The holotype and only known specimen of Pauropetalichthys magnoculus was collected from yellowish siltstone of the Chuandong Formation exposed on the slope of a hill close to Shuanghe Village, about 7 km southwest of Zhanyi township, Qujing, Yunnan. From the same layer of the same site were found fossils of Kenichthys campbelli Chang and Zhu, 1993, Heimenia Ørvig, 1969 and Tarachomylax multicostatus Qiao and Zhu, 2008 (Chang and Zhu, 1993, Qiao and Zhu, 2008, Wang, 1986 and Zhu et al., 2002). The Early Devonian non-marine strata of Qujing can be divided from bottom to top into the Xishancun, Xitun, Guijiatun, Xujiachong, and Chuandong formations. Biostratigraphic correlations involving spore, fish and bivalve fossils imply that the Lower-Middle Devonian boundary is located in the upper part of the Chuandong Formation, and that the fish-bearing horizon is Late Emsian in age (Liu et al., 2004 and Zhu and Wang, 1996a; Fig. 1). The specimen is housed at the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, in Beijing.

The specimen was prepared mechanically using pneumatic air scribes and needles under stereomicroscopes. To enhance contrast, the specimen was coated with ammonium chloride sublimate before it was photographed using Nikon D3X. We scanned the specimen using the 225 kV high-resolution computed tomography apparatus (developed by the Institute of High Energy Physics, Chinese Academy of Sciences) at the Key Laboratory of Vertebrate Evolution and Human Origins of the CAS. The specimen was scanned with a beam energy of 130 keV and a flux of 100 μA at a detector resolution of 8.6 μm per pixel, using a 720° rotation with a step size of 0.5° and an unfiltered aluminum reflection target. The same settings were used in a previous study carried out by Zhu et al. (2013). A total of 1440 transmission images were reconstructed in a matrix of 1608 slices each measuring 2048 × 2048 pixels, by means of two-dimensional reconstruction software (IVPP225KVCT_Reconstruction) developed by the Institute of High Energy Physics, CAS. Three-dimensional reconstructions were then generated using Mimics (Materialize version 16), and images exported from Mimics were processed in Adobe Photoshop and Adobe Illustrator.

The data matrix was composed with Mesquite version 2.73 (Maddison and Maddison, 2008). Phylogenetic analysis was performed with PAUP* v4.0b10 (Swofford, 2003). All characters were unordered and unpolarized a priori, and two arthrodire taxa were used as outgroups for state character polarization. The exhaustive search function was used, and synapomorphies were assigned to nodes according to DELTRAN (delayed transformation) optimization.

The length of the skull roof (from the anterior margin to the mesial articular process) is 13.6 mm. The maximum breadth, which occurs between the apexes of the arched lateral margins of the marginal plates, is 12.9 mm. The skull roof is broad, with a length-width ratio of 1.05. The orbit is anteriorly placed and not enclosed laterally by the skull roof. The length of the orbit is 4.4 mm, almost one third of the skull roof length. The orbit/skull roof length ratio varies from 0.12 (Macropetalichthys rapheidolabis; Norwood and Owen, 1846) to 0.16 (Lunaspis broilii; Gross, 1961) among previously described petalichthyids, so the value for Pauropetalichthys (Fig. 3) is the largest known within the group. The preorbital region is short, similar to those of arthrodires, quasipetalichthyids and Diandongpetalichthys. In contrast, macropetalichthyids have significantly longer preorbital regions (Liu, 1991, Stensiö, 1925, Stensiö, 1969 and Zhu, 1991). The skull roof is arched along the midline, its summit lying at the center of the nuchal plate (Fig. 2C).

The skull roof is thinner along the midline than along the margin. The skull roof plates are mostly fused, and few bone sutures can be traced. The restored dermal bone pattern (Fig. 3) is inferred from the positions of the sensory canal intersections, which are assumed to correspond to those of ossification centers, and from the concentrically arranged ornament. The skull roof consists of three median bones (the rostral, pineal, and nuchal plates) and six pairs of laterally located bones (the preorbital, postorbital, central, marginal, anterior paranuchal and posterior paranuchal plates). A similar pattern can be seen in Quasipetalichthys and Eurycaraspis (Liu, 1991; Fig. 4).

The rostral plate (Ro, Fig. 3) is situated at the front of the skull, and curves downward anteriorly. However, its sutures with adjoining plates are indistinct. Together with the downward-curving preorbital plates, the anterior part of the skull roof envelopes the nasal capsules. In Macropetalichthys and Eurycaraspis, by comparison, the anterior part of the skull roof is flat and lacks a downward-curving lamina (Liu, 1991 and Stensiö, 1925).

Although the boundaries of the pineal plate (Pi, Fig. 3) are not clear, both the position of this plate and the approximate position of the pineal organ can be inferred from the location of the pineal foramen and fossa on the visceral surface of the plate. A pineal opening is absent in both Quasipetalichthys haikouensis (Liu, 1973) and Eurycaraspis incilis (Liu, 1991; Fig. 4).

The nuchal plate (Nu, Fig, 3) is the largest bone in the skull roof. The ratio of its length to the total skull roof length is 0.72. The dermal ornament on the nuchal plate shows more tiny tubercles, interspersed between the larger ones, than are present on the other skull roof bones The ossification center of the nuchal plate is located near the posterior point of trisection of the plate midline.

In the posterior half of the skull roof, a pair of foramina is identified as the external openings of the endolymphatic ducts (f.dend.e, Fig. 2 and Fig. 3). In visceral view, corresponding foramina that represent the internal openings of the ducts are apparent. Similar duct openings are also present in Eurycaraspis, but were formerly misinterpreted as foramina for cutaneous sensory cells (Liu, 1991).

The suture between the preorbital and postorbital plates is clearly visible. The preorbital plate (PrO, Fig. 3) curves downward anteriorly, and its mesial margin is in contact with the rostral plate anteriorly and the pineal plate posteriorly. A dermal antorbital process (pr.dao, Fig. 5A) forms the anterior margin of the orbit. Moreover, a dermal eminence (th.PrO, Fig. 5) can be seen at the anterior part of the preorbital plate. The HRCT scan shows that the dermal eminence encloses the supraorbital canal (Fig. 5).

The suture between the postorbital (PtO, Fig. 3) and marginal plates (Mg, Fig. 3) is difficult to distinguish. The outline of the postorbital plate is inferred from its radiating ornament. Similarly, the point where the posterior pit line converges with the infraorbital canals is assumed to coincide with the ossification center of the marginal plate. The anterior postorbital notch (nla, Fig. 3) might indicate the position of the suture between the postorbital and marginal plates, whereas the posterior postorbital notch (nlp, Fig. 3) might indicate that of the suture between the marginal and posterior paranuchal plates. The mesial margin of the marginal plate contacts the central and anterior paranuchal plates.

The infraorbital lobe (l.io, Fig. 2C) of the postorbital plate is well developed and extends anteroventrally, but does not enclose the orbit. The tip of the lobe is slightly enlarged. In contrast, the lobe tapers gently in Eurycaraspis and Quasipetalichthys (Fig. 4). The infraorbital sensory canal passes through the infraorbital lobe, and exits at its anterior end (Fig. 2 and Fig. 3).

Both the central and the anterior paranuchal plates are proportionally small. The suture between the central plate (C, Fig. 3) and the anterior paranuchal plate (PNu.a, Fig. 3) is unclear. The central plate bears the middle pit line (mpl, Fig. 3) and the anterior part of the posterior pit line (ppl.a, Fig. 3). The point of convergence of the posterior part of the posterior pit line (ppl.p, Fig. 3) and the main lateral line canal (mpl, Fig. 3) coincides with the ossification center of the anterior paranuchal plate.

The posterior paranuchal plate (PNu.p, Fig. 3) forms the posterolateral part of the skull roof. Its ossification center is located where the main lateral line canal meets the occipital cross-commissure. On the visceral side of the posterior paranuchal plate, a smooth cucullaris depression (dp.cu, Fig. 5) is delimited by the ridge of the main lateral line canal anteriorly and the paranuchal crista (cr.PNu, Fig. 5A) laterally.

The posterior descending lamina of the nuchal plate (pdl, Fig. 3), which is known only in Pauropetalichthys and Eurycaraspis, is at the posterior end of the skull roof. It is a thin structure with a distinct posterior median process (pr.pm, Fig. 2 and Fig. 3). Lateral to the posterior descending lamina, the mesial articular process (pr.ma, Fig. 2 and Fig. 3) is well developed. The mesial articular process forms an angle of approximately 120° with the posterior margin of the posterior paranuchal plate in dorsal view.

On the visceral side of the posterior descending lamina, a nuchal thickening (th.Nu, Fig. 5) is present, bordering the posterior portion of the craniospinal process (pr.csp, Figs. 5B and 6). Lateral to the mesial articular processes are situated a pair of glenoid fossae (f.gl, Fig. 5) comparable to those of Eurycaraspis, Macropetalichthys and Wijdeaspis (Liu, 1991, Stensiö, 1969 and Young, 1978).

The ornamentation on the dorsal surface of the skull roof consists of densely set, spheroidal tubercles. In some areas, these tubercles are concentrically arranged, highlighting the ossification centers of the dermal bones on which they occur. The size of the tubercles varies across different areas. On the nuchal plate, the tubercles grow smaller near the ossification centers. Moreover, the tubercles along the skull roof margins are generally smaller than those located more centrally.

The sensory canals are enclosed within the dermal bones, with pores opening on the external surface. The pores along each canal are arranged in a single line. The sensory canals decrease in diameter near their exits, and taper to points where they end blindly with the bone of the skull roof. The sensory canals on the nuchal plate are arranged in an incomplete X-shaped pattern (Fig. 3): the supraorbital canals and posterior pit line canals converge toward the ossification center of the nuchal plate as in Eurycaraspis, but taper to points well before reaching the ossification center.

The supraorbital canal (soc, Fig. 3) begins at the anterior margin of the preorbital plate and extends posteromedially without meeting its counterpart on the opposite side. The supraorbital canal shows three moderate inflections. The anterior inflection is at the intersection with the ethmoid commissure, which coincides with the ossification center of the preorbital plate. The middle inflection occurs at the point of maximum curvature of the preorbital plate. The posterior inflection lies on the boundary between the preorbital and nuchal plates.

The ethmoid commissure (eth.c, Fig. 3) is situated on the anterior portion of the preorbital plate. The ethmoid commissure extends medially, but ends at the margin of the rostral plate. The commissure has been reported in some antiarchs, like Dianolepis liui Chang, 1965 and Byssacanthus dilatatus (Eichwald, 1844; see also Karatajūte-Talimaa, 1960), and thus seems likely to represent a primitive gnathostome feature.

The posterior part of the posterior pit line (ppl.p, Fig. 3) extends transversely and converges with its opposite counterpart and both supraorbital canals, similar to the condition in Quasipetalichthys, Eurycaraspis (Fig. 4) and Diandongpetalichthys (Liu, 1991 and Zhu, 1991). As in Lunaspis broilii, Gross, 1961, the left and right posterior pit lines fail to contact each other.

The infraorbital canal (ioc, Fig. 3) begins at the infraorbital lobe of the preorbital plate and is directed posteromedially. This canal shows an inflection just posterior to infraorbital lobe.

The main lateral line canal (lc, Fig. 3) bends through an angle of 140° in the middle of its course. The diameter of the lateral line canal is greatest near the junction with the posterior part of the posterior pit line, and gradually decreases in both directions. The posterior portion of the main lateral line canal and the posterior part of the posterior pit line meet at an angle of 90°.

The postmarginal canal (pmc, Fig. 3) extends posterolaterally from the intersection between the infraorbital and main lateral line canals. The postmarginal canal has multiple wide branches that exit the skull. The postmarginal and main lateral line canals are both comparatively narrow near the point where they converge.

The occipital cross-commissure (occ, Fig. 3) is short, a feature that is common in Arthrodira and Acanthothoraci but has not been reported in any petalichthyid other than Diandongpetalichthys (Zhu, 1991). Two pairs of short grooves, representing the middle pit line and the anterior part of the posterior pit line (mpl, ppl.a, Fig. 2 and Fig. 3), are clearly discernable on the central plates.

Based on the HRCT scan of the holotype, we reconstructed part of the endocranium. The endocranium of Pauropetalichthys is platybasic and perichondrally ossified as in other petalichthyids and indeed most placoderms (Janvier, 1996). There is no trace of either endochondral ossification or calcified cartilage. We can infer the contours of the endocranium from depressions on the visceral side of the skull roof, which is in intimate contact with the dorsal wall of the endocranium (Fig. 6).

The endocranial processes of Pauropetalichthys include, on each side of the skull, an antorbital process (pr.ant, Fig. 6), an anterior postorbital process (pr.poa, Fig. 5 and Fig. 6), a posterior postorbital process (pr.pop, Fig. 6), a supravagal process (pr.sv, Fig. 6) and a craniospinal process (pr.csp, Fig. 5 and Fig. 6). However, the supraorbital process is almost impossible to trace on the visceral side of the skull roof. In general, the processes can be closely compared to those of Eurycaraspis (Liu, 1991). The very short distance between the anterior and posterior postorbital processes is a noteworthy feature.

The endocranium consists of the ethmoid division and the postethmo-occipital bone, but the suture between these components is not traceable by HRCT scanning. The postethmo-occipital bone can be subdivided into orbitotemporal, otic and occipital regions.

The ethmoid division bears a pair of nasal capsules (cap.n, Fig. 6), a structure that has not been previously reported in any petalichthyid. Each nasal capsule opens ventrally as in the arthrodires Buchanosteus (Young, 1979) and Dicksonosteus (Goujet, 1984). An irregular foramen piercing the dorsomedial surface of the nasal capsule represents the foramina cribrosa (fc, Fig. 5B).

Behind the nasal capsule, the articular surface for the palatoquadrate (art.pq, Fig. 5 and Fig. 6) forms a hole in the perichondral envelope as in Romundina (Dupret et al., 2010, Dupret et al., 2014 and Goujet, 2001). Behind the articular surface, the perichondral bone around the orbital cavity forms a supraorbital vault (sov, Fig. 5 and Fig. 6). Beneath the infraorbital lobe, the perichondral bone encloses a canal for the hyomandibular branch of the facial nerve (VII.hm, Fig. 6). The hyomandibular branch of the facial nerve is not directed along the infraorbital lobe, and its foramen is at the tip of the antorbital process. The orbitotemporal region extends from the articular surface for the palatoquadrate to the front end of the ridge bordering the semicircular canals.

The otic region is characterized by a pair of depressions on the visceral side of the skull roof, corresponding to the ridges bordering anterior and posterior semicircular canals of the labyrinth (r.cs, Fig. 5). As in other petalichthyids, the otic region is moderately short. It is noteworthy that the anterior and posterior postorbital processes, and the otic region, vary among petalichthyids in their shapes and positions (Fig. 7).

The occipital region is the posterior part of the endocranium. The supravagal process (pr.sv, Fig. 6) is located posterior to the posterior extremity of the ridge bordering anterior and posterior semicircular canals.

The craniospinal process (pr.csp, Fig. 5 and Fig. 6) lies behind the supravagal process, as in Macropetalichthys (Stensiö, 1963). The paranuchal crest, which was described by Young (1978) as a ventral thickening developed beneath the ossification center of the posterior paranuchal plate in Wijdeaspis, contacts to the lateral wall of the craniospinal process. At the posterior extremity of the endocranium, the perichondral bone forms an infranuchal process (pr.iNu, Fig. 5B). On both sides of the infranuchal process, the craniospinal processes (Fig. 5 and Fig. 6) display subcircular depressions on their posterior walls (la.csp, Fig. 5 and Fig. 6) as in Macropetalichthys (Stensiö, 1969). These depressions represent articular facets for the trunk armor.

In order to resolve the systematic position of Pauropetalichthys magnoculus within the Petalichthyida, a computer-based phylogenetic analysis was performed. The ingroup consisted of six taxa including Diandongpetalichthys liaojiaoshanensis (P’an and Wang, 1978 and Zhu, 1991), Macropetalichthys rapheidolabis (Norwood and Owen, 1846 and Stensiö, 1969), Lunaspis broilii (Gross, 1937 and Gross, 1961), Pauropetalichthys magnoculus gen. et sp. nov., Quasipetalichthys haikouensis (Liu, 1973) and Eurycaraspis incilis (Liu, 1991). The outgroup consisted of the arthrodires Dicksonosteus arcticus (Goujet, 1975 and Goujet, 1984) and Kujdanowiaspis podolica (Brotzen, 1934 and Dupret, 2010). The full list of 22 characters is given in Appendix 1 (character state coding does not indicate any a priori designation of particular states as primitive or derived). The data matrix is given in Appendix 2.

The analysis yielded a single most parsimonious tree (Fig. 8). Among the monophyletic Quasipetalichthyidae, Pauropetalichthys is the sister group of Quasipetalichthys plus Eurycaraspis. Synapomorphies of the Quasipetalichthyidae (Node 4, Fig. 8) include the presence of a posterior descending lamina, the absence of a supraorbital process, the posterior position of the ossification center of the nuchal plate, the presence of a mesial articular process and the eminence of the preorbital plate. Synapomorphies of the Quasipetalichthys - Eurycaraspis clade (Node 5, Fig. 8) include contact between the contralateral posterior pit lines, and between the contralateral supraorbital canals. Synapomorphies of the quasipetalichthyid–macropetalichthyid clade (Node 2, Fig. 8) include the fact that the postnasal plate does not form part of the orbital margin, the absence of the postnasal plate, and the unforked morphology of the posterior postorbital process.

Pauropetalichthys presents many primitive characters with the potential to contribute to our understanding of petalichthyid interrelationships. Thus, we comment further on the morphology of petalichthyid endocrania with special reference to Pauropetalichthys.

The pineal foramen is positioned more posteriorly in Eurycaraspis and Macropetalichthys than in Dicksonosteus and Diandongpetalichthys (Goujet, 1984 and Zhu, 1991; Fig. 7). Pauropetalichthys retains the primitive state, having an anteriorly positioned pineal foramen. In most placoderms, the position of the pineal foramen roughly corresponds to the site where the pineal organ emerges from the diencephalon, because the flattened endocranium is close to the visceral surface of the skull roof. Accordingly, the posteriorly positioned pineal foramen seen in some taxa suggests a posteriorly placed diencephalon. In Macropetalichthys the posterior position of the diencephalon is inferred to have resulted mainly from elongation of the olfactory tract (fig. 31, Stensiö, 1963). Because of a lack of endocranial preservation, it remains uncertain whether the olfactory tract was also elongated in Eurycaraspis.

The position of the supravagal process varies in petalichthyids (pr.sv, Fig. 7). In Diandongpetalichthys, the process is close to the posterior extremity of the endocranium, similar to the morphology in arthrodires such as Dicksonosteus. By comparison, a significantly longer occipital part of the endocranium is situated posterior to the supravagal process in Pauropetalichthys, a condition reminiscent of that seen in Macropetalichthys and Eurycaraspis. The supravagal process itself also varies in shape, being nearly right-angled in Diandongpetalichthys and the arthrodire Dicksonosteus but forming an obtuse angle in Pauropetalichthys, Eurycaraspis and Macropetalichthys.

The posterior postorbital process is forked in Diandongpetalichthys (Zhu, 1991), but unforked and somewhat underdeveloped in Pauropetalichthys, Eurycaraspis and Macropetalichthys (Liu, 1991 and Stensiö, 1963). Outgroup comparison shows that the forked process is also present in primitive arthrodires such as Dicksonosteus (Goujet, 1984), and should therefore be considered a plesiomorphy. In addition, the posterior postorbital process is much closer to the anterior postorbital process in Pauropetalichthys than in Dicksonosteus and Diandongpetalichthys (Goujet, 1984 and Zhu, 1991). The short distance between anterior and posterior postorbital processes also appears in Eurycaraspis but to a lesser extent (Liu, 1991). In Macropetalichthys, the anterior and posterior postorbital processes are completely fused into a single complex (pr.poap, Fig. 8; Stensiö, 1963).

The extent of the otic region can be inferred from that of the labyrinths, or that of their imprints on the visceral surface of the skull roof. In Dicksonosteus and Diandongpetalichthys, the otic region is relatively long, forming 36 percent and 31 percent of the total length of the endocranium respectively (Goujet, 1984 and Zhu, 1991). In Pauropetalichthys and Eurycaraspis, the otic region is considerably shorter, forming 23 percent and 18 percent of the total length of the endocranium respectively (Liu, 1973 and Liu, 1991). The otic region of Macropetalichthys, forming only 15 percent of the total length of the endocranium, is the shortest of any petalichthyid (Stensiö, 1925 and Stensiö, 1963). Proportional shortening of the otic region in petalichthyids may actually reflect general elongation of both the orbitotemporal and occipital parts of the endocranium.

Before Pauropetalichthys was discovered, 17 petalichthyid genera had been reported from China, Iran, Australia, Siberia and Euramerica, ranging in age from Lochkovian to Late Frasnian (Fig. 9; Denison, 1978 and Zhu, 2000). A complete list of taxa and localities is given in Appendix 3.

Outside China, petalichthyids are known from the Emsian (Early Devonian) to the Late Frasnian (Late Devonian), and belong to the Macropetalichthyidae exclusively. By comparison, the Chinese petalichthyids appear earlier (Lochkovian, Earliest Devonian) and show a higher diversity. In Lochkovian - Pragian (Early Devonian) deposits, petalichthyids are restricted to the South China Block. The discovery of Quasipetalichthys in both North China and South China suggests paleogeographic proximity between the two blocks (Jia et al., 2010 and Ritchie et al., 1992). Although several Chinese petalichthyids are endemic, such as Diandongpetalichthys (P’an and Wang, 1978 and Zhu, 1991), Quasipetalichthys (Liu, 1973 and Pan et al., 1987) and Eurycaraspis (Liu, 1991), Chinese deposits have also yielded such macropetalichthyids as Xinanpetalichthys (P’an and Wang, 1978), Pampetalichthys (Zhu, 2000 and Zhu and Wang, 1996b), and Guangxipetalichthys (Ji and Pan, 1997). However, the purported discoveries of Lunaspis sp. (Liu, 1981) and Macropetalichthyidae gen. et sp. indet. (Wang and Cao, 1988) from China are doubtful. The former report is based on a partial coat of trunk armor that may belong to an arthrodire, while the latter is based on material that is too fragmentary to be assigned to the Macropetalichthyidae with certainty.

Plotting the distribution of petalichthyids on an Early Devonian global reconstruction (Fig. 10) indicates that macropetalichthyids may have dispersed along the nearly continuous coastline from South China to eastern Gondwana, Euramerica and Siberia, as may also have been true of sarcopterygians (Zhu and Zhao, 2006). The occurrence of Wijdeaspis in areas as widely separated as Spitsbergen, Severnaya Zemlya, Siberia and Iran (Denison, 1978, Heintz, 1929 and Heintz, 1937) suggests that macropetalichthyids might have possessed a strong dispersal capacity.

Two major bio-events affected Chinese petalichthyids. The first was the E’Em Event, which can be recognized as a faunal turnover from endemic taxa to more cosmopolitan ones (Walliser, 1995 and Zhu, 2000). The second was the Late Eifelian Event (Walliser, 1995), which resulted in the extinction of petalichthyids in China. Based on the analysis of Zhu and Wang (1996b), the Petalichthyida originated in the Lochkovian of South China and diversified in South China before the E’Em Event, and macropetalichthyids subsequently dispersed to other regions in the Early Emsian. Primitive arthrodires underwent a similar long-dispersal history from South China to Gondwana and Euramerica, but at an earlier time (Dupret, 2008 and Dupret and Zhu, 2008).