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Redescription of Yinostius Major (Arthrodira: Heterostiidae) from the Lower Devonian of China, and the Interrelationships of Brachythoraci

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Redescription of Yinostius Major (Arthrodira: Heterostiidae) from the Lower Devonian of China, and the Interrelationships of Brachythoraci bs_bs_banner Zoological Journal of the Linnean Society, 2015. With 10 figures Redescription of Yinostius major (Arthrodira: Heterostiidae) from the Lower Devonian of China, and the interrelationships of Brachythoraci YOU-AN ZHU1,2, MIN ZHU1* and JUN-QING WANG1 1Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing 100044, China 2University of Chinese Academy of Sciences, Beijing 100049, China Received 29 December 2014; revised 21 August 2015; accepted for publication 23 August 2015 Yinosteus major is a heterostiid arthrodire (Placodermi) from the Lower Devonian Jiucheng Formation of Yunnan Province, south-western China. A detailed redescription of this taxon reveals the morphology of neurocranium and visceral side of skull roof. Yinosteus major shows typical heterostiid characters such as anterodorsally positioned small orbits and rod-like anterior lateral plates. Its neurocranium resembles those of advanced eubrachythoracids rather than basal brachythoracids, and provides new morphological aspects in heterostiids. Phylogenetic analysis based on parsimony was conducted using a revised and expanded data matrix. The analysis yields a novel sce- nario on the brachythoracid interrelationships, which assigns Heterostiidae (including Heterostius ingens and Yinosteus major) as the sister group of Dunkleosteus amblyodoratus. The resulting phylogenetic scenario suggests that eubrachythoracids underwent a rapid diversification during the Emsian, representing the placoderm response to the Devonian Nekton Revolution. The instability of the relationships between major eubrachythoracid clades might have a connection to their longer ghost lineages than previous scenarios have implied. © 2015 The Linnean Society of London, Zoological Journal of the Linnean Society, 2015 doi: 10.1111/zoj.12356 ADDITIONAL KEYWORDS: Brachythoraci – Heterostiidae – morphology – phylogeny – Placodermi. INTRODUCTION misidentified and given names such as Ichthyosauroides Kutorga, 1835, Asterolepis asmussi Agassiz, 1844–45 The Arthrodira Woodward, 1891 is the largest and most and Chelonichthys asmusii Agassiz, 1844–45. Asmuss diverse group within the Placodermi McCoy, 1848 (Carr, (1856) for the first time referred these Baltic speci- 1995; Young, 2010). The morphological disparity of mens to fish correctly, and named them Heterostius and arthrodires is perhaps best illustrated by the Homostius, respectively, under several specific names. Heterostiidae Jaekel, 1903, an enigmatic arthrodire Shortly thereafter, Pander (1857) gave a more de- taxon known for its large body size and extremely ex- tailed description of the Baltic species of Heterostius tended anterior lateral plates. The type genus of the including a reconstruction of part of the head and tho- family, Heterostius Asmuss, 1856, was among the first racic armour. group of early vertebrates to be described scientifical- Woodward (1891) redefined Chelonichthys asmusii ly. The highly visible, large bony plates from the soft as Heterosteus asmussi. Subsequently, the two names matrix found in the Baltic states had long been noted ‘Heterostius’ and ‘Heterosteus’ seemed to be used inter- by early palaeontologists. However, they were often changeably. To give an incomplete list of examples, the name ‘Heterosteus’ was applied in Heintz (1928), Denison (1978, 1984) and Janvier (1996), while ‘Heterostius’ was *Corresponding author. E-mail: [email protected] applied in Heintz (1930), Gross (1933), Stensiö (1963) © 2015 The Linnean Society of London, Zoological Journal of the Linnean Society, 2015 1 2 Y.-A. ZHU ET AL. and Ørvig (1969). E. Mark-Kurik (pers. commu.) stated chapter on pachyosteomorphs. However, he stated that that the suffix ‘-ostius’ is derived from the adjective Heterostius ‘may turn out to be a coccosteomorph’, based latinized Greek word ‘ostinos’ (bony, having promi- on its elongated occipital region shared with Homostius, nent bones), while the suffix ‘-osteus’ is derived from which he assigned into Coccosteomorphi Stensiö 1944. the substantive ‘osteon’ (bone). Accordingly, the suffix Denison (1978) considered Heterostiidae as the sister ‘-ostius’ should not be corrected as ‘-osteus’ by reason group of all brachythoracids, although he acknowl- of an ‘inadvertent error’ (International Commission on edged that the thoracic armour of Heterostiidae ‘re- Zoological Nomenclature (ICZN), 1999: Art. 32.5.1). We sembles that of Pachyosteina’. He later moved the taxon accept this explanation and only use the names to a higher phylogenetic position by including it in ‘Heterostius’ and ‘Homostius’ in the following text. Brachythoraci Gross, 1932 (Denison, 1984), partly fol- Heintz (1928, 1930) described the Baltic specimens lowing Young (1981), who provided a revised cladogram of Heterostius in detail. Gross (1933) described a showing heterostiids among basal brachythoracids. heterostiid anterior lateral plate from Sötenich, Janvier (1996) described Heterostiidae as a Germany, and assigned it to Heterostius. Ørvig (1969) eubrachythoracid taxon, but beyond that is ‘of uncer- erected the second heterostiid genus, Herasmius, from tain phylogenetic position’. the Wood Bay Group (upper Lower Devonian or lower In the first published attempt to include any Middle Devonian) in Spitsbergen. The only species of heterostiid in a computerized analysis of brachythoracid the genus, Herasmius granulatus, is represented by phylogeny (Zhu & Zhu, 2013), Heterostius ingens is as- an incomplete skull roof. Herasmius granulatus Ørvig, signed to a much more crownward position. It is nested 1969 differs from Heterostius ingens Asmuss, 1856 in within Dunkleosteidae Stensiö, 1963, being the sister having a shorter and broader skull roof, broader and group of Dunkleosteus Lehman, 1956. However, the mor- longer central plates, thinner dermal bones, and much phological evidence that supports this scenario is mostly finer tubercular ornaments. The unidentified plate confined to the external aspects of the skull roof. (Ørvig, 1969: fig. 4C) should be the extended anteri- Here we provide a detailed redescription of the or lateral plate, which is integrated but not fused to holotype of Yinostius major, which yields new the anterior dorsolateral plate, and possibly belongs phylogenetically significant information, especially that to Herasmius granulatus.InHeterostius ingens the two of the neurocranium morphology. We then conduct an plates are fused. updated phylogenetic analysis, which for the first time Wang & Wang (1984) reported the third heterostiid includes basal brachythoracids in the ingroup. Also, genus Yinosteus from the South China block, which new characters are defined based on the comparative represents the first heterostiid outside Euramerica and anatomy of arthrodires, with particular emphasis on illustrates the wide distribution of the group. The fusion the visceral surface of the skull roof and neurocranium of the dermal plates and relatively extensive ossifica- morphology. With a revised data matrix, we hope to tion compared with most other brachythoracids suggest render a more convincing scenario that could lead to that the holotype of Yinosteus major Wang & Wang, a better understanding of not only the position of 1984 belongs to an adult individual. Yinosteus differs heterostiids, but also the brachythoracid phylogeny as from Heterostius and Herasmius in having a smaller a whole. body size, proportionately broader nuchal plate, larger nuchal gap and coarser tubercular ornaments. Part of GEOLOGICAL BACKGROUND the description in Wang & Wang (1984) needs to be properly illustrated. Also, characters that are The holotype material of Yinosteus major was collect- phylogenetically informative were not fully exploited. ed from black shale near Wuding in 1979 (Fig. 1). The The members of the Heterostiidae show similar- horizon was identified by Wang & Wang (1984) as the ities to the basal brachythoracid Homostiidae Jaekel, Black Shale Member of the Middle Devonian Haikou 1903, such as the large body size, anteriorly placed Formation. Shortly thereafter, Wang (1984) included orbits, elongated occipital region, anteriorly extended the horizon within the newly erected Jiucheng For- anterior lateral plate and reduction of ventral thorac- mation and maintained its Middle Devonian diagno- ic armour. However, heterostiids also resemble the ad- sis, which was supported by Liu (1994). The dating vanced eubrachythoracids in the position of the of the Jiucheng Formation was modified by Wang & posterolateral corner of the skull roof, and the highly Zhu (1995) to be late Emsian, Early Devonian, under developed posterior carinal process of the ventral keel the following arguments: first, the Jiucheng Forma- of the median dorsal plate (Stensiö, 1963; Denison, 1978; tion and the Pojiao Formation below it were continu- Young, 1981; Zhu & Zhu, 2013). ously deposited, and the latter is dated to be early This mosaic character complement of heterostiids has Emsian based on the invertebrate evidence; second, resulted in competing hypotheses of their phylogenetic the fish fauna from the Jiucheng Formation is com- assignment. Stensiö (1963) discussed Heterostius in the parable with European and Australian Emsian faunas. © 2015 The Linnean Society of London, Zoological Journal of the Linnean Society, 2015 EARLY DEVONIAN ARTHRODIRE FROM CHINA 3 Figure 1. Devonian sequence in the Wuding area (Yunnan, China), showing the stratigraphic position of Yinostius
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