Almost all aspects of conodont research rely on a sound taxonomy based on comparative analysis. This is founded on hypotheses of homology which ultimately rest on knowledge of the location of elements in the apparatus. Natural assemblages—fossils that preserve the articulated remains of the conodont skeletal apparatus—provide our only direct evidence for element location, but very few are known from the Late Triassic. Here we describe fused clusters (natural assemblages) from the late Norian limestone beds of the Nanshuba Formation in Baoshan, Yunnan Province, southwestern China. Recurrent arrangements and juxtaposition of S and M elements in multiple clusters reveal the composition of the apparatus of Mockina and, probably, Parvigondolella. They indicate that these taxa had a standard 15 elements ozarkodinid apparatus, and provide new insights into the morphology of the elements occupying the P2, M and S locations of the apparatus. The apparatus comprised a single alate (hibbardelliform) S0 element, paired breviform digyrate (grodelliform) S1 and (enantiognathiform) S2 elements, paired bipennate (hindeodelliform) S3 and S4 elements, paired breviform digyrate (cypridodellifrom) M elements, paired, modified-angulate P2 elements (with reduced or lacking 'posterior' process) and segminiplanate (mockiniform and parvigondolelliform) P1 elements. Our results will allow testing of the hypothesis that Mockina, Parvigondolella and Misikella—critical taxa in Late Triassic biostratigraphy—are closely related and possessed morphologically similar elements in homologous locations.
Electronic Supplementary Materials : Supplementary materials, compressed 3D videos of the clusters are available in the online version of this article at https://doi.org/10.1007/s12583-021-1459-2.
Conodonts were eel-shaped chordates that, other than their phosphatic feeding apparatus, lacked a biomineralized skeleton (Aldridge et al., 1995; Gabbott et al., 1995; Purnell et al., 1995; Aldridge and Theron, 1993; Purnell, 1993; Briggs et al., 1983). This feeding apparatus was composed of a number of elements, typically occupying 15 distinct locations, with the occupants of particular locations conforming to more or less distinct shape categories (Purnell et al., 2000; Purnell and Donoghue, 1998). Hundreds of fossils preserve the remains of articulated conodont apparatuses, but the fossil record as a whole is composed almost entirely of millions of disassociated remains. It is upon these isolated conodont elements that the applications of conodonts rely, most notably in Paleozoic and Triassic biostratigraphy.
Almost all aspects of conodont research rely on a sound taxonomy based on comparative analysis founded on hypotheses of homology. Ultimately, hypotheses of homology in conodonts rest on knowledge of their location in the apparatus derived from articulated remains (topological homology; Purnell et al., 2000; Purnell and Donoghue, 1997; Purnell, 1993), but for the majority of conodonts known only as isolated elements, hypotheses of homology can be inferred on the basis of the relationship between element locations and the shapes of their occupants (see Purnell et al., 2018 for discussion).
The situation for Late Triassic conodonts is rather different, with little articulated material known from the final 36 million years of conodont evolution. The majority of Late Triassic conodonts are known from disarticulated elements, and their multielement composition and homologies are inferred (Kolar-Jurkovšek and Jurkovšek, 2010; Kolar-Jurkovšek et al., 2005; Orchard, 2005; Kozur and Mock, 1991). However, the inference of homology may be problematic where morphological categories of element do not correspond closely between taxa, or where similar morphological categories are found in multiple locations of the apparatus. To test the accuracy of components and homologies of reconstructed apparatuses based on isolated elements and to reveal their skeletal architectures, the corresponding natural assemblages, in the form of articulated skeletons in clusters and on bedding planes, are the direct evidence (Purnell et al., 2018; Purnell and Donoghue, 2005). This is important because Late Triassic conodonts are of primary importance in providing evidence for biostratigraphic and chronostratigraphic subdivision; a stable and reliable taxonomy and phylogenetic framework, which in an ideal world should take into account the nature of the whole skeletal apparatus, thus have wider significance.
The few known natural assemblages from the Norian or Rhaetian stages of the Late Triassic, were mainly discovered in Italy and Hungary. Budai and Kovács (1986) discovered 2 clusters of fused P element pairs corresponding to the form genus "Metapolygnathus" slovakensis in the Rezi Dolomite Formation in the Keszthely Mts. of Hungary. Kozur (1989, pl. 18, fig. 6) presented a picture of paired articulated P elements of "Epigondolella" slovakensis. Roghi et al. (1995) mentioned more than 20 clusters and 3 bedding plane assemblages from the Dolomia di Forni in the Seazza Creek Section of Northeast Italy. Unfortunately, Roghi illustrated only a single articulated bedding plane assemblage of "Epigondolella"
slovakensis. Although not published, Demo (2017) recently restudied the Dolomia di Forni (Formation) in the Seazza Creek valley and reconstructed the apparatus of Mockina slovakensis based on 6 fused clusters. Mastandrea et al. (1997) reported the presence of articulated P element pairs of Misikella hernsteini and "Epigondolella" slovakensis, and fused clusters of subparallel ramiform elements (hindeodelliform elements and clusters of Grodella delicatula) in the Colle del Crapio Section in Monte Cocuzzo of Italy. Subsequently, Mastandrea et al. (1999) discovered well-preserved clusters of Norian-Rhaetian age in the Calabrian "Catena Costiera" (southern Italy) comprising P elements assigned to Misikella ultima,
Misikella hernsteini and Misikella posthernsteini, fused with ramiform element arrays. However, the illustrations of this material were presented without any further description or reconstruction of the skeletal apparatuses.
Here we report the first late Norian conodont clusters from China. Twelve well-articulated clusters preserve a consistent arrangement of ramiform elements, while 3 clusters preserve paired segminiplanate elements. This material provides direct evidence for the multielement composition and topological homology of Mockina and, probably Parvigondolella (hereafter referred to as probable Parvigondolella).
1.
GEOLOGICAL SETTINGS AND BIOSTRATIGRAPHY OF THE XIQUELIN SECTION
The Xiquelin Section is located at Jinji Village, about 20 km northeast of Baoshan City, Yunnan Province, southwestern China (Fig. 1). The section extends from the foot to the top of the hill, about 75.5 m in total (Fig. 1c).
Figure
1.
(a) Geological map showing the Xiquelin Section of the study area in China (modified after Bao et al., 2012); (b) road map showing the location of the Xiquelin Section; (c) lithological column of the Xiquelin Section, showing the sampling horizons yielding conodont clusters; (d) lateral view of one specimen of Mockina bidentata (Mosher, 1968), which first occurs at the lowermost section (specimen from sample XQL1); (e) lateral view of the specimen of Parvigondolella andrusoviKozur and Mock, 1972 (from sample XQL39).
This section, situated at the Baoshan Block, primarily consists of medium-thick grayish-white and grey limestone with a small number of banded or nodular cherts (Fig. 1c). The Upper Triassic strata exposed in Baoshao Block were traditionally represented mainly as the Dashuitang Formation (T3d) and the Nanshuba Formation (T3n), in ascending order (BGMRY, 1990; Wang and Dong, 1985). It is generally agreed that the Nanshuba Formation comprises Norian strata and is younger than the Dashuitang Formation (Du et al., 2020; Wang et al., 2019; Zhao et al., 2012). However, there is long standing confusion concerning the distribution of the two formations in the Baoshan region caused by the dramatic lateral variation in their thickness.
The Xiquelin Section was initially assigned to the Dashuitang Formation (BGMRY, 1990, 1980), but the Dashuitang Formation was subsequently incorporated into the Nanshuba Formation in view of the variable thickness and limited distribution of the Dashuitang Formation in the Baoshan Block (Zhang et al., 1996). The recent geological surveys (BGMRY, 2008) followed this approach, assigning the Upper Triassic strata in the Baoshan region to the Nanshuba Formation, abandoning the Dashuitang Formation altogether (Bao et al., 2012). Moreover, the lowermost Xiquelin Section yields Mockina bidentata (Fig. 1d) in sample XQL1, indicating a Norian age for strata, and that they therefore should probably be assigned to the Nanshuba Formation. Consequently, we use the recent division scheme of Upper Triassic strata in this region, attributing the Xiquelin Section to the Nanshuba Formation (Figs. 1a, 1c).
This section is preliminarily divided into the M. bidentata Zone and the Parvigondolellaandrusovi Zone based on the lowermost occurrence of M. bidentata (Mosher, 1968) (Fig. 1d) and the first occurrence of Pa. andrusovi in sampling horizon XQL39 (Figs. 1c, 1e), which implies a late Norian Sevatian age.
2.
MATERIALS AND METHODS
A total of 66 samples, varying between 4.6 and 14 kg, were taken from the Xiquelin Section. These samples are mainly limestones or siliceous limestones. All the samples were crushed into pieces of approximately 2–3 cm3 and processed with 10% dilute acetic acid to dissolve carbonate. The acid-insoluble residues were carefully collected, and were density separated using lithium heteropolytungstate (a water-miscible, nontoxic, high- density and inorganic heavy liquid for conodont separation) (Yuan et al., 2015). Initially, sample processing was intended to recover isolated conodonts to investigate the conodont biostratigraphy, but picking under stereomicroscopy recovered many fused clusters of elements. The study presented here is based on 12 well-preserved clusters, selected on the basis of regular arrangement of ramiform elements and repeated patterns of element juxtaposition (see Table 1, Figs. 2–5).
Table
1.
Composition of conodont clusters and topological homologies of elements present. Clusters 2.4 and 4.2 preserve fused elements, but they are not natural assemblages; we report them for completeness, but they are not referred to in the text.
Figure
2.
Clusters and isolated P2 elements of Mockina from samples XQL25 and XQL26.1a. dextral-dorsal view of cluster 2.1; 1b. dorsal view; 1c. sinistral- ventral view; 1d. ventral view; 2a. cluster 2.2; 2b. upper view of the P1s element in cluster 2.2; 1–2. from sample XQL25; 3a. lower view of the P1d element in cluster 2.3; 3b. upper view of the P1d element in cluster 2.3; 3c. cluster 2.3, from XQL26; 4. cluster 2.4 from XQL25, showing a P2 element; 5. lateral view of an isolated P2 element from XQL26; 6a. sinistral view of the S1s element in cluster 2.6; 6b. dorsal view of cluster 2.6; 6c. ventral view; 6d. dextral-dorsal view; 6e. sinistral view; from sample XQL26. See Electronic Supplementary Materials: compressed 3D videos of the clusters.
Figure
3.
Clusters of Mockina and isolated P2 and S0 elements from sample XQL32.1a. Dextral-dorsal view of cluster 3.1; 1b. dorsal view, the P1d element is segmented out from cluster 3.1 so as to better observe the S-M architecture; 1c. sinistral-ventral view; 1d–1h, successively, sinistral view of the Ms, S4s, S3s, S2s and S1s elements in cluster 3.1; 1i–1m. successively, dextral view of the S1d, S2d, S3d, S4d and Md elements in cluster 3.1; 1n, 1o. respectively, upper view and lower view of the P1d element in cluster 3.1, identified as Mockina bidentata (Mosher, 1968). 2a–2c. successively, dextral view, dorsal view and sinistral view of cluster 3.2; 3a, 3b. respectively, dorsal view and sinistral view of cluster 3.3; 4. an isolated specimen of the P2 element from sample XQL8; 5. an isolated specimen of the S0 element from sample XQL32. See Electronic Supplementary Materials: compressed 3D videos of the clusters.
Figure
4.
Clusters of probable Parvigondolella and isolated P2 and S0 elements from sample XQL39.1a. Sinistral-dorsal view of cluster 4.1; 1b. dorsal view; 1c. dextral view; 1d. a pair of P2 elements in cluster 4.1; 1e–1h. successively, sinistral view of the S4s, S3s, S2s and S1s elements in cluster 4.1; 1i. lateral view of the S0 element in cluster 4.1; 1j–1m. successively, dextral view of the S1d, S2d, S3d and S4d elements in cluster 4.1; 2a. the Ms elements of cluster 4.2; 2b. cluster 4.2 which irregularly fused with a P1s element and a Ms element; 2c, 2d. respectively, upper view and lower view of the P1s element in cluster 4.2; 3a, 3c. upper view of the P1d and P1s element in cluster 4.3; 3b. cluster 4.3 which fused with a pair of displaced P1 elements; 3d. lateral view of a randomly fused P1 element in cluster 4.3, identified as Mockina bidentata (Mosher, 1968); 4, 5. two specimens of isolated S0 element from sample XQL39; 6a, 6b. lateral view and lower view of an isolated P2 element from XQL39. See Electronic Supplementary Materials: compressed 3D videos of the clusters.
Figure
5.
Conodont clusters and isolated P2 elements from sample XQL39.1a–1c. Successively, dextral view, sinistral view and dorsal view of cluster 5.1; 1d. dextral view of the S1d element in cluster 5.1; 1e. sinistral view of the S2d element in cluster 5.1; 1f, 1g. dextral view of the S3d and S4d elements in cluster 5.1; 2a–2c. successively, dextral view, sinistral view and dorsal view of cluster 5.2; 2d–2g. successively, dextral view of the S1d, S2d, S3d and S4d elements in cluster 5.2; 2h. upper view of the P1d element in cluster 5.2, the specimen bears only one marginal denticle; 3a–3b. successively, dextral view, dorsal view and sinistral view of cluster 5.3; 4a–4c. successively, dextral view, dorsal view and sinistral view of cluster 5.4; 5a, 5b. respectively, dextral view and dorsal view of cluster 5.5; 6a–6c. successively, dorsal view, dextral view and sinistral view of cluster 5.6; 7a, 7b. upper view and lower view of an isolated P1 element from sample XQL39; 8a, 8b. lateral view and lower view of an isolated P2 element from sample XQL39. See Electronic Supplementary Materials: compressed 3D videos of the clusters.
The details of element morphology and arrangement in the clusters were revealed through Micro-CT carried out in the Micro-CT Lab at the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan, using a Bruker-MicroCT system—a SkyScan1172F. Method see Zhang et al. (2017). The 3D models of each cluster were generated using CTvox; CTAn was used to segment each element of the fused cluster. All figured specimens and Micro- CT data are housed in School of Earth Sciences, China University of Geosciences, Wuhan.
We follow the terms for orientation and anatomical notation introduced by Purnell et al. (2000). To avoid ambiguity in descriptions of element morphology, as recommended by Purnell et al. (2000), we use 'anterior' and 'posterior' in the traditional sense, to indicate the disposition of processes independently of biological orientation in vivo.
3.
RESULTS
Twelve clusters (Figs. 2.1, 2.6, 3.1–3.3, 4.1, 5.1–5.6) preserve repeated arrangements of ramiform elements of similar morphology. This arrangement is closely comparable to that described in older natural assemblages and reflects in-situ preservation of the original composition and, in large part, architecture of the apparatuses. Table 1 includes these natural assemblages together with 3 clusters of paired P elements (Figs. 2.2, 2.3, 4.3) and 2 randomly fused clusters (Figs. 2.4, 4.2), showing their composition and the topological homologies of the preserved elements, also displayed in different colors in Figs. 2–5 (S0 orange, S1 yellow, S2 green, S3 purple, S4 blue, M red, P1 grey and P2 pink).
Clusters 2.1, 2.2 and 2.4 are from sample XQL25 (Figs. 1c, 2.1, 2.2, 2.4). This limestone layer mainly yields conodont species of Mockina, P1 elements of which are characterized by 2–3 'anterior' platform marginal denticles, unornamented or weakly-ornated lateral-posterior platform margins, 'anteriorly' shifted pit, 'posteriorly' prolonged keel and pointed, truncated or obliquely truncated keel end. Cluster 2.2 contains a pair of displaced P1 elements of Mockina bidentata. Therefore, it is probable that cluster 2.1 preserves the S-M array of Mockina. Cluster 2.1 preserves an intact sinistral S array and dextral S array. The S0 element and M element are present in the cluster but are too broken for their basic morphological characteristics to be discerned. The S0 element is always sandwiched between the sinistral S array and dextral S array. The proximal part of the long 'posterior' process of the M element is commonly fused obliquely to the outer side of the proximal part of the long 'posterior' process of the S4 element, as is evident in the well-preserved cluster 3.1 (Fig. 3.1). The same oblique relationship between the M and S4 elements is preserved in the clusters of Nicoraella illustrated by Huang et al.(2019a, b), and in a number of bedding plane assemblages (Purnell and Donoghue, 1998). Cluster 2.2 (Fig. 2.2a) preserves the clearest evidence for M element morphology. Cluster 2.4 preserves a P2 element (Fig. 2.4); two additional isolated elements from sample XQL25 have identical morphological characteristics with the P2 elements from sample XQL26 (Fig. 2.5). The 3 P2 elements are characterized by a small basal cavity, a broad elongate cusp and an 'anterior' process bearing 3–4 discrete denticles (Figs. 2.4, 2.5).
Cluster 2.3 and 2.6 are from sample XQL26, adjacent to sampling horizon XQL25 (Figs. 1c, 2.3, 2.6). The majority of isolated and fused pairs of P1 elements (Fig. 2.3) from sample XQL26 belong to Mockina, and the isolated specimens of the P2 element from sample XQL26 are nearly identical to the P2 elements from sample XQL25 (Figs. 2.4, 2.5). This indicates that cluster 2.6 is probably also the remains of a species of genus Mockina. Cluster 2.6 is composed of complete sinistral S array (Figs. 2.6b, 2.6c, 2.6e), a broken S0 element, located between S1d and S1s elements (Figs. 2.6b, 2.6c) and most of the dextral S array (Fig. 2.6d). Two incomplete ramiform elements are fused successively outside the S2d element; only the more distal portions of their 'posterior' process is preserved, but they are probably remnants of the S3d and S4d elements. The Ms element (Fig. 2.6e) is slightly dislocated from the position expected in an intact S-M array.
Cluster 3.1, 3.2 and 3.3 are from sample XQL32 (Figs. 1c, 3.1–3.3), in which the majority of isolated P1 elements also belong to Mockina. Cluster 3.1 preserves a complete S-M array, except for a missing S0 (Fig. 3.1), the morphology of which can be inferred from the isolated alate elements from the same sampling layer (Fig. 3.5). The cluster also preserves a P1d element of M. bidentata, fused to the outer surface of the sinistral S and M elements, and we interpret this cluster as the remains of an apparatus of Mockina. Sample XQL32 also contained four isolated P2 elements. These angulate elements have an 'anterior' process with 4 to 6 denticles, a large broad cusp and a small denticle almost fully fused behind the large cusp, like the specimen illustrated in Fig. 3.4. Another P2 element possesses a long 'anterior' process with 7 'posteriorly' inclined denticles, but lacks a 'posterior' process behind the large broad cusp (Fig. 7.12). This form of P2 element is also presented in the apparatus of Mockina slovakensis (Demo, 2017).
Figure
6.
The overall distribution of the collected P2 elements from the Xiquelin Section. Above the sampling layer of XQL50, no specimens of P2 elements were collected.
Figure
7.
Part of Misikella apparatus reconstructed in Mastandrea et al.(1999, 1997), showing the basic architecture of the S series, and the morphologies of numerous types of P2 element from the Xiquelin Section. 1. A cluster with fused grodelliform S1 element from sample CC8 in Mastandrea et al. (1997, pl. 2, fig. 14); 2. a cluster showing the sinistral view of the S4s element and three parallelly arranged ramiform elements which are probably successively be the S3s element, S3d element and S4d element, from sample CC8 of the Colle del Crapio Section in Mastandrea et al. (1997, pl. 2, fig. 12); 3. a hibbardelliform S0 element from sample CC10 of the Colle del Crapio Section in Mastandrea et al. (1997, pl. 1, fig. 12); 4. a cluster of ramiform elements from sample FP1 of the Fosso Pantanelle Section in Mastandrea et al. (1999, pl. 3, fig. 4), showing the basic ventral morphological outline of the dextral S elements; 5a–5b, 6a–5b, 7a–7b, 8a–8b, 9a–9b, 10a–10b, 11a–11b, 12a–12b, P2 elements, successively from samples XQL50, XQL40, XQL39, XQL37, XQL39, XQL39, XQL36 and XQL32; a. lateral view; b. lower view. The scale bar corresponds to the P2 elements.
Clusters 4.1–4.3, and 5.1–5.6 are from sample XQL39 which contains P1 elements assigned only to Mockina and Parvigondolella. Three types of P1 element dominate sample XQL39. One is identified as M. bidentata (258 elements, Fig. 4.3d); another form is similar to M. bidentata (Figs. 4.2b–4.2d), including elements with an extremely reduced platform and just one marginal denticle (114 elements, Figs. 5.2a, 5.2h, 5.7a, 5.7b); thirdly, elements with the morphological characteristics of Parvigondolella, bearing no marginal denticles and no platform (155 elements, Figs. 1e, 4.3a–4.3c). Therefore, clusters 4.1–4.3, 5.1–5.6 represent the remains of either Mockina or Parvigondolella.
Cluster 4.1 comprises a complete S array and a pair of P2 elements (Fig. 4.1), the arrangement of which indicates preservation of elements in their original configuration, reflecting lateral collapse with a slight rotation of the long axis of the conodont body. The small denticles on the 'anterior' process of the paired P2 elements are broken but their morphologies can be inferred from an isolated specimen in sample XQL39 (Figs. 4.6a, 4.6b), which has a similar ratio of the length of the 'anterior' process to the cusp. Cluster 4.3 (Fig. 4.3) consists of a pair of P1 elements of Parvigondolella fused to a P1 element of M. bidentata (Fig. 4.3d). Cluster 5.1 (Fig. 5.1) consists of four parallel dextral S elements. The parallel arrangement suggests very little post-mortem disturbance although the S2d element is missing the distal part of its long 'lateral' process and the S1d element has lost the terminal part of its long 'posterior' process. Cluster 5.2 preserves four dextral S elements and a P1d element with only one marginal denticle. The S1d element is a little dislocated from what was originally a sub-parallel alignment with the other S elements.
4.
DISCUSSION
4.1
Composition of Late Norian Conodont Apparatuses and Implications for Biological Taxonomy
The clusters from samples XQL25, XQL26, XQL32 and XQL39, provide direct evidence that the apparatus of Mockina and that which is probably Parvigondolella consist of a single alate (hibbardelliform) S0 element, paired breviform digyrate (grodelliform) S1 elements, paired breviform digyrate (enantiognathiform) S2 elements, paired bipennate (hindeodelliform) S3 and S4 elements, paired breviform digyrate (cypridodelliform) M elements, paired segminiplanate P1 elements and angulate (ozarkodiniform) P2 elements: 7 distinct morphological categories occupying 15 locations in the apparatus. Demo (2017) independently reached similar conclusions regarding Mockina slovakensis in an unpublished thesis (of which we were unaware), the only differences being in the length of the 'anterior' process and the number of 'anterior' denticles of the P2 elements. Our clusters and those of Demo (2017) confirm Orchard's (2005) hypothesis of apparatus composition and element homology. The types and numbers of the elements in these late Norian conodont apparatuses provide direct confirmation that they possessed a standard, 15-element Ozarkodinid plane (Purnell et al., 2000; Purnell and Donoghue, 1998), indicating that the fundamental architecture and dental formula of conodont apparatuses remained remarkably stable for more than 250 million years (Purnell et al., 2018; Zhang et al., 2017), from the Early Ordovician (Tolmacheva and Purnell, 2002) to the Late Triassic.
The S0 elements possess two short, slightly downward projected and symmetrical lateral processes, a longer 'posterior' process and a prominent, backwardly inclined and sharp- edged cusp, these morphological features conform to form genus Hibbardella.
The S1 elements have a long,
'anterior' process which also gently flexed into an open curve or shallow "C" shape. The cusp is long, large, terminal and transitions smoothly into the basal edge, with only a slight break in this smooth transition curve at the base of the cusp. The denticles are thin, discrete, laterally compressed, and bend inwards (adaxially with respect the apparatus). One to two denticles close to the cusp are much longer than others. In form taxonomy, this category of breviform digyrate S1 element was classified as Cypridodella (Mosher, 1968) and identified as Cypridodella delicatula (Mosher, 1968); later it was attributed to Grodella (Kozur and Mostler, 1970) and identified as Grodella delicatula (Kozur and Mostler, 1970). The S1 elements can be easily discriminated from the M elements by having a terminal cusp.
S1 elements with similar grodelliform morphology to those of Mockina and probable Parvigondolella are present in Anisian Nicoraella (Huang et al., 2019a, b), Ladinian Pseudofurnishius murcianus (Kolar-Jurkovšek et al., 2018) and Neogondolella (Golding, 2018; Orchard, 2005; Orchard and Rieber, 1999), Norian Mockina slovakensis (Demo, 2017) and many other reconstructed apparatuses (e.g., Orchard, 2005, figs. 3J, 4G, 15F), indicating that the occupants of S1 locations of the apparatuses of some conodont genera were morphologically stable from the Middle to the Late Triassic (Anisian to late Norian age).
The S2 elements are characterized by a long 'posterior' process, a largest cusp and a short 'lateral' process, a category of element previously assigned to the form genus Enantiognathus. The S2 elements in all the clusters show nearly identical morphological features.
The M elements, preserved in clusters 2.6 and 3.1 as well as being present among the isolated elements recovered have a long, recured and caudally projected lateral process, a short excurved lateral process and a distinct cusp, resulting in a twisted morphology. The denticles are discrete, laterally compressed, sharp-edged and are inclined relative to the process. The basal edge bears a shallow groove, extending along the length of each process. Beneath the cusp, the element possesses a small basal pit. Morphologically, the M element preserved in the apparatuses of Mockina and probably Parvigondolella resembles elements previously assigned to the form species Cypridodella conflexa (Mosher, 1968); it is distinguished from the form species Cypridodella muelleri (Tatge) in having a groove extending along both processes and from Cypridodella spengleri (Huckriede) in having a lateral flexed short process.
The S3–4 elements have two prominent long denticles near the junction between the relatively short 'anterior' process and long 'posterior' process (Figs. 2.1b, 2.6d, 3.1a, 3.1k, 3.1l, 3.2a, 3.3a–3.3b, 4.1a–4.1c, 4.1e–4.1f, 4.1l–4.1m, 5.2a, 5.2e–5.2f, 5.3a–5.3b, 5.4a–5.4c, 5.5a–5.5c, 5.6a). Of these long denticles, the longest is the cusp, the other being located about 2–3 denticles to the 'anterior'. This category of elements was previously referred to the form species Hindeodella andrusovi andrusovi subsp. Kozur and Mostler, 1972. The occupants of S3 and S4 locations in the apparatus are morphologically similar, but in lateral view the angle at which the 'anterior' and 'posterior' processes meet is always larger in the former.
The morphologies of the elements occupying S0, S1, S2, S3–4 and M locations in Mockina and probable Parvigondolella are quite similar to those of Nicoraella (Huang et al., 2019a, b). In contrast to this relative morphological stability in S and M elements through time, the P1 elements evolved rapidly, and the P2 elements also display apparent changes. Two forms of P2 elements are present in the Mockina apparatuses: one has a short 'posterior' process bearing just one small denticle (Figs. 3.4, 5.8), the other entirely lacks the 'posterior' process (Figs. 2.4, 2.5). Both have a small basal cavity. Between sampling horizon XQL37 which yields the first specimen of Parvigondolella (not Pa. andursovi) and the top of the section, only 2 isolated P2 elements with a short 'posterior' process were collected but 23 isolated P2 elements with no 'posterior' process were recovered (Fig. 6, Table 2). Because many elements of Parvigondolella occur, it seems that the short 'posterior' process form which frequently occurs in conodont faunas with Mockina becomes less frequent, and the form lacking the process dominates. Moreover, 22 of the 23 isolated P2 elements have 1 to 3 'anterior' denticles (the other having 4). Therefore, the P2 elements in Parvigondolella apparatuses probably have no 'posterior' process and possess a short 'anterior' process bearing 3 or fewer denticles (Figs. 7.5–7.8); the paired P2 elements with no 'posterior' process and about 2 'anterior' denticles in cluster 4.1 (Figs. 4.1c, 4.1d) belonged to Parvigondolella. It can be seen that the P2 elements in the Parvigondolella apparatus are quite similar to the P2 elements bearing no 'posterior' process in Mockina apparatuses (Figs. 2.4, 2.5). Combined with the evidence of similar S and M elements, this supports the hypothesis of a close evolutionary relationship between M. bidentata and Pa. andrusovi as proposed by Karádi et al. (2020). On the basis of the occurrence in the Xiquelin Section of the P2 elements with no 'posterior' denticles (Table 2), it can be inferred that the shortening of the 'anterior' process and the decreasing of the 'anterior' denticles began within the M. bidentata Zone. This form of P2 elements was present in the clusters of Mockina slovakensis documented by Demo (2017) and was reported to occur in Rhaetian strata where it is probably the P2 element of cooccurring "Epigondolella" postera s.f. (Ryley and Fåhraeus, 1994, table 2, fig. 3.24). The P2 elements of M. slovakensis possess no 'posterior' denticles but a relatively long 'anterior' process with 6 'posteriorly' inclined denticles, which are quite similar to the specimen presented in Fig. 7.12. Here, in the Xiquelin Section, this form of P2 element is widely distributed within the M. bidentata Zone and Pa. andrusovi Zone (Table 2, Figs. 2.4, 2.5, 4.1d, 4.6a–4.6b, 7.5–7.10) and always co-occurs with P1 elements of Mockina. This co-occurrence suggests that these P2 elements with no 'posterior' process belong to Mockina, suggesting a new lineage characterized by loss of the short 'posterior' process and the small 'posterior' denticle of the P2 elements. This Mockina lineage may further split, with one branch leading to Parvigondolella, and the Mockina branch finally disappearing in the Rhaetian.
Table
2.
Stratigraphic distribution and statistic results of the number of P2 elements from the Xiquelin Section
The other form of P2 elements with one 'posterior' denticle and several 'anterior' denticles ranges from Early Triassic to Late Triassic. Dienerian Neospathodus (Orchard, 2005, fig. 14B), Anisian Nicoraella (Huang et al., 2019a, b; Sun et al., 2009), Carnian Mosherella (Orchard, 2005) all have the P2 elements with one 'posterior' denticle and about 6 'anterior' denticles. Similar P2 elements with 5–7 'anterior' denticles frequently occurred within the M. bidentata Zone of the Xiquelin Section (Figs. 7.11a, 7.11b, Table 2) but 2 specimens recovered from the lower Pa. andrusovi Zone only develop 2–3 'anterior' denticles (Figs. 5.8a, 5.8b, Table 2), which may indicate a trend among the last Mockina of shortening the 'anterior' process and decreasing the 'anterior' denticles while still preserving one 'posterior' denticle in the P2 elements. Furthermore, no specimen of this form of P2 element has hitherto been discovered within the Mi. hernsteini Zone or co-occurring with species of Misikella, which may imply that a lineage of Mockina conodonts having this form of P2 elements disappeared within the Pa. andrusovi Zone or went extinct in the Mi. hernsteini Zone. In short, the P2 elements may show great significance for conodont taxonomy and evolutionary phylogeny.
Orchard (2005) previously reconstructed the apparatus of Cypridodella, the name of which derives from the cypridodelliform M element. The author equated it with the apparatus of Mockina based on the P1 element of "Mockina" multidentata. However, the definition and scope of the main Norian genera (Ancyrogondolella, Epigondolella,
Mockina and Orchardella) are controversial and have been changing (Karádi, 2018; Orchard, 2018; Moix et al., 2007; Budurov and Sudar, 1990). The P1 element of multidentata was respectively assigned to Mockina (Karádi et al., 2020; Orchard, 2005), Orchardella (Orchard, 2018; Kozur, 2003) and Epigondolella (Karádi, 2018; Rigo et al., 2018; Orchard et al., 2001; Lai and Mei, 2000; Orchard, 1991; Mosher, 1970). If we accept the components of Cypridodella determined by Orchard (2005) as correct (putting aside the swapping of the S1 and S2 elements), the P2 element (Orchard, 2005, fig. 4B) and the M element (Orchard, 2005, fig. 4C) are obviously different from those in the reconstructed Mockina apparatus here and the apparatus of Mockina slovakensis presented by Demo (2017). The species multidentata should not belong to Mockina, but, as Orchard (2018) proposed, developed along a separate line from Mockina.
4.2
Architecture of Late Norian Conodont Apparatuses
The relative orientation and arrangement of the S1–4 and the M elements can be unequivocally constrained based on the recurrent clusters of fused with ramiform elements. And by comparing the orientation and arrangement with the apparatuses preserved in the fossils that preserve the remains of the conodont body, we can determine absolute orientations and architecture (Purnell et al., 2000; Aldridge and Theron, 1993). The S0 element is located in the sagittal plane and is flanked by dextral and sinistral S arrays, within which the long axes of elements are parallel (based on clusters 2.1, 2.2 and 4.1). However, the S0 elements in the three clusters are either broken or dislocated, and their precise orientation cannot be determined. As for S1 and S2 locations, based on clusters 2.1, 2.6, 4.1, 5.1 and 5.2 the breviform digyrate occupants of S1 locations are always grodelliform, and the occupants of S2 locations are always enantiognathiform. Previously, the grodelliform (or cypridodelliform) elements were interpreted as occupying S2 (Sb2) locations in the superfamily Gondolelloidea (Sun et al., 2009; Orchard, 2005; Koike et al., 2004; Orchard and Rieber, 1999), but were subsequently shown to occupy S1 locations in apparatuses of Novispathodus (Goudemand et al., 2012, 2011), Nicoraella (Huang et al., 2019a, b) and Mockina slovakensis (Demo, 2017). Our evidence provides further confirmation that grodelliform elements and enantiognathiform elements respectively occupy S1 and S2 locations. Moreover, the overall orientation of processes in the breviform digyrate S1 and S2 elements contrasts strongly with that of the bipennate S3 and S4 elements, with the terminal cusp of the S1 element and the long cusp of the S2 element directed caudally while the cusps and the 'anterior' processes of the S3 and S4 elements are more rostrally located. This is also a characteristic of Nicoraella (Huang et al., 2019a, b) and Mockina slovakensis (Demo, 2017) and was illustrated by Orchard and Rieber (1999). This orientation may indicate that the S1–2 elements worked together and that their function was somewhat different to that of S3–4 elements. Furthermore, the paired S1 elements in clusters 2.1, the S1d element in cluster 3.1 and the S1s element in cluster 4.1 show that the S1 elements are slightly longer than the S3–4 elements. The S3 and S4 elements are similar in length (Figs. 2.1b, 4.1b–4.1c, 4.1e–4.1f, 4.1l–4.1m). The dextral array of cluster 3.1, the sinistral array of cluster 4.1, and clusters 5.1–5.6 show the cusps of the S3–4 elements are parallel and oriented in the same direction, which indicates that the S3 and S4 elements are closely parallel to each other (Figs. 2.1a–2.1c, 5.1a–5.1c, 5.2a, 5.3–5.6) despite the most rostral position of the S3–4 elements being slightly different in the well-preserved clusters 3.1 and 4.1. In addition, clusters 4.1 and 5.2 show the long denticle of the S3–4 elements, about 2–3 small denticles away from the cusp, is also parallel to the longest denticle of the distal part of the long process of the S2 element.
From cluster 3.1, we can see the pair of M elements are symmetrically and obliquely located outside the pair of S4 elements, and the inner side of the M element always faces the S series. The S-M architecture of late Norian conodont apparatuses, including Mockina slovakensis (Demo, 2017), is nearly identical to that of the Anisian Nicoraella apparatus, both conforming to the standard 15-element Ozarkodinid plan, indicating no major change occurred in the overall function of the feeding apparatus of conodonts in the Late Triassic.
4.3
Implications for Evolutionary Relationships and Upper Triassic Biostratigraphy: Comparisons with Misikella
Mockina species are important in Upper Triassic biozonation, and M. bidentata has been proposed as the progenitor of both Parvigondolella and Misikella (Karádi et al., 2020), with the first appearance of Misikella posthernsteini identified as a possible marker for the base of the Rhaetian Stage (Rigo et al., 2018). The phylogeny of Karádi et al. (2020) was explicitly articulated as a framework for taxonomic improvements and biostratigraphic studies, and although the analysis employed rigorous cladistic approaches, they were nonetheless limited in drawing on data from P1 elements alone, which can be misleading (e.g., Atakul-Özdemir et al., 2012). The details of the morphology of all the elements of Mockina presented here offers a potential test of the closeness of the relationship between Misikella and Mockina because the phylogenetic hypothesis implies close similarity of the skeletal apparatus, not just the P1 elements. This test requires a comparison of the apparatuses of the two genera, and evidence for the apparatus of Misikella exists in the form of both fused clusters and isolated elements illustrated by Mastandrea et al.(1999, 1997).
Mastandrea et al. (1997) presented a cluster of grodelliform S1 element (pl. 2, fig. 14; Fig. 7.1), two clusters of hindeodelliform S3–4 elements (pl. 2, figs. 12, 13; Fig. 7.2) from the sample CC8 of the Colle del Crapio Section. Aside from ramiform elements, their sample CC8 and the layers above it yielded only P1 elements of the genus Misikella, so the associated ramiform elements are likely to be components of the Misikella apparatus, including hindeodelliform (=S3–4 elements), enantiognathiform (=S2 element), grodelliform (=S1 element), hibbardelliform (=S0 element) and prioniodiniform elements (=M? element).
The S0 (Fig. 7.3), S1 (Fig. 7.1) and S3–4 elements (Fig. 7.2) from sample CC8 in Mastandrea et al. (1997) undoubtedly belong to Misikella. However, although they recovered associated enantiognathiform and prioniodiniform elements Mastandrea et al. (1997) did not illustrate them. Mastandrea et al. (1999) subsequently reported conodonts from the Fosso Pantanelle Section which also yielded collections in which P1 elements were represented almost exclusively by Misikella. The associated ramiform clusters (Mastandrea et al., 1999, pl. 3, fig. 4; Fig. 7.4) show that the basic outline of S3–4 elements is similar to the S3–4 elements of Mockina and probable Parvigondolella but exhibit some differences compared to the S3–4 elements figured by Mastandrea et al. (1997). The S3–4 elements presented in Mastandrea et al. (1997) are shorter or smaller, bearing a nearly straight lower profile of the 'posterior' process. It is uncertain whether these differences between the two form of S3–4 represent variation within a or between taxa; they may be nothing more than different growth stages, as they all are associated only with P1 elements of Mockina.
Unfortunately the illustrations of these clusters reveal few details of the morphology of the S2 and S3–4 elements. Without this information we cannot currently determine whether the similarities between the apparatuses of Mockina and Misikella are sufficient to support the closeness of relationship suggested by Karádi et al. (2020), but further study of these elements and comparison with our evidence for Mockina and probable Parvigondolella will resolve the question.
5.
CONCLUSIONS
The well-preserved conodont clusters described here exhibit recurrent arrangements of ramiform elements and paired P1 and P2 elements that furnish direct evidence of the composition and skeletal architecture of the youngest known conodont apparatuses. These specimens provide another example of the standard 15-element template for conodont skeletal architecture. On the basis of four clusters with nearly complete preservation of the rostral S-M array, supplemented with other clusters and isolated conodont collections from the same limestone horizon, the apparatus of Mockina and a probable Parvigondolella apparatus have been reconstructed. These late Norian conodont apparatuses have similar composition, consisting of a single hibbardelliform S0 element, paired grodelliform S1 elements, paired enantiognathiform S2 elements, paired hindeodelliform S3–4 elements, paired cypridodelliform M elements, paired ozarkodiniform P2 elements and paired P1 elements (mockiniform, parvigondolelliform).
The grodelliform elements and the enantiognathiform elements respectively occupies the S1 and S2 positions. These elements have their cusps positioned more caudally relative to the hindeodelliform S3–4 elements, which have the cusp and the 'anterior' process positions more rostrally. The S1 elements are slightly longer than the S3–4 elements which are the same in length and closely parallel. S1–4 elements exhibit parallel arrangement and are symmetrically disposed about the axial S0 element, with all the denticles slightly curved toward the rostral-caudal axis. Our evidence that the standard 15 element apparatus remained stable right into the late Norian confirms the remarkable stability of the ozarkodinid apparatus from the Ordovician to the final days of the conodonts.
ACKNOWLEDGMENTS:
This work was supported by the National Natural Sciences Foundation of China (Nos. 41830320, 41972033, 41572324). We thank Prof. James G. Ogg for his valuable comments on earlier version of this manuscript. Thanks are also due to Binxian Qin, Hanxishuo Dong, Yifan Gong and Zichen Fang for their assistance in field sampling. We are grateful to Xulong Lai, Jinyuang Huang and Michael J. Orchard for their valuable suggestions and constructive remarks. Micro-CT scanning and imaging and all SEM pictures were undertaken at the State Key Laboratory of Biogeology and Environmental Geology, Wuhan. The final publication is available at Springer via https://doi.org/10.1007/s12583-021-1459-2.
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Table
1.
Composition of conodont clusters and topological homologies of elements present. Clusters 2.4 and 4.2 preserve fused elements, but they are not natural assemblages; we report them for completeness, but they are not referred to in the text.
Figure 1. (a) Geological map showing the Xiquelin Section of the study area in China (modified after Bao et al., 2012); (b) road map showing the location of the Xiquelin Section; (c) lithological column of the Xiquelin Section, showing the sampling horizons yielding conodont clusters; (d) lateral view of one specimen of Mockina bidentata (Mosher, 1968), which first occurs at the lowermost section (specimen from sample XQL1); (e) lateral view of the specimen of Parvigondolella andrusoviKozur and Mock, 1972 (from sample XQL39).
Figure 2. Clusters and isolated P2 elements of Mockina from samples XQL25 and XQL26.1a. dextral-dorsal view of cluster 2.1; 1b. dorsal view; 1c. sinistral- ventral view; 1d. ventral view; 2a. cluster 2.2; 2b. upper view of the P1s element in cluster 2.2; 1–2. from sample XQL25; 3a. lower view of the P1d element in cluster 2.3; 3b. upper view of the P1d element in cluster 2.3; 3c. cluster 2.3, from XQL26; 4. cluster 2.4 from XQL25, showing a P2 element; 5. lateral view of an isolated P2 element from XQL26; 6a. sinistral view of the S1s element in cluster 2.6; 6b. dorsal view of cluster 2.6; 6c. ventral view; 6d. dextral-dorsal view; 6e. sinistral view; from sample XQL26. See Electronic Supplementary Materials: compressed 3D videos of the clusters.
Figure 3. Clusters of Mockina and isolated P2 and S0 elements from sample XQL32.1a. Dextral-dorsal view of cluster 3.1; 1b. dorsal view, the P1d element is segmented out from cluster 3.1 so as to better observe the S-M architecture; 1c. sinistral-ventral view; 1d–1h, successively, sinistral view of the Ms, S4s, S3s, S2s and S1s elements in cluster 3.1; 1i–1m. successively, dextral view of the S1d, S2d, S3d, S4d and Md elements in cluster 3.1; 1n, 1o. respectively, upper view and lower view of the P1d element in cluster 3.1, identified as Mockina bidentata (Mosher, 1968). 2a–2c. successively, dextral view, dorsal view and sinistral view of cluster 3.2; 3a, 3b. respectively, dorsal view and sinistral view of cluster 3.3; 4. an isolated specimen of the P2 element from sample XQL8; 5. an isolated specimen of the S0 element from sample XQL32. See Electronic Supplementary Materials: compressed 3D videos of the clusters.
Figure 4. Clusters of probable Parvigondolella and isolated P2 and S0 elements from sample XQL39.1a. Sinistral-dorsal view of cluster 4.1; 1b. dorsal view; 1c. dextral view; 1d. a pair of P2 elements in cluster 4.1; 1e–1h. successively, sinistral view of the S4s, S3s, S2s and S1s elements in cluster 4.1; 1i. lateral view of the S0 element in cluster 4.1; 1j–1m. successively, dextral view of the S1d, S2d, S3d and S4d elements in cluster 4.1; 2a. the Ms elements of cluster 4.2; 2b. cluster 4.2 which irregularly fused with a P1s element and a Ms element; 2c, 2d. respectively, upper view and lower view of the P1s element in cluster 4.2; 3a, 3c. upper view of the P1d and P1s element in cluster 4.3; 3b. cluster 4.3 which fused with a pair of displaced P1 elements; 3d. lateral view of a randomly fused P1 element in cluster 4.3, identified as Mockina bidentata (Mosher, 1968); 4, 5. two specimens of isolated S0 element from sample XQL39; 6a, 6b. lateral view and lower view of an isolated P2 element from XQL39. See Electronic Supplementary Materials: compressed 3D videos of the clusters.
Figure 5. Conodont clusters and isolated P2 elements from sample XQL39.1a–1c. Successively, dextral view, sinistral view and dorsal view of cluster 5.1; 1d. dextral view of the S1d element in cluster 5.1; 1e. sinistral view of the S2d element in cluster 5.1; 1f, 1g. dextral view of the S3d and S4d elements in cluster 5.1; 2a–2c. successively, dextral view, sinistral view and dorsal view of cluster 5.2; 2d–2g. successively, dextral view of the S1d, S2d, S3d and S4d elements in cluster 5.2; 2h. upper view of the P1d element in cluster 5.2, the specimen bears only one marginal denticle; 3a–3b. successively, dextral view, dorsal view and sinistral view of cluster 5.3; 4a–4c. successively, dextral view, dorsal view and sinistral view of cluster 5.4; 5a, 5b. respectively, dextral view and dorsal view of cluster 5.5; 6a–6c. successively, dorsal view, dextral view and sinistral view of cluster 5.6; 7a, 7b. upper view and lower view of an isolated P1 element from sample XQL39; 8a, 8b. lateral view and lower view of an isolated P2 element from sample XQL39. See Electronic Supplementary Materials: compressed 3D videos of the clusters.
Figure 6. The overall distribution of the collected P2 elements from the Xiquelin Section. Above the sampling layer of XQL50, no specimens of P2 elements were collected.
Figure 7. Part of Misikella apparatus reconstructed in Mastandrea et al.(1999, 1997), showing the basic architecture of the S series, and the morphologies of numerous types of P2 element from the Xiquelin Section. 1. A cluster with fused grodelliform S1 element from sample CC8 in Mastandrea et al. (1997, pl. 2, fig. 14); 2. a cluster showing the sinistral view of the S4s element and three parallelly arranged ramiform elements which are probably successively be the S3s element, S3d element and S4d element, from sample CC8 of the Colle del Crapio Section in Mastandrea et al. (1997, pl. 2, fig. 12); 3. a hibbardelliform S0 element from sample CC10 of the Colle del Crapio Section in Mastandrea et al. (1997, pl. 1, fig. 12); 4. a cluster of ramiform elements from sample FP1 of the Fosso Pantanelle Section in Mastandrea et al. (1999, pl. 3, fig. 4), showing the basic ventral morphological outline of the dextral S elements; 5a–5b, 6a–5b, 7a–7b, 8a–8b, 9a–9b, 10a–10b, 11a–11b, 12a–12b, P2 elements, successively from samples XQL50, XQL40, XQL39, XQL37, XQL39, XQL39, XQL36 and XQL32; a. lateral view; b. lower view. The scale bar corresponds to the P2 elements.
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