Historical Biology An International Journal of Paleobiology

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Falxcornus, a new genus of Tridensaspidae (Galeaspida, stem-Gnathostomata) from the Lower Devonian in Qujing, Yunnan, China

Xinyuan Meng & Zhikun Gai

To cite this article: Xinyuan Meng & Zhikun Gai (2021): Falxcornus, a new genus of Tridensaspidae (Galeaspida, stem-Gnathostomata) from the Lower Devonian in Qujing, Yunnan, China, Historical Biology, DOI: 10.1080/08912963.2021.1952198 To link to this article: https://doi.org/10.1080/08912963.2021.1952198

Published online: 27 Jul 2021.

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Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=ghbi20 HISTORICAL BIOLOGY https://doi.org/10.1080/08912963.2021.1952198

Falxcornus, a new genus of Tridensaspidae (Galeaspida, stem-Gnathostomata) from the Lower Devonian in Qujing, Yunnan, China Xinyuan Menga,b and Zhikun Gaia,b,c aInstitute of Vertebrate Palaeontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China; bUniversity of the Chinese Academy of Sciences, Beijing, China; cCAS Center for Excellence in Life and Paleoenvironment, Beijing, China

ABSTRACT ARTICLE HISTORY A new genus and species of galeaspid, Falxcornus liui gen. et sp. nov., is described from the Xishancun Received 25 April 2021 Formation (Lochkovian, Lower Devonian) in Qujing, Yunnan, China. The new taxon displays a suite of Accepted 27 June 2021 diagnostic characters of the Eugaleaspiformes and strikingly resembles the tridensaspids Tridensaspis and KEYWORDS Pterogonaspis in its sickle-like complex of cornual and inner cornual processes, leaf-like inner cornual Falxcornus; Lower Devonian; processes, and median dorsal opening posteriorly placed level with the centre of the orbital opening, but Galeaspida; Tridensaspidae; clearly differs from these genera in lacking the rostral process and laterally projecting cornual processes. An taxonomy; vertebrate extended phylogenetic analysis of the Galeaspida suggests that Falxcornus, Pterogonaspis and Tridensaspis palaeontology form a monophyletic group as the family Tridensaspidae, and Falxcornus is resolved as sister to Pterogonaspis and Tridensaspis. Therefore, Falxcornus liui gen. et sp. nov. now represents the most primitive and oldest known tridensaspid fish. Its subacute rostral margin and sickle-like complex of cornual and inner cornual processes likely represent an intermediate state of these highly specialised rostral and lateral projecting processes, respectively. The recurrent evolution of similar cephalic elaborations within the Galeaspida suggests that some galeaspids could manipulate water flow around their headshields to become active animals with higher manoeuvrability and versatility, similar to osteostracans.

Introduction the Tridensaspidae by Shan et al. (2020), as it formed a monophyletic group together with Tridensaspis magnoculus in The Galeaspida isa clade of armoured jawless ‘ostracoderms’ (stem- all parsimony-based cladograms (Zhu 1992; Gai et al. 2005; Zhu gnathostomes) restricted to East Asia during the Middle and Gai 2006; Shan et al. 2020; Jiang et al. 2021). Ferrón et al. Palaeozoic (Tarlo 1967; Zhu 1992; Janvier 1996, 2009; Zhu and (2020,2021) showed that these cephalic elaborations of headshields Gai 2006; Gai et al. 2018; Shan et al. 2020; Jiang et al. 2021). They represent adaptations for passive hydrodynamic control both in the are among the enigmatic clades of stem-gnathostomes key to osteostracans and galeaspids. Therefore, the Tridensaspidae is an understanding the transition from jawless to jawed vertebrates important clade for understanding the last adaptive radiation of (Janvier 1996; Donoghue et al., 2000; Sansom et al., 2010; Gai galeaspids during the Pragian stage of the Early Devonian. et al., 2011; Gai and Zhu, 2012; Donoghue et al. 2014; Keating However, the evolution of cephalic elaborations within the and Donoghue 2016; Gai et al., 2019). Research on ostracoderm Eugaleaspiformes remains enigmatic because there is no intermedi­ morphology and phylogeny could facilitate interpretation of the ate state of these highly specialised rostral and lateral projecting gradual assembly of gnathostome characters such as jaws, processes. This condition is considerably different from that of the a mineralised dermal skeleton, vertebrae, a mineralised braincase, Huananaspiformes. Here, we describe a new eugaleaspidiform paired nostrils, a differentiated gut, a complex inner ear, externally genus and species from the lower part of the Xishancun open endolymphatic ducts, and paired pectoral and pelvic appen­ Formation (lower Lochkovian, Lower Devonian). The new taxon dages (Donoghue et al. 2014; Keating and Donoghue 2016; Gai et al. is characterised by a subacute rostral margin and sickle-like com­ 2011). Among galeaspids, the family Tridensaspidae plex of cornual and inner cornual processes, which likely represent (Eugaleaspiformes) is of particular interest because their head­ an intermediate condition between Tridensaspis and Nochelaspis. shields exhibit significant convergence with those of the As the new taxon was collected from the lower Lochkovian, lower Huananaspiformes in possessing similar cephalic elaborations than the other members of the Tridensaspidae, it sheds new light on (elongated rostra, lateral expansions or processes), which were the early evolution of the Tridensaspidae. previously regarded as diagnostic characters of the Huananaspiformes (Janvier 1975). The Tridensaspidae was erected by Liu (1986) based on Tridensaspis magnoculus from the bottom of Anatomical abbreviations the Yukiang Formation (Pragian, Lower Devonian) of Guangxi, as br.f, branchial fossa; br.o, branchial opening; c, cornual process; da, the rostral process and lateral projecting processes of this taxon dorsal aorta; dcm, dorsal commissure; ic, inner cornual process; ifc, were unknown in all other members of the Eugaleaspiformes. infraorbital canal; ldc, lateral dorsal canal; ltc, lateral transverse Pterogonaspis yuhaii (Zhu 1992) from the Xujiachong Formation canal;md.o, median dorsal opening;mdc, median dorsal canal; (Pragian, Lower Devonian) of Qujing, Yunnan was later referred to obr.c, oralobranchial chamber; oesc,oesophagus; orb, orbital

CONTACT Zhikun Gai [email protected] Institute of Vertebrate Palaeontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China; University of the Chinese Academy of Sciences, Beijing, China; Institute of Vertebrate Palaeontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China; CAS Center for Excellence in Life and Paleoenvironment, Beijing, China; University of the Chinese Academy of Sciences, Beijing, China © 2021 Informa UK Limited, trading as Taylor & Francis Group

Published online 27 Jul 2021 2 X. MENG AND Z. GAI

opening; pi, pineal fossa; ro, rostral process; soc1, anterior supraor­ and Zhu 2010; Xue et al. 2018) (Figure 1(c)). The Xishancun bital canal; soc2, posterior supraorbital canal; pb.b, postbranchial Formation is mainly composed of hard and dense quartz sand­ bar; pb.w, postbranchial wall; vr, ventral rim. stone with yellowish green and black shale. A layer of greyish yellow fine-grained sandstone is in conformable or disconformable contact with the Silurian Yulongssu Formation at the bottom of Geological setting the Xishancun Formation, and the Xitun Formation conformably The material of Falxcornus liui gen. et sp. nov. was collected by overlies it (Lin et al. 1998; Ma et al. 2009). A large number of Professor Yuhai Liu in May 1980 under a water pipe on the south galeaspid, placoderm, sarcopterygian and actinopterygian fossils side of the road in front of the Qujing Maternal and Child Health have been found in the Xishancun Formation, and these constitute Care Hospital, Yunnan Province, China (Figure 1(a, b)). The an important early vertebrate fauna (Zhu et al. 2000; Zhao and Lower Devonian strata in eastern Yunnan Province are well devel­ Zhu 2010). Galeaspids described from the Xishancun Formation oped with a complete sequence, known as the Cuifengshan Group, include Eugaleaspis changi (Liu 1965, Liu 1980), Yunnanogaleaspis which is divided into four formations: the Xishancun, Xitun, major (Pan and Wang 1980), Nochelaspis maeandrine (Zhu 1992), Guijiatun and Xujiachong formations from bottom to top (Wang Polybranchiaspis liaojiaoshanensis (Liu 1965), P. minor, Laxaspis 1984; Zhu 1992; Liu 2002; Zhu and Gai 2006; Ma et al. 2009; Zhao qujingensis, L. yulongssus, Diandongaspis xishancunensis (Liu

Figure 1. Maps of the fossil locality of Falxcornus liui gen. et sp. nov. (a, b) and the fossil fish-bearing lithological column (c) in Qujing, Yunnan Province, China. HISTORICAL BIOLOGY 3

1975), Pseudolaxaspis rostrata (Liu 1975; Gai et al. 2018), a subtriangular headshield with cornual and inner cornual processes, ‘Dongfangaspis qujingensis’ (Pan and Wang 1981), Damaspis var­ a longitudinal oval median dorsal opening, and a typical eugaleaspid- tus (Wang and Wang 1982), Nanpanaspis microculus (Liu 1965), type sensory canal system. It differs from the Shuyuidae, Altigibbaspis huiqingae (Liu et al. 2018), Siyingia perlatuspinosa (Si Sinogaleaspidae and Eugaleaspidae in its leaf-like inner cornual pro­ et al. 2015), Pentathyraspis pelta (Pan 1992) and Stephaspis dipter­ cess, differs from Shuyuidae and Sinogaleaspidae in its slit-like median iga (Gai and Zhu 2007). This galeaspid assemblage occurs only in dorsal opening (length/width > 6) and absence of the fourth lateral the Qujing district of eastern Yunnan and is known as the transverse canal, and further differs from Shuyuidae in the presence of Polybranchiaspis–Laxaspis assemblage (Assemblage I) (Zhu et al. the median dorsal canal; it differs from Nochelaspis and 2000) or the Xishancun Assemblage (Zhao and Zhu 2010), which Yunnanogaleaspis in its median dorsal opening overstepping the ante­ is characterised by the radiation of Polybrachiaspidiformes. The rior margin of the orbital opening and tiny granular tubercle orna­ Xishancun Formation is considered to represent a foreshore–off­ mentations; it strikingly resembles Tridensaspis and Pterogonaspis of shore environment based on the greyish black sandstones and the Tridensaspidae in its sickle-like complex of cornual and inner shales (weathering colour is greyish yellow) (Ma et al. 2009; cornual processes, leaf-like inner cornual process, U-shaped median Zhao and Zhu 2010). According to evidence from graptolites dorsal canal, and the posterior end of the median dorsal opening being and the Emphanisporites micronatus–Strelispora newportensis level with the centre of the orbital opening, but differs from them in spore assemblage, the Xishancun Formation likely dates to the lacking the rostral process and laterally projecting cornual processes. early Lochkovian, Early Devonian (Cai et al. 1994; Zhao and Zhu Therefore, Falxcornus can be considered as a new genus of 2010; Qie et al. 2018). Eugaleaspiformes.

Material and methods Falxcornus liui gen. et. sp. nov The new material of Falxcornus liui gen. et. sp. nov. includes one (Figures 2–3) nearly complete headshield and its counterpart (IVPP V26,670.1A, Etymology After professor Yuhai Liu, who found the specimen B). The fossil was prepared mechanically using a vibro tool with of Falxcornus liui gen. et. sp. nov. in May 1980 and is the first a tungsten-carbide bit or a needle, and measured with digital vernier person to study galeaspids. callipers, studied under an Olympus SZ61 zoom stereo microscope, Holotype A nearly complete headshield and its counterpart, and photographed with a Canon EOS 5D Mark III camera coupled IVPP V 26670.1A, B. with a Canon EF 100 mm 1:2.8 L macro photo lens for the general Locality and Horizon The south side of the road in front of the morphology as well as a Canon MP-E 65 mm 1:2.8 1–5× macro Qujing Maternal and Child Health Care Hospital, Qilin District, photo lens and an Olympus SZ61 zoom stereo microscope for close- Qujing City, Yunnan Province; Xishancun Formation, Cuifengshan up images of the ornamentation on the headshield. The specimen is Group, lower Lochkovian, Lower Devonian. permanently housed and accessible for examination in the collec­ tions of the Institute of Vertebrate Palaeontology and Palaeoanthropology (IVPP), Chinese Academy of Sciences, Beijing. Measurements See Table 1. Systematic palaeontology Description The holotype (IVPP V26670.1 A, B) of Falxcornus Subclass Galeaspida Tarlo et al. (1967) liui gen. et. sp. nov. is a nearly complete headshield, the size and Order Eugaleaspiformes (Liu, 1965) Liu (1980) shape of which indicate that it is a medium-sized Eugaleaspiforme Family Tridensaspidae Liu (1986) (Gai and Zhu 2017). The headshield is subtriangular with clear Genus Falxcornus gen. nov. cornual and inner cornual processes. The preserved width of the (Figures 2–3) headshield is about 55 mm, but the maximum width should be Etymology Falx Latin, the shape of a sickle; cornus, from Latin located between the tips of paired cornual processes and is esti­ cornu, the horn of an animal, referring to the sickle-like complex of mated to be 60 mm (calculated based on bilateral symmetry). The the cornual and inner cornual processes. maximum length of the headshield is about 40.7 mm (Table 1) Type species Falxcornus liui gen. et. sp. nov. (distance between the tips of the rostral margin and inner cornual Diagnosis Medium- process), and the length of the headshield in the midline is approxi­ sized tridensaspid fish; subtriangular headshield; subacute rostral mately 30.4 mm (Table 1). The width/length ratio of the headshield margin with an apparent rostral angle, but lacking the rostral process; is estimated to be 1.6. The rostral margin of the headshield are cornual process projecting posterolaterally; inner cornual process slightly arched outward to form an apparent rostral angle, but the extending posteriorly, angle between the cornual and inner processes about 90 degrees (sickle-like); inner cornual process lobe-shaped in Table 1. Measurements of the holotype of Falxcornus liui gen. et sp. nov. (mm). outline that looks like a lanceolate leaf, and obviously larger than the cornual processes; margin of the headshield smooth; median dorsal Item Value opening slit-like (length/width > 6) with paired nearly parallel lateral Maximum length of the headshield 40.7 Maximum width of the headshield 60.0 margins, and its posterior end almost level with the centra of the Length of the headshield in midline 30.4 orbital openings; orbital opening large, round, and dorsally posi­ Maximum diameter of the orbital opening 5.8 tioned; pineal opening small, round, and located approximately level Distance between the orbital openings 18.6 with the posterior margins of the orbital openings; sensory canal Long axis of the median dorsal opening 10.4 system with developed median dorsal canal and three pairs of lateral Short axis of the median dorsal opening 1.3 Long axis of the pineal fossa 0.8 transverse canals; ornamentation composed of tiny granular tubercles, Short axis of the pineal fossa 0.7 about 15 tubercles per square millimetre. Length of the pre-pineal region 19.8 Remarks Falxcornus gen. nov. is a medium-sized tridensaspid fish Length of the post-pineal region 10.6 with a suite of diagnostic characters of the Eugaleaspiformes, including Distance between paired inner cornual processes 23.6 4 X. MENG AND Z. GAI rostral process is absent. The lateral margins of the headshield are preserved, located behind the median dorsal opening at the midline, smooth without serration. and roughly level with the posterior margins of the orbital open­ The headshield protrudes latero-posteriorly into a pair of corn­ ings. The long axis of the pineal fossa is about 0.8 mm, and the short ual processes and posteriorly into a pair of inner cornual processes. axis is about 0.7 mm. The length of the pre-pineal region is about The cornual processes (c, Figure 2 (b, e), 3 (a, b)), the left of which is 19.8 mm, whereas the length of the post-pineal region is about not preserved in the holotype, gradually taper off from the broad 10.6 mm (Table 1). base into a pointed end. The paired inner cornual processes (ic, The sensory canal system of Falxcornus liui gen. et. sp. nov. is of Figure 2 (b, d, e), 3 (a, b)) are completely preserved in the holotype. the typical eugaleaspid type, which consists of posterior supraorbi­ They are lobe-shaped with an outline that resembles a lanceolate tal canals (soc2, Figure 2 (b, e), 3 (a)), infraorbital canals (ifc, leaf and are larger than the cornual processes as they obviously Figure 2 (b, e), 3 (a)), lateral dorsal canals (ldc, Figure 2 (e), 3 overstep the posterior margin of the cornual processes. The dis­ (a)), lateral transverse canals (ltc, Figure 2 (b, e), 3 (a)), median tance between the paired inner cornual processes is 23.6 mm dorsal canals (mdc, Figure 2 (e), 3 (a)), and a dorsal commissure (Table 1). The angle between the cornual and inner processes is (dcm, Figure 2 (b, e), 3 (a)). Anterior supraorbital canals (soc1) about 90 degrees (sickle-like), much larger than those of might have existed; however, they do not appear to be preserved in Yunnanogaleaspis and Nochelaspis. the holotype. The posterior supraorbital canal (soc2, Figure 2 (b, e), The median dorsal opening (md.o, Figure 2 (a, b, e), 3 (a, b)) is 3 (a)) is partly preserved; its starting end is unclear, and it extends slit-like lengthwise in shape with nearly parallel lateral margins and posteriorly to connect with the median dorsal canals (mdc, Figure 2 subacute ends. The long axis of the median dorsal opening in the (e), 3 (a)). holotype is about 10.4 mm, whereas the short axis is about 1.3 mm The median dorsal canal (mdc, Figure 2 (e), 3 (a)) is U-shaped, (Table 1). The long axis/short axis ratio of the median dorsal open­ has a seamless connection with the posterior supraorbital canal ing is about 8. The anterior end of the median dorsal opening is anteriorly (soc2, Figure 2 (b, e)), and joins with the dorsal commis­ separated from the rostral margin of the headshield, and the dis­ sure posteriorly (dcm, Figure 2 (b, e), 3 (a)). The infraorbital canal tance between them is about 5.0 mm, which is clearly differentfrom (ifc, Figure 2 (b, e), 3 (a)) is located on the lateral side of the orbital the terminal position of Shuyuidae. The posterior end of the med­ opening, and then extends posteriorly as the lateral dorsal canal ian dorsal opening is roughly level with the centres of the orbital (ldc, Figure 2 (e), 3 (a)). In the holotype, three pairs of lateral openings. transverse canals (ltc, Figure 2 (b, e)) are observed in each side of The orbital openings (orb, Figure 2 (a, b, e), 3 (a, b)) are round in the headshield, and the third pair of the lateral transverse canal (ltc3, outline and large, with a diameter of about 5.8 mm (Table 1), and Figure 2 (b, e)) is the longest one, almost extending to the end of the are positioned dorsally on the headshield. In the holotype, the cornual process. The fourth lateral transverse canal is absent. distance between the two orbital openings is about 18.6 mm The ventral side of the headshield shows a large heart-shaped (Table 1). The pineal fossa (pi, Figure 2 (b, c, e), 3 (a, b)) is well oralobranchial chamber (obr.c, 2(b, e), 3(b)), which is encompassed

Figure 2. Photograph (a-d) and interpretive drawing (e) of Falxcornus liui gen. et. sp. nov., a nearly complete headshield, holotype, IVPP V26670.1. (a), Internal mould, in dorsal view; (b), external mould, in ventral view; (c), close-up of pineal opening (square box region of Figure 2(b)); (d), close-up of the inner cornual process and granular tubercles on it (rectangular box region of Figure 2(b)); (e), interpretive drawing of Figure 2(b). Artwork credit: Xiaocong Guo. HISTORICAL BIOLOGY 5

Figure 3. Restoration of Falxcornus liui gen. et. sp. nov. (a), in dorsal view; (b), in ventral view. Artwork credit: Xiaocong Guo.

by a pair of crescent-shaped ventral rims (vr, Figure 2(b, e), 3(b)). state where the projection direction of the cornual processes transi­ An extensive postbranchial bar (pb.b, Figure 2(b, e), 3(b)) covers tions from posterolateral to lateral. After re-examining all preserved the abdominal division of the headshield ventrally, and an equally cornual and inner cornual processes in all known galeaspids, we extensive vertical endoskeletal postbranchial wall (pb.w, Figure 2(b, found that the complex of cornual and inner cornual processes can e), 3(b)) closes the oralobranchial chamber posteriorly. The post­ be divided into three different types: (i) The cornual process pro­ branchial wall is penetrated by a large pore in the midline of the jects posterolaterally, the inner cornual process projects posteriorly, headshield, probably for the common passage of the dorsal aorta and the angle between them is much less than 90 degrees, as in the and oesophagus (da+oesc, Figure 2(b, e), 3(b)). The endoskeletal genera Sinogaleaspis, Yunnanogaleaspis, Nochelaspis and Sanqiaspis roof of the oralobranchial chamber was poorly preserved in the (Figure 4 (a), (b), (c), (g)). This type resembles a fork or hook and holotype. Thus, the number of branchial fossae remains unclear, can be defined as a fork-like or hook-like type. (ii) The cornual but six pairs of branchial fossae (br.f, Figure 3(b)) are known in all process projects posterolaterally or laterally and the inner cornual members of the Eugaleaspiformes (Zhu et al. 2015). process projects posteriorly, but the angle between them is close to The entire headshield of Falxcornus liui gen. et. sp. nov. is or more than 90 degrees, as in the genera Falxcornus, Pterogonaspis ornamented with dense, tiny granular tubercles (Figure 2(d)), and and Sanchaspis (Figure 4(e), (h), (f)). This type resembles a sickle there are about 15 tubercles per square millimetre. and can be defined as a sickle-like type. (iii) The cornual process projects posterolaterally or laterally, but the inner cornual process is absent, as in the genera Eugaleaspis, Huananaspis, Stephaspis and Phylogenetic analysis and results Macrothyraspis (Figure 4(i)). In some genera, such as To explore the phylogenetic position of Falxcornus liui gen. et. sp. Zhaotongaspis, Nanpanaspis and Jiaoyu (Figure 4(d)), the cornual nov. within Galeaspida, we conducted a new phylogenetic analysis processes are connected with the lateral margins of the headshield. based on the known dataset (Gai et al. 2005, 2018; Zhu and Gai It is still unclear whether they have inner processes. Therefore, this 2006; Shan et al. 2020; Jiang et al. 2021). In addition to Falxcornus, character [62] has been coded as ‘?’ in these taxa in our data matrix. the recently described genus Jiangxialepis (Liu et al. 2021) is incor­ [63] The inner cornual process exceeds the posterior edge of the porated as well. Jiangxialepis exhibits similar characters to other cornual process in the galeaspids with triangular headshields: (0) members of the Shuyuidae, such as a triangular headshield, slender absent; (1) present. longitudinal oval-shaped median dorsal opening, terminal anterior Among the Eugaleaspiformes, the inner cornual processes of end of the median dorsal opening, splayed posterior supraorbital Nochelaspis (Figure 4(c)), Falxcornus (Figure 4(e)) and canals, and three pairs of lateral transverse canals branching from Pterogonaspis (Figure 4(h)) are all leaf-like and so large that they the infraorbital canals. However, Jiangxialepis shows some differ­ exceed the posterior edge of the cornual process. The inner cornual ences from the genera Shuyu and Meishanaspis in the presence of processes are spine-shaped in the Shuyuidae and Sinogaleaspidae a median dorsal spine, an unclosed rostral margin disrupted by the (Figure 4(a)) and much smaller than the cornual processes, which anterior end of the median dorsal opening, and seven lateral trans­ do not exceed the posterior edge of the cornual process. Although verse canals on each side. the inner cornual processes of Yunnanogaleaspis (Figure 4(b)) are According to the new observations and definition, two new leaf-like, like those of Nochelaspis (Figure 4(c)), Falxcornus characters, [62] and [63], were incorporated into our dataset. (Figure 4(e)) and Pterogonaspis (Figure 4(h)), they do not extend [62] Sickle-like complex of cornual and inner cornual processes beyond the posterior edge of the cornual processes. In contrast, the (the angle of the cornual and inner cornual processes is equal to or inner cornual processes are totally lost in the Eugaleaspidae. more than 90 degrees): (0) absent; (1) present. Phylogenetic data entry and formatting were performed using Although the cornual processes of Falxcornus project poster­ Mesquite version 3.6 (Maddison and Maddison 2015) (S1_File). olaterally, rather than completely laterally as those of the The phylogenetic analysis was conducted using TNT and PAUP Tridensaspidae and Huananaspidae, the angle between the cornual 4.0 with the parsimony analysis package and the heuristic search and inner processes is about 90 degrees, a condition that is clearly option (1000 replicates, random addition sequence) (Swofford different from the non-tridensaspid Eugaleaspiformes, but very 2003; Goloboff and Catalano 2016). All characters were unordered similar to those of Tridensaspis, Pterogonaspis, and the sanchaspid and weighted equally, as in the earlier versions of this dataset. An Sanchaspis. This condition probably represents an intermediate early plesiomorphic osteostracan, Ateleaspis, was selected as the 6 X. MENG AND Z. GAI

Figure 4. The three different types of the complex of cornual and inner cornual processes in galeaspids. (a), Sinogaleaspis shankouensis; (b), Yunnanogaleaspis major; (c), Nochelaspis maeandrine; (d), Jiaoyu imperfectus; (e), Falxcornus liui; (f), Sanchaspis magalarotrata; (g), Sanqiaspis rostrata; (h), Pterogonaspis yuhaii; (i) Macrothyraspis longilanceus; (a-c, g) are fork-like or hook-like; (e-f, h) are sickle-like.

outgroup for our phylogenetic analysis because Osteostraci is Eugaleaspiformes was established by Liu (1965), and consists of a sister taxon to the galeaspids, and Ateleaspis is considered to be the families Shuyuidae, Sinogaleaspidae, Tridenaspidae and the ancestral taxon of osteostracans (Sansom 2009). This phyloge­ Eugaleaspidae, in addition to the genera Yunnanogaleaspis and netic analysis yielded 10 equally most-parsimonious trees (Figure5 Nochelaspis (incertae familiae) (Shan et al. 2020). Falxcornus bears with a tree length = 183, consistency index (CI) = 0.44, and reten­ a striking resemblance to Yunnanogaleaspis and Nochelaspis in the tion index (RI) = 0.77. general shape of the headshield, leaf-like inner cornual processes, and typical eugaleaspid-type sensory canal system, and a closer resemblance to Nochelaspis than Yunnanogaleaspis in its larger Discussion and conclusion inner cornual processes overstepping the posterior limit of the cornual process. However, it clearly differs from Yunnanogaleaspis Falxcornus can be referred to the Eugaleaspiformes because it and Nochelaspis in its sickle-like complex of cornual and inner exhibits a suite of diagnostic characters of the Eugaleaspiformes, cornual processes, more posteriorly located median dorsal opening, including a subtriangular headshield with cornual and inner corn­ and tiny granular tubercles. In addition, the pre-pineal region is ual processes, a longitudinal oval median dorsal opening, and longer than the post-pineal region in Falxcornus, but shorter in a typical eugaleaspid-type sensory canal system. The HISTORICAL BIOLOGY 7

Figure 5. Strict consensus tree of the 10 most parsimonious trees and cladistically based classification of the Galeaspida. Tree length = 183, consistency index (CI) = 0.4372, retention index (RI) = 0.7736. Numbers on branches denote bootstrap frequencies (above node) and Bremer support values (below node); bootstrap frequencies below 50 are not shown; Numbers on Nodes denote character (above node) and character state (below node).

Nochelaspis. Falxcornus more closely resembles the two tridensas­ Polybrachiaspidiformes, which consists of 11 polybrachiaspid pid members Tridensaspis and Pterogonaspis in its sickle-like com­ species, known as the Polybranchiaspis–Laxaspis assemblage plex of cornual and inner cornual processes, leaf-like inner cornual (Assemblage I) (Zhu et al. 2000) or Xishancun Assemblage process overstepping the posterior limit of the cornual process, the (Zhao and Zhu 2010). During this interval, galeaspids experi­ posterior end of the median dorsal opening level with the centre of enced a second rapid radiation with the modification of the the orbital opening, and tiny granular tubercles, although it clearly median dorsal ridge or spine. For example, except for the differs from these genera in lacking the rostral process and laterally typical one as in Polybranchiaspis, Laxaspis and Damaspis, projecting cornual processes. Our phylogenetic analysis indicates Siyingia altuspinosa and Hyperaspis acclivis each had a high that the genera Falxcornus, Tridensaspis and Pterogonaspis form upright and compressed spine, and S. perlatuspinosa had a monophyletic group, the family Tridensaspidae, and Falxcornus a broad spine, whereas Altigibbaspis huiqingae had a blade-like is resolved as sister to all other members of the Tridensaspidae median dorsal ridge (Liu et al. 2018). The occurrences of the (Figure 5). The new taxon was collected from the lower earliest tridensaspid, Falxcornus, and huananaspid, Stephaspis, Lochkovian, lower than the other members of the Tridensaspidae; in the Xishancun Formation indicate that the Eugaleaspiformes therefore, Falxcornus is now the most primitive and earliest known and Huananaspiformes began to differentiate in the early tridensaspid fish. The subacute rostral margin with a rostral angle Lochkovian (Figure 6)(Gai and Zhu 2007), but they did not and sickle-like complex of cornual and inner cornual processes in undergo adaptive radiation until the Pragian (Early Devonian). Falxcornus likely represent the precursors of the rostral process and In contrast, the diversity of the Polybranchiaspiformes suddenly laterally projecting cornual processes. declined in the Pragian, except that two unusually large poly­ The other members of the Tridensaspidae, Tridensaspis and branchiaspiforms survived: Dongfangaspis major from the Pterogonaspis, come from the Pragian Yukiang Formation and Pingyipu Formation of Sichuan and Nanningaspis zengi from Xujiachong Formation, whereas Falxcornus comes from the ear­ the Nakaolin Formation of Guangxi (Liu 1975; Gai et al. 2018). lier Lochkovian Xishancun Formation. The Xishancun During this time, galeaspids experienced a third and final rapid Formation is characterised by the radiation of the radiation with the modification of the rostral process and 8 X. MENG AND Z. GAI

peak of their diversity during the Pragian (Gai et al. 2018) (Figure 6).

Acknowledgments

The authors are grateful to Dr. Humberto G. Ferròn for constructive discussion on his computational fluid dynamics analysis. We would like to thank Professor Yuhai Liu for providing the specimen of Falxcornus, and Xiaocong Guo for drawing the illustrations and life restoration.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Funding

This work was supported by the National Nature Science Foundation of China [41972006, 42072026], the Key Research Program of Frontier Sciences, CAS, [QYZDB-SSWDQC040], the National Program for Support of Topnotch Young Figure 6. Life restoration of Falxcornus liui gen. et. sp. nov. Artwork credit: Professionals [W02070206], and the Strategic Priority Research Program of the Xiaocong Guo. The following information is supplied regarding the registration of CAS [XDB26000000]. a newly described genus and species: Publication LSID:urn:lsid:zoobank.org:pub: E0EC6328- EAB2-43BD-AC06-D09346A4638C Falxcornus gen. nov. LSID: urn:lsid: zoobank.org:act:2AEE917D-79D5-4E3A-8796-A2DBAF9CC6BA Falxcornus liui gen. et sp. nov. LSID: urn:lsid:zoobank.org:act:D8DE8F20-59DF-4968-B2FF- References 6112939AE9E1 Afanassieva OB. 1992. Some peculiarities of osteostracan ecology. In: Mark- Kurik E, editor. Fossil Fishes as Living Animals. Tallinn: Academy of Sciences of Estonia; p. 61–70. 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Agnatha. In: Harland WB, eds. The Fossil Record. London: [30] Median transverse canals (mtc): (0) two; (1) one; (2) more than two. The Geological Society of London; p. 629–636. [31] Short branches running from posterior supraorbital canal (soc1): (0) Voichyshyn V. 2006. New osteostracans from the Lower Devonian terrigenous absent; (1) present. deposits of Podolia. Ukraine. Acta Palaeontologica Polonica. 51(1):131–142. [32] Lateral transverse canal: (0) short; (1) long. Wang JQ. 1984. Geological and paleogeographical distribution of Devonian [33] Branching end of lateral transverse canal: (0) absent; (1) present. fishes in China. Vertebrata Palasiatica. 22(3):219–229. [34] Lateral transverse canals leaving from infraorbital canal: (0) present; Wang N, Wang JQ. 1982. A new Agnatha and its sensory systematic variation. (1) absent. Vertebrata Palasiatica. 20(4):276–281. [35] The lateral transverse canals leaving from infraorbital canal before Xue JZ, Huang P, Wang DM. 2018. Silurian-Devonian terrestrial revolution in ltca: (0) present; (1) absent. South China: taxonomy, diversity, and character evolution of vascular plants [36] Fourth lateral transverse canal (ltc4): (0) present; (1) absent. in a paleogeographically isolated, low-latitude region. Earth-Science Reviews. [37] Lateral transverse canals behind ltc4: (0) present; (1) absent. 180:92–125. [38] Anterior supraorbital canal (soc1): (0) absent; (1) present. Zhao WJ, Zhu M. 2010. Siluro-Devonian vertebrate biostratigraphy and biogeo­ [39] Posterior supraorbital canal (soc2): (0) present; (1) absent. graphy of China. Palaeoworld. 19(1–2):4–26. doi:10.1016/j. [40] Posterior supraorbital canals (soc2) meet with infraorbital canal (ifc): palwor.2009.11.007. (0) no; (1) yes. Zhu M. 1992. Two new eugaleaspids. With a Discussion on Eugaleaspid [41] Posterior supraorbital canals (soc2): (0) funnel-shaped; (1) parallel; Phylogeny. Vertebrata Palasiatica. 30(3):169–184. (2) V-shaped. 10 X. MENG AND Z. GAI

[42] Branching end of posterior supraorbital canals (soc2): (0) absent; (1) [54] Broad ventral rim: (0) absent; (1) present. From Gai et al. (2018) present. [55] The unclosed rostral margin: (0) absent; (1) present. [43] Medial dorsal canal (mdc): (0) present; (1) degenerated; (2) absent. [56] The ornamentation of the head-shield: (0) star- or snowflake-like [44] Medial dorsal canal and posterior supraorbital canal: (0) unjointed; tubercles; (1) tiny, dense granular tubercles; (2) coarse, sparse granular (1) contact. tubercles; (3) polygonal tubercles; (4) large tubercle surrounded by signifi­ [45] U-shaped medial dorsal canal: (0) absent; (1) present. cantly smaller tubercles. [46] Portion of headshield behind dorsal commissure proportionally: (0) [57] Pineal organ: (0) on front of or level with posterior margin of orbital long; (1) short. opening; (1) behind posterior margin of orbital opening. [47] Postbranchial wall: (0) short; (1) long. [58] Pre-pineal region longer than the post-pineal region in mid-line of [48] Elongated branchial region: (0) absent; (1) present. head-shield: (0) absent; (1) present. [49] Number of branchial fossae: (0) 5~7 pairs; (1) 9~17 pairs; (2) more [59] The preorbital commissure: (0) absent; (1) present. than 20 pairs. [60] Ventrally curved branchial fossae:(0) absent; (1) present. [50] Maximum width of headshield placed: (0) posteriorly; (1) medially. [61] Ventral rim of headshield: (0) absent; (1) present. [51] Width/length in oval-like headshield: (0) < 1; (1) >1. [62] Sickle-like complex of cornual and inner cornual process (the angle of [52] Nearly parallel lateral margins of headshield: (0) absent; (1) present. cornual and inner cornual process equal to or more than 90 degrees): (0) [53] Broad and large middle dorsal spine of headshield: (0) absent; (1) absent; (1) present. present. [63] The inner cornual process exceeds the posterior edge of the cornual [1]- [53] from Gai et al. 2006 process in the galeaspids with triangular headshield: (0) absent; (1) present.