus, the fossil evidence indicates that Leptomitus generally had an effectively cosmopolitan distribution and had a relatively long stratigraphic range over a timespan of ca. 16 m.y. 6. Conclusions Two leptomitid sponges, Jianhella obconica nov. gen., nov. sp. and Leptomitus teretiusculus, are respectively recognized from the Stage 4 Balang Fauna of South China

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based on the body forms and skeletal characters. The new taxon has a large obconical shape and possesses a skeleton consisting of isolated longitudinal and oblique monaxons without transverse spicules. Jianhella nov. gen. may be somewhat intermediate in form between leptomitids and basal protomonaxonids. In addition, the occurrence of Jianhella nov. gen. from a single deposit and its absence elsewhere may indicate that it was a monospecific, endemic genus, but may also suggest that it was a rare genus since it is currently present in very small numbers in a single site. The differences in spicule morphology, the degree of spicule bundling, and the skeletal architecture between Jianhella nov. gen. and Leptomitus

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suggest that a deep node separates Jianhella nov. gen. from the earlier ancestral members of Leptomitus. Fossil evidence confirms that Leptomitus has a relatively long stratigraphic range (Stage 3 to Drumian) and has an effectively cosmopolitan distribution during the early and

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middle Cambrian.

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Acknowledgments

We are grateful to Joseph P. Botting (Amgueddfa Cymru−National Museum Wales,

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Cathays Park, UK), Jonathan B. Antcliffe (University of Lausanne, Switzerland), Yu Liu (Yunnan University, China), an anonymous referee and the editors for their constructive and

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helpful comments that significantly improved the manuscript. We thank John Paterson (University of New England) for supplying an image of Emu Bay specimens, Lixia Li (Nanjing Institute of Geology and Palaeontology, CAS) for providing useful references, Tian Lan (Guizhou University) for discussing the age of Cambrian biotas, and Yuning Yang (Guizhou University) for helping to distinguish the skeletal layers of the present compressed specimens. Thanks are also due to Shuai Liu for his assistance in fieldwork. This research was financially supported by the National Natural Science Foundation of China (No. 41672005), the Guizhou Science and Technology Department Foundation of China (Nos. Gui. Sci. Z. [2014]4003, Gui. Sci. G. [2017]5788, Gui. Sci. [2019]1124), and the Research Foundation for the Introduced Talents of Guizhou University (No. 201535). References

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Figure captions Fig. 1. A. Map showing the distribution of the Balang Formation (grey areas) in Guizhou and the location of the fossil locality (star). B. Lithological column of the Lazizhai section. Fig. 2. Jianhella obconica nov. gen., nov. sp. A, B. Small additional fragment and its counterpart, JLSHM17010A, B, showing the longitudinal and oblique monaxons. C, D. overall view of the holotype JLSHM17009A, B, showing a typical obconical shape and a longitudinal skeleton with long longitudinal and oblique monaxons. Scale bars: 1 cm.

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Fig. 3. Jianhella obconica nov. gen., nov. sp.: enlargements of JLSHM17009, showing details of the skeletal architecture. A, B. Longitudinal skeleton with sub-longitudinal monaxons and opposing oblique monaxons as well as some projecting from the sponge margin at varied

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angles. C. Longitudinal monaxons and opposing oblique monaxons. D−F. Fine and coarse longitudinal monaxons, coarse oblique monaxons and the rods arranged tip to tip with a slight

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overlap (arrow). Scale bars: 2 mm (A−C), 1 mm (D−F).

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Fig. 4. Jianhella obconica nov. gen., nov. sp., photographed under a mixture of glycerol and ethyl alcohol: enlargements of JLSHM17009, showing details of the skeletal architecture.

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A−C. Longitudinal skeleton with sub-longitudinal and wall-parallel monaxons, (opposing) oblique monaxons as well as some projecting from the sponge margin at varied angles. D, F. Fine and coarse (sub-)longitudinal monaxons and the rods arranged tip to tip with a slight overlap (arrow). E. Fine and coarse (sub-)longitudinal monaxons and opposing oblique monaxons. Scale bars: 2 mm.

Fig. 5. A, B. Overall view of JLSHM18116A, B, possessing a similar skeletal architecture and probably referable to Jianhella obconica nov. gen., nov. sp.: longitudinal skeleton with sublongitudinal monaxons, and (opposing) oblique monaxons. C, D. Enlargements of JLSHM18116, showing fine and coarse (sub-)longitudinal monaxons and opposing oblique monaxons. Scale bars: 1 cm (A, B), 2 mm (C, D). Fig. 6. Leptomitus teretiusculus: fragmented specimens showing the elongated tubular shape and the longitudinal rods of the outer skeletal layer. A, B. JLSHM17435A, B. C, D.

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JLSHM2625A, B. E, I. JLSHM17104A, B. F, G. JLSHM17114A, B. H, J. JLSHM17247A, B. Scale bars: 5 mm. Fig. 7. Leptomitus teretiusculus. A, B. Enlargements of JLSHM17114 and JLSHM17104, showing the longitudinal rods of the outer skeletal layer and the faint transverse lines (indicated by the arrowheads) corresponding to the tiny, monaxial spicules of the inner skeletal layer. C, D. Enlargements of JLSHM2625 and JLSHM17435, showing the longitudinally coarse rods (arrows) and small oxeas (arrowheads) of the outer skeletal layer, and the tiny, monaxial spicules of the inner skeletal layer (indicated by the black arrow). E.

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enlargement of JLSHM2625, showing the longitudinal rods of the outer skeletal layer as well as the fine oxeas arranged tip to tip with a slight overlap (indicated by the arrows). Scale bars: 2 mm.

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Fig. 8. Phylogenetic hypothesis for the protomonaxonids, which includes basal protomonaxonids, leptomitids, hamptoniids and most choiids (modified after Botting et al.,

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2013). New genera A, B and C were described briefly in Botting et al. (2013). No scale of

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time or morphological transformation is implied vertically. Major character state transitions: 1, conical growth form, with longitudinal monaxons and short-rayed hexacts; 2, loss of

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hexactines; 3, regular helical monaxons; 4, loss of monaxons; hexact rays reduced to stubs; 5, transverse monaxons; 6, bundled longitudinal monaxons; 7, bundled transverse monaxons; some plumose arrays; 8, plumose arrays dominant; 9, open conical body form; 10, flattened body form, coronal spicules; 11, bimodal monaxon array, conical body form; 12, thickened organic sheath of major spicules; 13, spicules enlarged, fan-like body form; 14, inclined prostalia developed, tall conical body form; 15, minor spicules reduced; major spicules developed into organic-walled prostalial sclerites.

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