A new tubalepid (Antiarcha, Placodermi) from the Middle Devonian of Huize, Yunnan, China

doi:10.21203/rs.3.rs-5287718/v1

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A new tubalepid (Antiarcha, Placodermi) from the Middle Devonian of Huize, Yunnan, China

https://doi.org/10.21203/rs.3.rs-5287718/v1

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A new tubalepid antiarch, Tongdulepis concavus gen. et sp. nov., is described from the Middle Devonian (Qujing Formation, late Eifelian) of Huize County, Qujing, northeastern Yunnan, China. It differs from other antiarchs in the trapezoidal premedian plate, absence of postmarginal plate (except Tubalepis), spade-shaped preorbital recess, contorted infraorbital sensory line on the lateral plate, and the anterior median dorsal plate with broad anterior margin and concave posterior margin. This material confirms the absence of the postmarginal plate in Tubalepididae and adds to our understanding of the neurocranium and brachial process in the primitive bothriolepidoids. Our phylogenetic result places Tongdulepis within Tubalepididae, at the plesiomorphic position of the Tenizolepis-Dianolepis-Bothriolepis lineage.

Antiarcha

Tubalepididae

Middle Devonian

phylogeny

China

Antiarcha is a diverse group of Placodermi or the jawed stem-Gnathostomata during the Silurian and Devonian periods, mainly represented by three subgroups: Yunnanolepidoidei, Sinolepidoidei, and Euantiarcha (Denison, 1978; Janvier, 1995; Zhu, 1996; Ritchie et al., 1992; Pan et al., 2018; Wang & Zhu, 2018). Their morphological disparities are often known as the changes in the skull proportion, trunk armor pattern, and development of the pectoral appendage and its articulation with the trunk shield (Denison, 1978; Downs et al., 2019; Michael et al., 2019). However, unlike Arthrodira, another major placoderm group, Antiarcha showed few disparities in the skull roof pattern. Though their contour may change, the skull roofs of different antiarch groups remain the same constitution, consisting of a premedian plate, a rostral plate, a pineal plate, a postpineal plate, a nuchal plate, paired lateral plates, postmarginal plates, and paranuchal plates.

Moloshnikov (2011) established Tubalepididae based on Tubalepis (Sergienko, 1961; Panteleyev & Moloshnikov, 2003), which was found in the Upper Devonian of Krasnoyarsk, Kazakhstan, with a paranuchal plate that lacks the anterolateral margin. He suggested that Tubalepis might have lost its postmarginal plate, representing the first antiarch that changed its skull roof constitution. However, due to the rarity of head shield materials, the loss of the postmarginal plate in Tubalepis remains uncertain. Although the phylogenetic position of Antiarcha has received attention for decades, most of the previous studies placed Antiarcha as the basalmost group of Placodermi or the jawed stem-gnathostomes (Goujet & Young, 1995; Brazeau, 2009; Carr et al., 2009; Zhu et al., 2013; Qiao et al., 2016; Zhu et al., 2021; Zhu et al., 2022). The homology of the plates between antiarchs and some other placoderms, like Kujdanowiaspis (Stensiö, 1942; Dupret, 2010; Dupret et al., 2017) and Romundina (Goujet & Young, 2004), could be traceable. The postmarginal plates in Anitarcha and Arthrodira, or the posterior paranuchal plates in Acanthothoraci, often occupy the lateral sides of their skull roofs and form the lateral corners. Most primitive antiarchs tend to bear a branch from the principal section of the infraorbital sensory line on the lateral plate, which often extends to the postmarginal plate (preopercular sensory line), like other primitive placoderms (Zhang, 1980; Young & Zhang, 1996).

During the fieldwork in 2021, we discovered a new vertebrate fossil site from the Middle Devonian (lowermost part of the Qujing Formation, late Eifelian) of Luna township, Huize County, Qujing, Yunnan, China (Fig. 1), which includes onychodonts, tetrapodomorphs, dipnoans (Luo et al., 2022) and placoderms. Here, we describe Tongdulepis concavus gen. et sp. nov. from this fossil site, based on a well-preserved specimen with head shield, trunk armor, and pectoral appendages in articulation. The well-preserved fossil corroborates the diagnosis of Tubalepididae and adds to our understanding of plesiomorphic bothriolepidoids. Interesting features from Tongdulepis, like the wide orbital fenestra, more vertical pectoral appendages, and ridge-less trunk armor without a regular median ventral plate, outline Tubalepididae as a unique group among Bothriolepidoidei.

Material

The holotype of Tongdulepis concavus (IVPP V33248) was collected from the calcareous mudstone of the Qujing Formation at Huize in Qujing City, Yunnan Province, China (Luo et al., 2022). It is currently housed at the IVPP. With GE v|tome|x m300&180 micro-computed tomography scanner from the Key Laboratory of Vertebrate Evolution and Human Origins of the Chinese Academy of Sciences, the fossil was scanned with a resolution of 38.823µm and carried out at 180 kV/100 mA. The images were optimized and converted with VGStudio Max v.3.0, and the result optimization and further reconstruction were completed in Mimics v.18.01. Nikon D3X camera was used to take the photographs of Tongdulepis concavus.

Phylogeny

To explore the phylogeny of Tongdulepis among the antiarchs, we compiled a matrix (see ‘Supplementary Materials’) in Mesquite V.3.61 (Maddison & Maddison, 2019). We performed our phylogenetic analysis in TNT v.1.5 (Goloboff & Catalano, 2016). Our matrix herein mainly follows Young (1984b, 1988), Zhu (1996), Ritchie et al. (1992), Wang and Zhu (2018), and Plax and Lukševičs (2023), but introduced new taxa and replaced genera with species as the smallest OTU (Operational Taxonomic Unit) to launch a deeper analysis. We added 51 ingroup taxa and 11 new characters (Ch. 81–91) that have not been regarded in previous parsimony analyses. Our grand matrix consists of 2 outgroup taxa (Kujdanowiaspis and Romundina), 85 ingroup taxa, and 92 characters (Ch. 18, 48, and 49 are marked as ordered). To get a more resolved result, we removed taxa whose completeness is lower than 40%, and 46 ingroup taxa remain in our final matrix (see ‘Supplementary Materials’).

Systematic paleontology

Class Placodermi McCoy, 1848

Order Antiarcha Cope, 1885

Suborder Euantiarcha Janvier and Pan, 1982

Infraorder Bothriolepidoidei Gross, 1965

Family Tubalepididae Moloshnikov, 2011

Tongdulepis gen. nov.

Type and only species - Tongdulepis concavus sp. nov.

Etymology - The generic name ‘Tongdu-’ is after the epithet of Huize County, which means ‘The Copper City’ in Chinese. People from Huize started copper mining and smelting in the 12th century BC and invented the cupronickel by the 3rd century AD. The copper industrial of Huize funded the Chinese dynasties till the end of the Feudal Age.

Diagnosis - A middle-sized tubalepid antiarch resembling Tubalepis in tubercular ornamentation, anterior median dorsal plate with wide anterior margin, absence of postmarginal plate, and elongated trunk shield. It can be further distinguished by the following combination of characters: isosceles trapezoidal premedian plate; spade-shaped preorbital recess; contorted infraorbital sensory lines on lateral plates; nuchal plate without orbital facets; wide and narrow orbital fenestra; anterior median dorsal plate overlapping mixilateral plates; anterior median dorsal plate with concave posterior margin; ridge-less trunk armor; large axillary foramen.

Tongdulepis concavus sp. nov.

Etymology - The specific epithet ‘concavus’ refers to the highly concave posterior margin of the anterior median dorsal plate.

Diagnosis - As for the genus.

Holotype - IVPP V33248, a flattened individual with head shield, trunk armor, and pectoral appendages.

Type locality and horizon - Luna, Huize County, northeastern Yunnan Province, China; Qujing Formation; late Eifelian, Middle Devonian (Fig. 1).

Description

General features

The holotype (IVPP V33248) has an almost complete head shield, trunk armor, and pectoral appendages. Its surface is covered with sporadic and shallow tubercular ornamentation. The posterior median dorsal and median ventral plates are missing, and the mixilateral, the posterior ventrolateral plates, and the pectoral appendage’s distal segment are incomplete. The whole fossil was intensively recrystallized, making the CT scanning result unsatisfactory.

Head shield

Premedian plate - The premedian plate (PrM, Figs. 2–4, 7C) is isosceles trapezoidal, short and broad. Its rostral margin is about 31.6 mm wide, and the orbital margin of the PrM plate is slightly convex, about 48.3 mm wide. The PrM plates in other antiarchs are inverted trapezoidal or fan-shaped, with their rostral margins mostly greater than or equal to the orbital margins. The trapezoidal PrM plate in Tongdulepis may be apomorphic compared with Microbrachius (Fig. 7A; Hemmings, 1978), which also have an introvertive part of the lateral margin on their PrM plates. Besides, the length of the PrM plate is about 11.3 mm, longer than that of the lateral plates, supporting the PrM may protrude from the rostral margin of the head shield (Figs. 2–4). The contorted principal section of the infraorbital sensory line on the head shield (ifc₁, Figs. 2B, 4B, 7B) extends from the lateral plate (L, Figs. 2B, 3B, 4) and hardly reaches viscerally, making a shallow anterior section of the supraorbital sensory line (soc, Figs. 2B, 4B). On the ventral side of the PrM plate, a spade-shaped preorbital recess (prh, Figs. 3B, 4D) reaches anteriorly to the centroid of the PrM plate and extends aside to the L plates. The remaining part of the preorbital recess on the PrM plate is an approximate triangle (Figs. 3B, 4D, 7B).

Lateral plate - The paired L plates (Figs. 2–4, 7C) contact the lateral margins of the PrM plate. The relevant complete left one is about 31.0 mm wide and 32.7 mm long, with an anterolateral angle of the head shield (alc, Figs. 2B, 4B) on its anterolateral corner. The contorted infraorbital sensory line runs through the dorsal surface of the L plate and extends to the PrM plate. On the rostral margin of the right L plate, an edge-fold process (pr.alm, Figs. 3B, 4D) is on the anterolateral angle and occupies the notch between the PrM and the right L plate. The edge-fold process is deltoid-shaped and forms the anterolateral margin of the oral fenestra. It shares topological similarities with the prelateral plates in Bothriolepis (Young, 1984c, 1988; Béchard et al., 2014), Grossilepis (Stensiö, 1948; Miles, 1968; Olive, 2015; Lukševičs, 2001), and Nawagiaspis (Young, 1990). The branch of the infraorbital sensory line (ifc₂, Figs. 2B, 4B) of Tubalepis is absent on the edge-fold process but extends directly to the submarginal plate (SM, Figs. 2B, 3B). Considering the branch of the infraorbital sensory line in other antiarchs primarily extending to the prelateral plate (Young & Zhang, 1996; Young, 2008) and the possible prelateral notch (nprl?, Fig. 4B) on the corresponding position of the left L plate, we have a reservation to suggest that the edge-fold process may be a fused prelateral plate. The area overlapped by the paranuchal plate (oa.PNu, Fig. 4B) occupies the posterior margin of the L plate, indicating the configuration with the paranuchal plate (PNu, Figs. 2B, 4).

On the visceral side of the L plate, the preorbital recess extends from the PrM plate and surrounds the orbital fenestra (fe.orb, Figs. 2B, 3B, 4). The preorbital recess on the L plates are arches, forming a spade-shaped with the PrM part. The middle part of the preorbital recess on the PrM plate is similar to that of Wudinolepis (Zhang, 1965). Still, the flank part of the preorbital recess extends to the lateral margin as Jiangxilepis (Zhang & Liu, 1991), which may be a transitional feature. A narrow plane near the anterolateral corner of the head shield may represent the attachment area for the prelateral plate (a.PrL?, Fig. 4D). A triangular transverse lateral groove of the head shield (tlg, Fig. 4D) is outlined by the prelateral and posterolateral cristae (cr_1 − 2, Fig. 4D) next to the attachment area. The lateral pit (p, Fig. 4D) is anteromedial of the transverse lateral groove, almost clinging to the lateral lobe of the preorbital recess, and much smaller than that of Bothriolepis (Young, 1988; Luo et al., 2023). The anterior postorbital process (p.apo, Figs. 3B, 4D) extends anteriorly to the level of the orbital notch (between the posterior margin and the anterior margin of the orbital fenestra) and is close to the posterior of the preorbital recess. The anterior attachment area for the submarginal plate (a₁SM, Fig. 4D) is anterolateral of the transverse lateral groove and connects the anterior articular process of the SM plate (ad₁, Figs. 2B, 3B). The posterior attachment area for the submarginal plate (a₂SM, Fig. 4D) is short and only occupies less than one-third of the lateral margin between the posterolateral cristae and the preobstantic corner. The posterior attachment area in the bothriolepid lineage mainly extends from the anterior attachment area to the lateral end of the L plate. Because of the absence of the postmarginal plate, the depressions of inframarginal crista (cr.im, Fig. 4D) and subobstantic area (soa, Fig. 4D) are also on the visceral side of the L plate.

Submarginal plate - The only remaining SM plate (Figs. 2, 3) is about 15.4 mm wide and 21.7 mm long, and contacts the lateral margin of the left L plate. Although the infraorbital sensory line is absent on the L plate, the sensory line still extends ventrally onto the SM plate (Fig. 2B). A well-developed anterior articular process (ad₁, Figs. 2B, 3B) is on the anteromesial margin of the SM plate like the bothriolepids (Stensiö, 1948; Young, 1988), while the posterior articular process is either lost or undeveloped.

Postpineal plate - The postpineal plate (PP, Figs. 2–4, 7C) is oval, about 13.8 mm short and 30.2 mm wide. Its anterior margin is conspicuously convex into the orbital fenestra, and its posterior margin contacts the L plates, jointly forming a short and broad orbital fenestra that extends two-thirds of the width of the head shield (Figs. 4, 7B). On the visceral side of the PP plate, paired postorbital cristae (cr.po, Fig. 4D) are developed, and lay in approximately horizontally, each occupying one-third of the PP’s width. Because the developed preorbital recess nearly surrounds the orbital fenestra, the postorbital cristae meet the preorbital recess on the L plates at the lateral margins of the PP plate. A broad and gradually narrowing median ridge (mr, Fig. 4D) is between these postorbital cristae.

Nuchal plate - The nuchal plate (Nu, Figs. 2, 4, 7C) is about 42.9 mm wide and 24.1 mm long, and contacts the posterior margin of the PP plate. The posterior pit line grooves (pp, Fig. 4B) run posterolaterally through the dorsal surface of the Nu plate. The obtected nuchal area of the head shield (nm, Figs. 2B, 4B) is on the posterior margin of the Nu and extends sidewards to the PNu plate (Figs. 2B, 4B). The obtected area is relatively deep, which may support a strong coupler between the head and trunk shields. The available specimens of Tubalepis and the dianolepids often lack the neurocranium part; our reconstruction of the head shield reveals some transitional features between the neurocrania of the bothriolepids and plesiomorphic antiarchs. The visceral side of the Nu plate (Figs. 4C, 4D) is relatively flattened, and mainly occupied by the otico-occipital depression (ood, Fig. 4D). The depressions caused by the semicircular canals (dsc, Fig. 4D) are shallow, and slightly horizontal, showing the semicircular canals of Tongdulepis less oblique than that of the bothriolepids (Wang & Zhu, 2018). The supraotic thickening (sot, Fig. 4D) of Tongdulepis is small and on the posterior portion of the nuchal plate, while that of the bothriolepids is often large and on the center of the Nu plate (Young, 1983; Wang & Zhu, 2018; Luo et al., 2023). The transverse nuchal crista (cr.tv, Fig. 4D) runs through the posterior margin of the Nu plate with a continuous posterior edge, without the extended posterior process and median occipital crista in the bothriolepids (Wang & Zhu, 2018).

Paranuchal plate – The left PNu plate (Figs. 2, 4, 7C) is about 21.2 mm wide and 14.3 mm long, nearly rectangular, and lacks the anterolateral margin in other antiarchs. Its mesial and anterior margins only contact with the L and Nu plates. The shape of the PNu plate is like that of Tubalepis (Moloshnikov, 2011), which led Moloshnikov to presume there was no postmarginal plate in Tubalepis and rely on it to establish Tubalepididae. The infraorbital sensory line that extends from the L plate goes through the dorsal surface of the PNu plate and diverges a short segment of the occipital cross commissure (occ, Fig. 4D) near the posterolateral margin of the PNu plate. On the visceral side of the PNu plate, the otico-occipital depression (ood, Fig. 4D) is extensive and almost extends to half the width of the PNu plate. The otico-occipital depression in other antiarchs (e.g., Phymolepis and Bothriolepis; Wang & Zhu, 2018; Luo et al., 2023) only achieves one-third of the anterior margins of their PNu plates. This difference is mainly caused by the size of the PNu plate in other antiarchs, which is more extensive than that of Tongdulepis. The PNu plate of Tongdulepis is relatively narrow, and hardly achieves half the width of the Nu plate. The transverse nuchal crista on the Nu plate is developed with the paranuchal trochlea (pnt, Fig. 4D). The crista and trochlea jointly form a roughened ridge that extends to the postobstantic corner of the head shield.

Trunk armor

Anterior median dorsal plate - The anterior median dorsal plate (AMD, Figs. 2, 5, 7C) is a hexagon, about 71.9 mm wide and 81.3 mm long. Its length is approximately 1.13 times the width, providing a robust central shield for the trunk armor. The index between the width of the anterior margin and the maximum width of the AMD plate is about 0.56, and the index between the anterior and posterior divisions of the AMD plate is about 2.07. The posterior oblique dorsal sensory line grooves (dlg₂, Fig. 2B) are anterior to the AMD plate, marking the tergal angle of the trunk armor (dma, Fig. 2B) and the anterior ventral pit (pt₁, Fig. 5B) on its visceral side. The AMD plate resembles Tubalepis in overall shape, like the wide anterior margin and the posteriorly positioned lateral corners (lc, Figs. 2B, 5B). Tongdulepis bears a strong postlevator thickening (alr, Fig. 5B), much slender levator fossa (f.retr, Fig. 5B), and lacks the postnuchal notch of the bothriolepids (Young, 1983; Moloshnikov, 2004, 2010; Olive, 2015). Tongdulepis is also similar to Grossilepis (Olive, 2015) and Asterolepis (Michael et al., 2019) in the contact relationship between the AMD and surrounding plates. On the visceral side of the AMD plate, there are areas overlapping the mixilateral and anterior dorsolateral plates (cf.MxL, cf.ADL, Fig. 5B). The AMD plate is overlapped by the posterior median dorsal plate (oa.PMD, Fig. 2B). The overlapped area by the PMD is about 44.2 mm wide, suggesting a broad PMD plate with a notably convex anterior margin (Fig. 7B). The PMD plate of Tongdulepis differs from those of other antiarchs, which only have a mildly convex anterior margin or anterior angle and never reach anteriorly more than half of the posterolateral margin of the AMD plate (Young, 1984a; Lukševičs, 1991; Zhu, 1996; Moloshnikov, 2012; Olive, 2015). Considering the elongated trunk armor of Tubalepis, the PMD plate of Tongdulepis may have a similar length as the PVL plate, we can further presume that the PMD plate is larger than that of other antiarchs (Fig. 7).

Anterior dorsolateral plate - The left ADL plate (Figs. 2B, 3B, 7C) is about 33.6 mm wide with posterior fractures, and the length of the complete right ADL plate is about 87.5 mm (covered by the AMD plate). The dorsolateral ridge is poorly developed on its surface, representing a smooth cross-section of the trunk shield. The obstantic process of trunk armor (pro, Fig. 2B) substantially extends anteriorly and articulates with the PNu plate to form the gill opening. The length of the obstantic process is about half of the width of the ADL plate, which is like that of Tubalepis (Moloshnikov, 2012). The advanced obstantic process may represent a less flexible cranio-thoracic connection, compared with the taxa with short obstantic process (e.g., Bothriolepis and Tenizolepis; Moloshnikov, 2012) or without obstantic process (e.g., Stegolepis, Sherbonaspis and Kirgisolepis; Malinovskaya, 1973; Young & Gorter, 1981; Panteleyev, 1992, 1993; Moloshnikov, 2011). The main lateral line groove (lcg, Fig. 2B) is on the ventral margin of the ADL plate and extends backward to its midpiece.

Mixilateral plate - Only part of the right MxL plate (Figs. 2B, 7C) is preserved, about 47.8 mm in width. The area overlapped by the anterior median dorsal plate (oa.AMD, Fig. 2B) is on its dorsolateral margin, and the area overlapped by the anterior dorsolateral plate (oa.ADL, Fig. 2B) is on its anterior margin. The overlapped area marks the connection between the MxL and ADL plates at the level of the lateral corner of the AMD plate and suggests the AMD plate overlaps the ADL and MxL plates.

Anterior ventrolateral plate - Both the anterior ventrolateral plates (AVL, Figs. 3, 6) are incomplete. The right AVL is about 72.5 mm wide, 113.2 mm long, and overlaps the left one like most of the antiarchs (Ritchie et al., 1992; Lukševičs, 2001; Panteleyev & Moloshnikov, 2003; Moloshnikov, 2008; Jia et al., 2010). The lateral lamina of the AVL plate is incomplete, and about 23.3mm in height. The remaining length of the right AVL plate is already more than twice the anterior margin of the head shield. Assuming the length of the AVL plate close to that of the MxL plate as other antiarchs, Tongdulepis may exhibit an elongated trunk shield, which is like Tenizolepis and Tubalepis (Moloshnikov, 2012). The remaining part of the posterior margin of the AVL plate is incomplete, in contact with the posterior ventrolateral plate (PVL, Fig. 3B), and lacks the area overlapping the median ventral plate. The median ventral plate is either very small or just absent.

Pectoral appendage articulation - The pectoral appendage articulation of Tongdulepis (Figs. 3, 6) is developed, but differs from the bothriolepids, showing the transformation in the euantiarchs. The prepectoral corner (prc, Figs. 3B, 6B) is unobtrusive, placed anterior of the brachial process, anterolateral of the AVL plate. The pedal part of the brachial process (pe, Figs. 3B, 6B) is short and stout, which may limit the range of articulation. The insertion area for the abductor muscle, which is often observed in the bothriolepids (Young, 1988; Young & Zhang, 1992; Lukševičs, 2001), is missing in Tongdulepis, suggesting its pectoral appendage less flexible. The short and straight dorsal and ventral articular depressions for the dermal processes of the pectoral appendage (art.d, art.v, Fig. 6) are placed nearly in a vertical line, which differs from the crescent articular depressions in the bothriolepids. This articular structure might suggest a constrained rotation ability of Tongdulepis. The brachial process (pbr, Figs. 3B, 6) is developed as helmet-shaped, but its orientation is distinctively pointed posteriorly, making the funnel pit (fp, Fig. 6) on the articular surface of the brachial process invisible in lateral view. The anterior dorsal and ventral muscle insertions (a.sup, a.inf, Fig. 6) are beneath the brachial process. Inside the anterior dorsal muscle insertion, the canal for nerves and/or vessels (c.nv, Fig. 6) is present, while the posterior dorsal muscle insertion (p.sup, Fig. 6) is aside from the anterior dorsal muscle insertion. Although the lateral lamina is incomplete, the ventral edge of the axillary foramen (f.ax, Fig. 6) still can be observed posterior to the brachial process. The remaining edge is about 34.7 mm long, indicating a large axillary foramen, longer than the length of the brachial process. That foramen tends to be smaller than the brachial process in most antiarchs (Young & Zhang, 1992), which may be better for supporting more robust pectoral appendages in Tongdulepis.

Semilunar plate - The semilunar plate (Sm, Fig. 3B) is unpaired, about 26.9 mm wide and 12.7 mm long, with a convex anterior margin. The Sm plate is fusiform and shapes a shallow notch on the AVL plates, similar to that of Tenizolepis (Moloshnikov, 2012) and Tubalepis (Panteleyev, 2001; Panteleyev & Moloshnikov, 2003).

Pectoral appendage - Both pectoral appendages are fragmented. The only remaining proximal segment of the left pectoral appendage includes disjoined dorsal central plate 1 (CD₁, Figs. 2B, 3B), ventral central plate 1 (CV₁, Figs. 3B, 7) with a treble row of spines, and mesial marginal plate 1 (Mm₁, Fig. 3B). The right proximal segment is distorted and preserved with slightly more details. The proximal segment is thick, the length/width index is about 5. The dorsal central plate 2 (CD₂, Figs. 2B, 7C) extends anteriorly. It may have a narrow or point contact with the CD₁ (Fig. 7C). The right distal segment bears an incomplete central plate 3 (CD₃, Figs. 2B, 7C), about 27.3 mm in length, which may indicate an elongated distal segment.

Phylogenetic analysis.

The first comprehensive matrix to analyze the phylogeny of antiarchs was established by Zhu (1996), following an initial matrix for Bothriolepidoidei (Zhang & Young, 1992). Due to the diversity, convergences, and reversals in the evolution of Antiarcha, the interrelationship of antiarchs tends to be unstable. Thus, most earlier studies on Antiarcha didn’t conduct the phylogenetic analysis or used synapomorphies to discuss the antiarch phylogeny without parsimony analysis. Here we followed the matrix that was gradually enhanced in previous studies (Jia et al., 2010; Pan et al., 2018; Wang & Zhu, 2018; Plax & Lukševičs, 2023) and revised some definitions and codings. The high diversity of the antiarchs often leads to multiple species under a single genus. As such, the type species cannot represent the polytropic characters of the genus. Hence, we further refined the ingroup taxa, replaced genera with species, introduced and coded 51 ingroup taxa not incorporated in previous phylogenetic analyses, and added a few new characters. Our phylogenetic analysis yields 20 most parsimonious trees of length 270 steps (TBR), with a consistency index (CI) of 0.356, and a retention index (RI) of 0.718.

With additional characters brought by the new species, the result of our analysis improved our earlier understanding of Antiarcha (Fig. 8). The clade of Chuchinolepis (Node 1, Fig. 8) is characterized by their anterior postorbital process extending in front of the orbital notch (Ch. 35, state 0). The rest of the yunnanolepidoids are placed into a clade (Node 2, Fig. 8) for the nerves and vessels to the pectoral fin passing through multiple axillary foramina (Ch. 91, state 0). The Yunanolepis-Parayunnanolepis-Mizia lineage (Node 3, Fig. 8) is assembled by the internal posterior transverse crista of trunk shield turning anteriorly and in front of posterior ventral process and pit (Ch. 60, state 1) and absence of the ventrolateral fossa of trunk shield (Ch. 63, state 0). Mizia and Parayunnanolepis formed a monophyletic group (Node 4, Fig. 8) characterized by the absence of the postmarginal canal (Ch. 83, state 0).

Microbrachiidae is resolved as the sister group of the remaining euantiarchs (Node 5, Fig. 8), characterized by three synapomorphies: ridged ornamentation (Ch. 2, state 1), central sensory canal (Ch. 29, state 1) and occipital cross-commissure long and extending onto nuchal plate (Ch. 34, state 1). Microbrachius (Node 6, Fig. 8) is defined by four synapomorphies: the subparallel ridges on dorsal wall of trunk shield (Ch. 3, state 1); anterior median dorsal plate partly overlapping anterior dorsolateral plate (Ch. 53, state 1); absence of brachial process (Ch. 72, state 1); postmarginal plate posterolateral of paranuchal plate (Ch. 82, state 1). Wudinolepis and Hohsienolepis form a monophyletic group (Node 7, Fig. 8), characterized by their fused posterior ventrolateral and posterior lateral plates (Ch.65, state 1).

The remaining euantiarchs (Node 8, Fig. 8) are supported by three synapomorphies: absence of the preorbital depression (Ch. 13, state 0); short and broad postpineal and nuchal plates (Ch. 21, state 1); supraotic thickening on the visceral view of nuchal plate (Ch. 25, state 1). The main branch of the ‘Bothriolepidoidei’ that consists of the Tubalepis-Tenizolepis-Dianolepis-Bothriolepis clade (Node 9, Fig. 8) is characterized by four synapomorphies: preorbital recess extending laterally to the lateral plates (Ch. 16, state 1); postorbital crista extending obliquely to nuchal plate (Ch. 23, state 1); elongated pectoral appendage (Ch. 77, state 1); elongated lateral marginal plate 2 (Ch. 78, state 1). Tongdulepis is placed as the sister taxon of Tubalepis among Tubalepididae (Node 10, Fig. 8), defined by the absence of the postmarginal plate (Ch. 81, state 0). Due to the absence of the postmarginal plate and the median ventral plate, Tubalepididae is reasonably a diverging lineage in Bothriolepidoidei (Miles, 1968) and is plesiomorphic of the Tenizolepis-Dianolepis-Bothriolepis lineage (Node 11, Fig. 8), which are supported by anterior median dorsal plate completely overlapping anterior dorsolateral plate (Ch. 53, state 0), and ventral tuberosity on posterior median dorsal plate (Ch. 86, state 1). Dianolepididae is paraphyletic in our result, and Bothriolepididae (Node 12, Fig. 8) is characterized by 2 synapomorphies that their nuchal plate with orbital facets (Ch. 24, state 1) and branch of infraorbital canal diverging on lateral plate (Ch. 32, state 1).

Nawagiaspis, used to be the nearest sister group of the Asterolepidoidei (Zhu, 1996) or referred to as bothriolepids (Jia et al., 2010), is placed outside of the Tenizolepis-Dianolepis-Bothriolepis lineage in recent studies (Plax & Lukševičs, 2023; Wang & Zhu, 2018). Our research also supports Nawagiaspis at the most basal position of the asterolepidoid lineage (Node 13, Fig. 8). Nawagiaspis is united with other asterolepidoids by three synapomorphies: narrow lateral plate (Ch. 12, state 0); high and short trunk shield (Ch. 47, state 1); index between the width of the anterior margin and maximum width of anterior median dorsal plate between 35 to 55 (Ch. 49, state 1). The main asterolepidoid lineage (Node 14, Fig. 8) is characterized by a combination of five synapomorphies: occipital cross-commissure long and extending onto nuchal plate (Ch. 34, state 1); absence of submarginal articulation (Ch.42, state 0); absence of postsuborbital plate (Ch. 43, state 0); centrally or posteriorly placed tergal angle of anterior median dorsal plate (Ch. 51, state 0); absence of anterior ventral process and pit on anterior median dorsal plate (Ch.56, state 0).

Wang and Liu (1991) established Jiangxilepidae in the bothriolepidoid lineage, mainly for the anterior median dorsal plate with broad anterior margin and median spine, jointed and elongated pectoral fins. Jia et al. (2010) redefined Jiangxilepididae and still placed it in the bothriolepidoids. Our result assigns Jiangxilepididae to the asterolepidoid lineage, in accordance with Wang and Zhu (2018). Jiangxilepis, Ningxialepis and Kirgisolepis form a monophyletic group (Node 16, Fig. 8), characterized by the four synapomorphies: dorsal spine of anterior median dorsal plate (Ch. 52, state 1); small axillary foramen (Ch. 74, state 0); elongated pectoral appendage (Ch. 77, state 1); elongated lateral marginal plate 2 (Ch. 78, state 1). Walterilepis is assigned to Jiangxilepidae (Node 15, Fig. 8) for their reticular ornamentation (Ch. 1, state 1), but the material is highly fragmented, here we hold our reservation for considering it as a member of Jiangxilepidae.

Comparison within Bothriolepidoidei.

Skull roof - Moloshnikov (2011) established the family Tubalepididae for the PNu plate of Tubalepis lacking the anterolateral margin that usually connects with the postmarginal plate (PM, Fig. 7). He assumed that Tubalepis may lose its PM plate, but without feather materials support. Besides the Tubalepis, a few Chinese taxa, like Liujianglepis (Wang, 1987) and Hohsienolepis (P'an & Wang, 1978; Pan, 1984), were described to lack the PM plate. However, the basic skull pattern of those taxa is rarely changed, and the expanded L plate occupies the anterolateral sides of the PNu plates. Thus, the loss of the PM plates can be regarded as some fusion of the head shield. The shape of the PNu plate in Tongdulepis indicates that the loss of PM plates cannot just be considered as fusion but a stable feature.

Microbrachiidae is usually suggested as the most primitive group of the euantiarchs for its preorbital depression (pr.d, Fig. 7A). In our phylogenetic analysis, it is also plesiomorphic and the close group with the Tubalepididae lineage. If we ignore the PM plate in Microbrachius (Fig. 7A), the shape of its PNu plate is almost the same as that of Tubalepididae for the lack of anterolateral margin. Meanwhile, the Tenizolepis-Dianolepis-Bothriolepis lineage inherits the topological relationship between the PM, PNu, and L plates, the relative location among those plates remains stationary, and the preobstantic corners are always on their PM plates (Fig. 7). Our phylogeny analysis also suggests those two directions of evolution, the Tubalepididae lineage forms a sister group with the Tenizolepis-Dianolepis-Bothriolepis lineage (Fig. 8).

As mentioned before, the orbital fenestra is wide and occupies two-thirds of the width of the head shield (Figs. 4, 6). The wide orbital fenestra spreads eyeballs aside, making the horizon visual field much broader than the antiarchs with smaller orbital fenestra. Researchers suggest the large orbital fenestra is plesiomorphic, which may be a coincidence for similar habitat between the outgroup taxa and Tongdulepis.

Trunk armor - Besides the notable pattern of the head shield, interesting features on the trunk armor of Tubalepididae also isolated it from other Bothriolepidoidei. The ridges on the trunk shield are conducted as stiffeners, enhancing the strength of the trunk shield for more protection. The dorsolateral and dorsal median ridges are absent in the tubalepids, which are commonly observed within the bothriolepidoids. Besides, unlike the round trunk armor in other antiarchs, the tubalepids own elongated and slimmer trunk shields (Moloshnikov, 2011). Those features may jointly make their trunk armor drop-shaped, which helps reduce the water drag. Suppose we consider the contact margin with the median ventral plate (MV, Fig. 7) may be absent at the AVL of Tongdulepis, which has been confirmed in Tubalepis (Fig. 3). In that case, it is reasonable to believe the MV plate is either degenerated or lost in Tubalepididae. Tubalepis lost their MV plate without leaving a large rectangular aperture like the sinolepids (Ritchie et al., 1992) and left the four ventral plates covering each other (Fig. 7). In all the groups of the antiarchs bearing the MV plate, the MV plates are always overlapped by the surrounding ventral plates, mechanically making it a weakness (Fig. 7). In tubalepids, the ventral plates articulate each other, strengthen the ventral side of the trunk armor, and better resist the threat from beneath.

Brachial process - During the evolution of the antiarchs, the development of the brachial process is one of the key innovations (Young & Zhang, 1992). Compared with the primitive groups, the helmet-shaped articulation in the euantiarchs essentially increases the flexibility of the pectoral appendages. The helmet-shaped articulation is well-developed in Tongdulepis, but it can be further distinguished from the well-described brachial process of Bothriolepis (Young & Zhang, 1992). Compared to that of Bothriolepis, the dorsal and ventral articular depressions are less curved in Tongdulepis, which indicates its pectoral appendages have less rotation than the species with well-curved dorsal and ventral articular depressions around the helmet-shaped articulation. The insertion area for the abductor muscle in the bothriolepids (Young, 1988) that controls the protraction of the pectoral appendages is also absent in Tongdulepis, indicating its pectoral appendages may have a weaker protraction than the bothriolepids.

The flexibility in the radial direction is barely outlined by the articulation between the CD₁, CV₁, and pedal parts (Figs. 3, 6), making the vertical flexibility of the pectoral appendage limited, the rotation angle is less than the horizon protracted angle (Béchard et al., 2014). The pedal part of Tongdulepis tilts downside and marks the pectoral appendages of Tongdulepis pointed much downwards than known euantiarchs. Most of the antiarchs bear horizon-placed pectoral appendages, which fit well for their demersal habitat. The abdominal pectoral appendages may indicate that Tubalepis has a different habitat from the antiarchs bearing more horizon pectoral appendages. Similar phenomena are common in several fish groups. The sharks, for example, the species from the sea floor like Zebra shark Stegostomatidae (Dudgeon et al., 2008), tend to bear level pectoral appendages, while the epipelagic species like Great White Shark Carcharodon (Bruce, 2008) have more vertical fins to make it cruising more stable. Moreover, the large axillary foramen posterior of the brachial process (Fig. 6) suggests that Tongdulepis tends to have stronger pectoral appendages than the bothriolepids. Alternatively, the robust pectoral appendages may be also used to sustain its body on the sea floor, like the tripod fish Bathypterois grallator (Davis & Chakrabarty, 2011) or ‘walk’ like the frogfish Antennariidae (Pietsch, 1984).

A new tubalepid antiarch, Tongdulepis concavus, is described from the Middle Devonian (Qujing Formation, late Eifelian) of Huize County. It resembles Tubalepis in similar tubercular ornamentation, skull roof pattern, the shape of the AMD plate, and elongated trunk armor. Tongdulepis confirms the loss of the PM plates in Tubalepididae and fills our understanding of the neurocranium and brachial process in the primitive bothriolepidoids. The wide and narrow orbital fenestra, ridge-less trunk armor, extreme concave posterior margin of the AMD plate, and large axillary foramen outline Tongdulepis a distinctive taxon among the bothriolepidoids. To analyze its phylogenetic position among antiarchs, we established a large matrix with additional characters and species for an optimized analysis. Our result suggests Tongdulepis as a member of Tubalepididae, which is more plesiomorphic than the Tenizolepis-Dianolepis-Bothriolepis lineage within the Bothriolepidoidei.

Institutional abbreviation - IVPP, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China.

Geological abbreviation - Ch., Chuandong Formation; Em., Emsian; Fm., Formation; Sh., Shangshuanghe Formation.

Anatomical abbreviation - ad₁, anterior articular process on submarginal plate; ADL, anterior dorsolateral plate; a.fac, auxiliary articular facet for pectoral appendage; a.inf, ventral muscle insertion; alc, anterolateral angle of head shield; alr, postlevator thickening of anterior median dorsal plate; AMD, anterior median dorsal plate; a.PrL, attachment area for prelateral plate; art.d, dorsal articular depression for dermal process of pectoral appendage; art.v, ventral articular depression for dermal process of pectoral appendage; a₁SM, anterior attachment area for submarginal plate; a₂SM, posterior attachment area for submarginal plate; a.sup, anterior dorsal muscle insertion on brachial process; a.un, unornamented area beneath articular fossa of pectoral; AVL, anterior ventrolateral plate; Cd_1-3, dorsal central plate 1-3; cf.ADL, area overlapping anterior dorsolateral plate; cf.MxL, area overlapping mixilateral plate; c.nv, canal for nerves and/or vessels; cr.im, inframarginal crista of head shield; cr₁, prelateral crista of transverse lateral groove of head shield; cr₂, posterolateral crista of transverse lateral groove of head shield; cr.po, postorbital crista; cr.tv, transverse nuchal crista; Cv_1-2, ventral central plate 1-2; dsc, depression caused by semicircular canal; dlg₁, anterior oblique dorsal sensory line groove dlg₂, posterior oblique dorsal sensory line groove; dlr, dorsolateral ridge of trunk armor; dma, tergal angle of trunk armor; dmr, dorsal median ridge of trunk armor; f.ax, axillary foramen of anterior ventrolateral plate; fe.orb, orbital fenestra; fp, funnel pit of brachial process; f.retr, levator fossa of anterior median dorsal plate; ifc₁, principal section of infraorbital sensory line on head shield; ifc₂, branch of infraorbital sensory line diverging on lateral plate; L, lateral plate; lc, lateral corner of anterior median dorsal plate; lcg, main lateral line groove; Ml_1-4, lateral marginal plate 1-4; Mm_1-2, mesial marginal plate 1-2; mr, median ridge of postpineal plate; MV, median ventral plate; MxL, mixilateral plate; nm, obtected nuchal area of head shield; nprl, prelateral notch of head shield; Nu, nuchal plate; oa.ADL, area overlapped by anterior dorsolateral plate; oa.AMD, area overlapped by anterior median dorsal plate; oa.PMD, area overlapped by posterior median dorsal plate; oa.PNu, area overlapped by paranuchal plate; occ, occipital cross commissure; ood, otico-occipital depression of head shield; p, lateral pit of head shield; p.apo, anterior postorbital process; pbr, brachial process; pe, pedal part of brachial process; PMD, posterior median dorsal plate; pnt, paranuchal trochlea; PNu, paranuchal plate; PP, postpineal plate; pp, posterior pit line groove; pr.alm, process on anterolateral margin of lateral plate; prc, prepectoral corner of anterior ventrolateral plate; pr.d, preorbital depression of head shield; prh, preorbital recess of head shield; pro, obstantic process of trunk armor; proc, preobstantic corner of head shield; PrM, premedian plate; p.sup, posterior dorsal muscle insertion; pt₁, anterior ventral pit of dorsal wall of trunk armor; ptoc, postobstantic corner of paranuchal plate; PVL, posterior ventrolateral plate; sar, supra-articular ridge; soa, subobstantic area of head shield; soc, anterior section of supraorbital sensory line on premedian plate; sot, supraotic thickening of head shield; SM, submarginal plate; Sm, semilunar plate; tlg, transverse lateral groove of head shield.

Availability of data and materials

All data generated or analyzed during this study are included in this published article (and its additional files).

Acknowledgements

We thank Xindong Cui and Yingtian Zhao for their help during the fieldwork, Wei Gao for taking photos, and Xiufen Lu for the specimen preparation.

Funding

This work was supported by the National Natural Science Foundation of China (42130209, 92255301), and the Youth Innovation Promotion Association CAS (2021070).

Author information

Authors and Affiliations

University of Chinese Academy of Sciences, Beijing 100049

Yanchao Luo (https://orcid.org/0000-0002-2555-5016) & Min Zhu (https://orcid.org/0000-0002-4786-0898)

Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing 100044

Yanchao Luo, Zhaohui Pan (https://orcid.org/0000-0003-4321-3834), and Min Zhu

Contributions

MZ and YL organized the fieldwork. YL conducted the fieldwork, reconstructed the model, drew the figures, and wrote the manuscript. YL and MZ compiled the character matrix and performed the phylogenetic analyses. MZ and ZP contributed interpretation of the results, discussion and figures. All authors approved the final version of the manuscript.

Corresponding author

Correspondence to Min Zhu: [email protected]

Ethics declarations

Competing interests

We have no competing interests.

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A new tubalepid (Antiarcha, Placodermi) from the Middle Devonian of Huize, Yunnan, China

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