An early Daonella from the Middle Anisian of Guangxi, southwestern China, and its phylogenetical significance

Abstract

The late Middle Anisian Daonella fengshanensis sp. nov. from the upper part of the Lower Member of the Banna Formation near Jinya in northwestern Guangxi, southwestern China, represents the earliest appearance of Daonella in this region. With regard to stratigraphical occurrence as well as morphological features the new species is in a transitional position between Enteropleura and more derived species of Daonella. The new species is an early representative of the genus Daonella and regarded as phylogenetically being near the genus’ origin. It is characterised by a small posterior auricle similar to Enteropleura and flat-topped radial ribs resembling later species of Daonella. Its anterior shell sector is large. The shell sculpture of juvenile stages closely resembles both Daonella and Enteropleura, supporting the hypothesis that Daonella may have evolved from Enteropleura.

Introduction

Species of Enteropleura, Daonella, and Halobia of the family Halobiidae are among the most short-lived and widely distributed flat-shelled bivalves of the Triassic, making them excellent biostratigraphical index fossils in certain facies (e.g. Campbell 1994; McRoberts 1997, 2000, 2010; Schatz 2004, 2005; Waller in Waller and Stanley 2005; Chen and Stiller 2007). Waller (in Waller and Stanley 2005, p. 17, fig. 5) proposed a phylogeny for Triassic “flat clam” genera including Bositra, Enteropleura, Daonella, Aparimella, Halobia, and others, in which Daonella was considered to be derived from Enteropleura in the early Middle Triassic and to have evolved into Aparimella in the latest Middle Triassic. Enteropleura is a short-ranged genus, occurring within a narrow stratigraphical interval of late Middle Anisian age in Nevada, Europe, and southern China (Waller in Waller and Stanley 2005; Chen and Stiller 2007; McRoberts 2010). Daonella is younger than Enteropleura and generally considered to have its maximum abundance during Late Anisian and Ladinian times (e.g. Kittl 1912; Chen et al. 1992; Campbell 1994; McRoberts 2010). However, recognition of the first appearance of Daonella remains controversial (e.g. Campbell 1994).

Mojsisovics (1874) regarded the first appearance of Daonella to be in the Lower Muschelkalk. According to Schatz (2005, p. 116) the earliest occurrence of Daonella in Alpine Europe is in the Late Anisian Daonella sturi Zone that is correlated with the upper Paraceratites trinodosus Zone. Silberling and Nichols (1982) showed that Daonella makes its first appearance in Nevada, USA, during the early Late Anisian, represented by Daonella americana Smith in the lower part of the Gymnotoceras rotelliformis Zone. Campbell (1994, p. 34) recognised the first appearance datum (FAD) of Daonella in Arctic Svalbard also in the early Late Anisian, represented by Daonella lindstroemi Mojsisovics. Chen (1982) and Gan (1983) regarded Daonella guizhouensis Gan to be the earliest Daonella in southern Guizhou, southwestern China. In the Guizhou succession, D. guizhouensis occurs in a stratigraphical position about 20–30 m below D. americana (Gan 1983); therefore, it is slightly older than the latter species, but most probably also of early Late Anisian age.

On the other hand, based on Begg (1981), Campbell (1994, p. 25) mentioned that the FAD of Daonella in New Zealand lies within the Middle Anisian, thus being earlier than the Daonella FAD in Nevada, Alpine Europe, Svalbard, and Guizhou. Unfortunately, the study of Begg (1981) is unpublished and was not available for reference. Therefore, it is difficult to correlate the Middle Anisian Daonella of New Zealand with the Chinese specimens.

In this contribution we report a new species, Daonella fengshanensis sp. nov., from the Jinya section in Fengshan County, northwestern Guangxi, southwestern China (Fig. 1), providing new information on the early evolution of the genus Daonella. Although the available specimens of D. fengshanensis are relatively poorly preserved, they are of phylogenetical significance and represent the earliest appearance of Daonella in southern China (Figs. 2, 3). In the Jinya section, the late Middle Anisian D. fengshanensis occurs in a transitional position between Enteropleura and more derived species of Daonella, about 90–100 m below the Daonella americana–Daonella guizhouensis beds, and about 9–11 m above the Enteropleura bed. Significantly, the morphological features of the new taxon are intermediate between Enteropleura and more derived species of Daonella.

Fig. 1

Location of the fossil site near Jinya in Fengshan County, northwestern Guangxi, southwestern China. a General map of China with the area of map b marked. b Map of northwestern Guangxi and southwestern Guizhou with location of the fossil site and reconstructed Anisian (Middle Triassic) sedimentary facies, indicated by standard symbols (palaeogeographical reconstruction after Chen and Komatsu 2002)

Fig. 2

Stratigraphical subdivision and correlation of the Middle Triassic in northwestern Guangxi and southwestern Guizhou, southwestern China (Fm Formation, Mb Member) (not to scale with regard to unit thicknesses)

Fig. 3

Stratigraphical and lithological columns of the Middle Triassic in the Fengshan area, northwestern Guangxi, and occurrences of bivalves, with emphasis on the Anisian Banna Formation (Fm Formation, Mb Member; note the different scales for Lower Member and Upper Member of the Banna Formation); taxonomic identifications of the Late Anisian and Ladinian bivalves by Chen et al. (1992)

All specimens described in this paper are housed in the collections of the Nanjing Institute of Geology and Palaeontology (NIGP), Chinese Academy of Sciences, in Nanjing, P.R. China.

Stratigraphical setting

The Middle Triassic in the Fengshan area comprises the Banna Formation (Anisian) and the Lanmu Formation (Ladinian) (Figs. 2, 3). The Banna Formation is subdivided into two members, and the Lanmu Formation into three members (Geological Bureau of Guangxi Zhuang Autonomous Region 1985; Chen et al. 1992). The Lower Member of the Banna Formation consists of finely laminated mudstones, siltstones, and limestones, is about 45–60 m thick, and ranges from Early to Middle Anisian in age. These strata are considered to have been deposited in the central area of the Nanpanjiang Basin (Chen et al. 1992; Chen and Komatsu 2002). Chen and Stiller (2007) further divided the Lower Member of the Banna Formation into five stratigraphical units (Unit A to Unit E in ascending order).

Units A and B consist of limestones and marlstones with intercalations of black calcareous mudstones, in total 15.8 m thick, and yield ammonoids, conodonts, and bivalves. The conodonts and ammonoids indicate an Early Anisian age of these units (Chen and Komatsu 2002; Chen and Stiller 2007).

Unit C is composed of grey marlstones and mudstones, 7 m thick, and yields bivalves of the genus Bositra and ammonoids. It may be correlated with the early Middle Anisian Acrochordiceras hyatti Zone of Nevada (Chen and Stiller 2007).

Unit D consists of grey siltstones and mudstones, is 8 m thick, and yields the ammonoids Balatonites cf. balatonicus and Gymnites sp. and the bivalve Enteropleura walleri. It is of late Middle Anisian age (Chen and Stiller 2007) and may be correlated with the Balatonites balatonicus Zone of Alpine Europe and the Balatonites shoshonensis Zone of Nevada (Bucher 1992; Waller and Stanley 2005).

Unit E is made up of brown and grey mudstones and siltstones, 15 m thick, and yields the ammonoids Balatonites sp. and Gymnites sp. The bivalve Daonella fengshanensis sp. nov. occurs in the middle part of this unit, about 9–11 m above the Enteropleura walleri bed of Unit D. The ammonoid specimen from this unit assigned to Balatonites sp. is poorly preserved, but its general features are very close to those of Balatonites cf. balatonicus from Unit D. Therefore, the age of Unit E is considered to be late Middle Anisian as well (Chen and Stiller 2007).

The Upper Member of the Banna Formation consists of sandstones, siltstones, and mudstones, is in the Fengshan area about 300 m thick, and of Late Anisian age. During Late Anisian times, the region of today’s Fengshan area was located in the basin slope area of the Nanpanjiang Basin (Chen et al. 1992; Chen and Komatsu 2002; Chen and Stiller 2007). Chen et al. (1992) recognised two Daonella zones in this member, the Daonella producta–Daonella americana Zone in its lower part, and the Daonella moussoni Zone in its upper part. The latter may be correlated with the Daonella moussoni–Daonella dubia beds of the latest Anisian Frechites occidentalis Zone of Nevada (Silberling and Nichols 1982). The D. producta–D. americana Zone spans the earlier parts of the Late Anisian. Chen et al. (1992, pp. 407–408) suggested that this zone may be subdivided into two beds: The Daonella americana bed in the lower part may be correlated with the D. americana beds of the middle Late Anisian Gymnotoceras rotelliformis Zone of Nevada (Silberling and Nichols 1982), and the Daonella producta–Daonella elongata bed in the upper part may be correlated with the D. elongata beds of the late Late Anisian Parafrechites meeki Zone of Nevada. In the Jinya section of Guangxi, Daonella guizhouensis, previously assigned to “Daonella elongata” by Chen et al. (1992), occurs about 10 m below the D. americana bed.

Systematic palaeontology

Class:

Bivalvia Linnaeus 1758

Subclass:

Autolamellibranchiata Grobben 1894

Superorder:

Pteriomorphia Beurlen 1944 (emend. Waller 1978)

Order:

Pterioida Newell 1965 (emend. Waller 1978)

Suborder:

Pterioidina Newell 1965 (emend. Waller 1978)

Superfamily:

Posidonioidea Frech 1909 (emend. Waller, in Waller and Stanley 2005)

Family:

Halobiidae Kittl 1912 (emend. Waller, in Waller and Stanley 2005)

Genus:

Daonella Mojsisovics1874

Type species—Halobia lommeli Wissmann 1841; subsequent designation by Diener 1923, p. 46.

Remarks—Taxonomy and systematics of the Middle Triassic “flat clam” genera Bositra De Gregorio 1886, Enteropleura Kittl1912, and Daonella Mojsisovics1874 have been discussed by various authors (e.g. Kittl 1912; Campbell 1994; Waller in Waller and Stanley 2005) and remain controversial. Due to preservation reasons, internal features such as ligament, resilifer, and muscle scars are unknown in most species of these genera, making reliable assignments sometimes difficult. In general, ligament features are particularly important for bivalve taxonomy, but in some of the taxa discussed here they are less significant for genus-level assignment; for example, the alivincular ligament of Enteropleura is very similar to that of Bositra, and the lamellar ligament of Daonella resembles that of Aparimella Campbell1994 and HalobiaBronn 1830 (Waller in Waller and Stanley 2005, pp. 17–18). Although the external shell sculpture may vary ontogenetically and is susceptible to preservation issues, external shell features nevertheless may provide clues for taxonomy and systematics in the case of well-documented taxa. Waller (in Waller and Stanley 2005) and Hopkin and McRoberts (2005, p. 797) also regarded the external shell sculpture as an important trait for distinguishing the genera Bositra, Enteropleura, and Daonella. The new taxon described herein and regarded as a very early representative of the genus Daonella shows external shell features intermediate between Enteropleura and Daonella, and its internal shell features are unknown.

Bositra is characterised by regular commarginal ribs; radial shell sculpture is lacking, or the radial riblets are fine and weak, weaker than the commarginal sculpture. Anterior and posterior auricles may be present or absent. Following Waller (in Waller and Stanley 2005) and not adopting the view of McRoberts (2010), Peribositria Kurushin & Trushchelev 1989 is regarded as a junior synonym of Bositra. Waterhouse (2008) erected the family Bositridae for Triassic and Jurassic Bositra-like bivalves. Here, Bositridae Waterhouse2008 is regarded as a junior synonym of Posidoniidae Frech 1909, and Bositra is considered to be a posidoniid.

Enteropleura shows commarginal ribbing and in most species radial threads or ribs; the commarginal sculpture is stronger than the radial sculpture; Enteropleura walleri Chen & Stiller 2007 lacks radial sculpture. The radial ribs, if present, are fine and rounded (Mojsisovics 1874; Kittl 1912; Waller in Waller and Stanley 2005; Hopkin and McRoberts 2005). No anterior auricle is developed. A narrow and small posterior auricle is present in some species, in other species the posterior auricle has not (yet) been observed (maybe due to insufficient preservation or documentation). Here, Enteropleura and Daonella are placed in the family Halobiidae (order Pterioida).

Waterhouse (2008) erected the genus Wallerobia for species of Enteropleura with radial shell sculpture and small posterior auricles, and placed the new genus in his new family Bositridae. Enteropleura sensu Waterhouse (2008) lacks anterior and posterior auricles as well as radial shell sculpture. Herein, the genus Wallerobia Waterhouse 2008 is rejected and regarded as a junior synonym of Enteropleura, because the morphological features emphasised by Waterhouse (2008) to distinguish Wallerobia from Enteropleura are only of species-level significance, and the character of the morphological differences does not justify the formal erection of a new genus-level taxon (also see discussion on species-groups in Enteropleura in Chen and Stiller 2007, p. 58). The always very small posterior auricle in Enteropleura varies in size in different species (Waller in Waller and Stanley 2005; Hopkin and McRoberts 2005; Chen and Stiller 2007) and is, due to its tiny and fragile character, not easily preserved. Some species of Enteropleura are not well documented, and therefore it remains unclear whether they had a tiny posterior auricle or not, because the auricle may have been overlooked or not been preserved. The somewhat varying external shell sculpture with variably distinct radial riblets is only of species-level significance. For example, Enteropleura walleri has a small posterior auricle but lacks radial shell sculpture (Chen and Stiller 2007). Waterhouse (2008, p. 178) described this species as being characterised by “close-set commarginal wrinkles and very faint radial ribs”, but he had not seen the specimens. His “very faint radial ribs” are preservation artefacts, E. walleri lacks radial shell sculpture. If following the genus-level classification of Waterhouse (2008), E. walleri with its mosaic of morphological features characteristic for Enteropleura and “Wallerobia”, would be difficult to assign on the genus-level.

Daonella is characterised by commarginal and radial ribbing with the radial sculpture being (much) stronger than the commarginal sculpture in adult ontogenetical stages; the radial ribs in general are relatively strong, broad, and flat-topped. Daonella lacks auricles. However, there are few exceptions in early representatives of this genus: Daonella boeckhi Mojsisovics1874 shows commarginal shell sculpture and only weak radial sculpture; it lacks auricles. Daonella fengshanensis described herein has a narrow and small posterior auricle but exhibits typical Daonella-type shell sculpture.

Daonella fengshanensis sp. nov. (Figs. 4, 5, 6, 7)

Fig. 4

Daonella fengshanensis sp. nov. All specimens composite moulds, from Unit E of the Lower Member of the Banna Formation (late Middle Anisian, early Middle Triassic), Jinya, Fengshan County, northwestern Guangxi, southwestern China. a, b NIGP 148209, paratype, right valve; b detail of a, showing posterodorsal part of right valve with posterior auricle (deformed and damaged). c NIGP 148213, paratype, external mould of left valve. d NIGP 148210, right valve. e NIGP 148214-1, right valve. f, g NIGP 148211, paratype, external mould of left valve; counterpart of NIGP 148212; f detail of g. h, i NIGP 148212, holotype, left valve; counterpart of NIGP 148211; i detail of h. All scale bars 5 mm; arrows pointing to posterior auricles

Fig. 5

Daonella fengshanensis sp. nov. All specimens composite moulds, from Unit E of the Lower Member of the Banna Formation (late Middle Anisian, early Middle Triassic), Jinya, Fengshan County, northwestern Guangxi, southwestern China. a NIGP148214-2, paratype, dorsal part of right valve, showing the shell sculpture of the pre-adult growth stage D; arrow pointing to posterior auricle. b NIGP 148216-1, right valve, showing the shell sculpture of the pre-adult growth stage D. c NIGP 148215, left valve, showing shell sculpture intermediate between the pre-adult growth stages B and C. d NIGP 148216-3, external mould of left valve, showing shell sculpture intermediate between the pre-adult growth stages B and C. e NIGP 148218, left valve, showing the shell sculpture of the pre-adult growth stage B. f NIGP 148216-2, left valve. All scale bars 5 mm

Fig. 6

Daonella fengshanensis sp. nov. All specimens composite moulds, from Unit E of the Lower Member of the Banna Formation (late Middle Anisian, early Middle Triassic), Jinya, Fengshan County, northwestern Guangxi, southwestern China. a NIGP 148222-1, left valve, showing the shell sculpture of the pre-adult growth stage A. b NIGP 148222-2, left valve, showing the shell sculpture of the pre-adult growth stage B. c NIGP 148223-2, right valve, showing the shell sculpture of the pre-adult growth stage A. d NIGP 148223-3, right valve, showing the shell sculpture of the pre-adult growth stage B. e NIGP 148220-1, left valve, showing the shell sculpture of the pre-adult growth stage B. f NIGP 148225, right valve, showing shell sculpture intermediate between the pre-adult growth stages C and D. g NIGP 148221-1, left valve, showing the shell sculpture of the pre-adult growth stage D. All scale bars 5 mm

Fig. 7

Daonella fengshanensis sp. nov. All specimens composite moulds, from Unit E of the Lower Member of the Banna Formation (late Middle Anisian, early Middle Triassic), Jinya, Fengshan County, northwestern Guangxi, southwestern China. a NIGP 148224, valves of juvenile individuals, external and internal moulds. b NIGP 148224-5, external mould of right valve, showing the shell sculpture of the pre-adult growth stage C. c NIGP 148224-2, right valve, showing the shell sculpture of the pre-adult growth stage B. d NIGP 148224-7, right valve, showing the shell sculpture of the pre-adult growth stage A; note the short radial riblets on the ventral part of the central sector. e NIGP 148224-4, external mould of left valve, showing the shell sculpture of the pre-adult growth stage B. f NIGP 148224-3, right valve, showing the shell sculpture of the pre-adult growth stage A. g NIGP 148224-6, external mould of left valve, showing the shell sculpture of the pre-adult growth stage C. h NIGP 148224-1, (?) right valve. All scale bars 5 mm

1992 Daonella spp.—Chen et al.: p. 405.

2002 Daonella dianana Guo—Chen and Komatsu: p. 436, p. 442.

2007 Daonella sp.—Chen and Stiller: p. 55, p. 56.

Etymology—Named after Fengshan County in which the type locality is located.

Types—Holotype, NIGP 148212 (Fig. 4h–i); paratypes, NIGP 148209 (Fig. 4a–b), NIGP 148211 (Fig. 4f–g), NIGP 148213 (Fig. 4c), NIGP 148214-2 (Fig. 5a). All type specimens from Unit E of the Lower Member of the Banna Formation (late Middle Anisian, early Middle Triassic), Jinya, Fengshan County, northwestern Guangxi, southwestern China.

Other material examined—More than 55 external and internal moulds of left and right valves, and numerous fragments, from the same strata and locality as the type specimens (NIGP 148209–148225).

Measurements—Holotype (NIGP 148212) length about 43 mm, height >20 mm (specimen ventrally incomplete).

Occurrence—Hitherto only known from Unit E of the Lower Member of the Banna Formation (late Middle Anisian, early Middle Triassic), Jinya, Fengshan County, northwestern Guangxi, southwestern China.

Diagnosis—Daonella with a small and narrow posterior auricle; anterior sector large and smooth, central sector with flat-topped radial ribs; pre-adult growth stages with both rounded and flat-topped radial ribs.

Description—Shell outline subcircular to ovate; size small to medium, with length generally 20–30 mm, but up to about 40 mm; shell longer than high with height/length ratio about 0.8–0.9, of low convexity. Beaks small, orthogyrous, located anterior to mid-length, slightly protruding above the dorsal margin. Dorsal margins gently convex or sometimes nearly straight. Hinge line long, length equalling about two-third of the shell length or more, commonly asymmetrical, with posterior hinge line longer than anterior hinge line. Anterior auricle absent. Posterior auricle small and narrow, weakly inflated, separated from disk by narrow sulcus; dorsal and posterior margin of auricle forming obtuse angle of about 145°–165°, angle subtended by dorsal margin and base of auricle about 5°–10°; surface of auricle smooth (Figs. 4f–i, 5a). Ligament and muscle scars unknown due to preservational reasons. Prodissoconch subcircular, smooth, moderately sized, length about 0.5–0.8 mm.

Description: Surface of shell—(Not including smooth surfaces of prodissoconch and small posterior auricle.) Shell surface divisible into three sectors (triangular fields): (1) anterior sector, distinguished only by lack of radial shell sculpture, its ventral margin subtending an angle of about 60°–80° with the dorsal margin; (2) central sector, sculptured by radial ribs and commarginal undulations (pre-adult stages) or lirae (adult stages), sector margins forming an angle of about 90°–100°; and (3) posterior sector, slightly inflated, sculptured by commarginal undulations/lirae only or sometimes additionally by very weak radial furrows, its ventral margin subtending an angle of about 20°–30° with the dorsal margin.

Description: Shell sculpture—Broad commarginal rugae or undulations present in early ontogeny, variably developed but generally more distinct on the posterior sector than on the anterior and central sectors; adult shell portions only with weak to indistinct commarginal lirae. Radial ribs in pre-adult and adult growth stages restricted to central sector, straight, flat-topped, regularly and densely spaced, generally about equal in strength; radial ribs separated by narrow and shallow furrows of two orders; primary furrows starting at about 3–5 mm from beak; secondary furrows beginning at 4–6 mm from beak.

Four pre-adult growth stages distinguishable based on the surface sculpture of specimens with shell lengths of less than 15 mm:

• 1. Stage A, shell very small, length 3–6 mm (Figs. 6a, c, 7d, f; some specimens in Fig. 7a), sculptured by commarginal undulations or both commarginal undulations and radial riblets; radial riblets, if present, very weak and short, rounded, restricted to the marginal ventral part (Figs. 6c, 7d);

• 2. Stage B, shell length about 4–9 mm (Figs. 5e, 6b, d–e, 7c, e), sculptured by commarginal undulations and radial riblets; radial riblets very fine, thread-like, densely spaced, rounded, restricted to the ventral quarter to third of the central sector;

• 3. Stage C, shell length 7–13 mm (Figs. 5c–d, 7b, g); commarginal ribs becoming finer ventrally; both flat-topped radial ribs and rounded radial riblets present, limited to the ventral half of the central sector; flat-topped radial ribs narrowly spaced and separated by shallow furrows, increasing in number by branching; rounded riblets very fine and weak; specimens of Fig. 5c and d showing features intermediate between stages B and C;

• 4. Stage D, shell length 8–15 mm (Figs. 5a–b, 6f–g); radial flat-topped ribs relatively distinctly developed, covering more than the ventral half of the central sector; rounded riblets disappearing; specimen of Fig. 6f showing features intermediate between stages C and D.

Remarks—The shell sculpture of Daonella fengshanensis sp. nov., consisting mainly of flat-topped radial ribs, is basically similar to that of later, more derived species of the genus Daonella (e.g. Mojsisovics 1874; Kittl 1912; Campbell 1994; McRoberts 2000). The new species is characterised by the presence of a posterior auricle (Figs. 4a–b, f–i, 5a), in contrast to all later species of Daonella that lack auricles (e.g. Campbell 1994; Waller in Waller and Stanley 2005). The posterior auricle of D. fengshanensis is very similar to the posterior auricles in some species of Enteropleura (Waller in Waller and Stanley 2005; Hopkin and McRoberts 2005; Chen and Stiller 2007). This indicates that D. fengshanensis is closely related not only to Daonella but also to Enteropleura.

Daonella boeckhi also is an early representative of the genus Daonella and shows some morphological features different from later congeneric species (see above). It differs from D. fengshanensis in lacking a posterior auricle and flat-topped radial ribs.

Guo (1985, pl. 15: 11, 12) reported a “primitive” Daonella, assigned to Daonella dianana Guo 1985, from the Anisian of Funing, Yunnan, southwestern China. The shell sculpture of the anterior and central sectors of D. dianana is similar to that of D. fengshanensis. But D. dianana has no posterior auricle, and its radial ribs on the central sector are more closely spaced than those in D. fengshanensis. Moreover, D. dianana probably does not represent adult growth stages but juvenile stages of a Daonella, because the documented shells are fairly small (less than 20 mm long) and their radial ribs are limited to the ventral half of the shell surface.

Concerning shell shape and sculpture, D. fengshanensis is closest to Daonella luganensis Rieber 1969 and Daonella pseudomoussoni Rieber 1969 from the latest Anisian of the southern Alps (Rieber 1969, 1973). It differs from the latter two species in the presence of a small and narrow posterior auricle. Furthermore, in D. fengshanensis the anterior sector is larger and the radial ribs and furrows on the central sector in early ontogeny are less distinctly developed than in the Alpine species.

Daonella fengshanensis is a taxon somewhat intermediate between Enteropleura and Daonella with regard to its morphological features and its stratigraphical occurrence. If regarding the presence of a small posterior auricle as taxonomically more important than the shell sculpture, D. fengshanensis may be assigned to Enteropleura as well. However, the species apparently lacks the internal ridges typical for Enteropleura, and its external shell sculpture is not of the Enteropleura type but of the Daonella type. Following Waller (in Waller and Stanley 2005) and Hopkin and McRoberts (2005) and regarding the external shell sculpture as an important trait, the new species is assigned to Daonella. The small posterior auricle most probably represents a character retained from the ancestors in the evolutionary lineage from Bositra via Enteropleura to Daonella. It is regarded to represent a species-level character.

The pre-adult ontogeny of D. fengshanensis also indicates that this species is closely related to more derived species of Daonella and at the same time phylogenetically connected to Enteropleura and Bositra. The shell sculpture of stage A (Figs. 6a, c, 7d, f), characterised by commarginal undulations, sometimes additionally with short radial riblets on the marginal ventral part (Figs. 6c, 7d), resembles that of Bositra. The radial sculpture of stage B (Figs. 5e, 6b, d–e, 7c, e), consisting of fine, rounded riblets, is very similar to that of some species of Enteropleura, e.g. Enteropleura guembeli Mojsisovics 1874 (Mojsisovics 1874; Kittl 1912). The shell sculpture of stage C (Figs. 5c–d, 7b, g), combining Daonella-type ribs with Enteropleura-type riblets, may indicate phylogenetical linkage between Enteropleura and Daonella. The shell sculpture of stage D (Figs. 5a–b, 6f–g), consisting only of well developed Daonella-type radial ribs, is already very close to that of the adult stage of the species (Figs. 4a–i, 5f, 7h). These features indicate that D. fengshanensis is an early representative of the genus Daonella, phylogenetically situated near the origin of this genus. Daonella may have evolved from a species of Enteropleura, and this lineage may be ultimately rooted in Bositra, thus suggesting an evolutionary lineage from Bositra via Enteropleura to Daonella.

Some authors referred to a dorsal shell portion in Halobia as a “posterior auricle” (e.g. Campbell 1994, p. 38, fig. 3.1; McRoberts 2000, p. 600, fig. 2). However, this shell part is not homologous with the posterior auricles of Daonella fengshanensis and Enteropleura, and also not homologous with the posterior auricles of alate pterioids and pectinoids. It rather is merely a part of the shell disk (Waller in Waller and Stanley 2005, p. 18).

Conclusions

With regard to stratigraphical occurrence as well as external shell morphology the late Middle Anisian Daonella fengshanensis sp. nov. from southwestern China is in a transitional position between Enteropleura and more derived species of Daonella. The new species is an early representative of the genus Daonella and regarded as phylogenetically being near the genus’ origin.

The characteristics of the external shell sculpture and those regarding the auricles in combination with the stratigraphical succession (see Fig. 3; for general stratigraphical succession of the genera see Waller in Waller and Stanley 2005 and McRoberts 2010) suggest an evolutionary lineage from Bositra via Enteropleura to Daonella, in which the morphological development may be summarised as (1) gradual strengthening of the radial shell sculpture (2) rounded radial ribs developing into flat-topped radial ribs and (3) gradual reduction or loss of the auricles. In earliest Daonella more ancestral features occasionally are retained in single species, e.g. very weak radial sculpture in D. boeckhi (resembling Bositra) or small posterior auricle in D. fengshanensis (resembling Enteropleura). This may be interpreted as supporting the hypothesis of this phylogenetical lineage. However, whether these morphological changes really mark an evolutionary lineage needs to be further explored, taking also into consideration internal shell features such as ligament and resilifer characteristics that hitherto are unknown for most of the species concerned.

The sequence of ontogenetical stages observed in D. fengshanensis may suggest a peramorphosis in the evolution of radial ribbing characteristics in this assumed lineage. Early ontogenetical stages of D. fengshanensis appear to preserve ancestral states with the earliest ontogenetical stage resembling Bositra, followed by stages resembling Enteropleura, and finally leading to typically Daonella-type radial ribbing in adult stages. Although similar ontogenetical sequences can be found in many other bivalves, and external shell sculpture features generally are somewhat variable and susceptible to preservation issues, this ontogenetical sequence in D. fengshanensis may be interpreted as a further clue to the supposed evolutionary lineage, especially when considered in connection with the stratigraphical occurrences of the genera concerned.