Meemannia Eos, a Basal Sarcopterygian Fish from the Lower Devonian of China – Expanded Description and Significance
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Morphology, Phylogeny and Paleobiogeography of Fossil Fishes D. K. Elliott, J. G. Maisey, X. Yu & D. Miao (eds.): pp. 199-214, 8 figs. © 2010 by Verlag Dr. Friedrich Pfeil, München, Germany – ISBN 978-3-89937-122-2 Meemannia eos, a basal sarcopterygian fish from the Lower Devonian of China – expanded description and significance Min Zhu, Wei Wang and Xiaobo Yu Abstract Additional morphological and histological features of a stem-group sarcopterygian fish Meemannia eos (Zhu et al. 2006) are provided, including dermal bone features, endocranial structures in the oto-occipital region, and details of the superimposed enamel + odontode layers bearing on the stepwise origin of cosmine in crown- group sarcopterygians. A lower jaw characterized by six infradentary foramina, a relatively straight dentary profile, and absence of parasymphysial tooth whorls is tentatively assigned to Meemannia. The distribution and phylogenetic significance of the lateral cranial canal, the endolymphatic duct of supraotic cavity, the horizontally-positioned coronoid-supporting face of the Meckelian bone, and the pore-canal network in dermal bone surface (integration of pore-canal network with multi-layered odontodes) are discussed in the context of the sequential acquisition of characters leading from stem-group osteichthyans to basal sarcopterygians. The histological condition in Meemannia indicates that stem-group sarcopterygians share two important histologi- cal features with stem-group osteichthyans and basal actinopterygians, i. e., the ability of an earlier generation of odontodes to induce the formation of future odontodes, and the absence of resorption. The multi-layered odontodes coexisting with the pore-canal network bring cosmine into alignment with surface covering in stem osteichthyans and actinopterygians. Introduction Recent studies on basal actinopterygians (e. g., Dialipina and Ligulalepis ) (Basden & Young 2001, Basden et al. 2000, Schultze & Cumbaa 2001) and basal sarcopterygians (e. g., Psarolepis, Achoania, and Styloichthys) (Zhu & Yu 2002; Zhu et al. 1999, 2001), have greatly improved our understanding of the origin and early diversification of osteichthyans. The earlier report of Meemannia eos, combining an actinopterygian-like skull roof and a cosmine-like dermal surface, outlined a possible morphotype for the common ancestor of actinopterygians and sarcopterygians, and highlighted its phylogenetic position as the most basal sarcopterygian fish so far known (Zhu et al. 2006). Due to space limitation, the earlier report focused on major features such as the actinopterygian-like dermal skull roof and the unique histological condition of the dermal covering. The present paper provides additional morphological and histological information such as endocranial features in the oto-occipital region and details of the surface covering, and describes a lower jaw specimen tentatively assigned to Meemannia. The paper discusses the implications of the unique histological condition in Meemannia and other features in the context of the stepwise acquisition of characters underlying the diversification of early sarcopterygians from their actinopterygian relatives. Material and faunal assemblage. The Meemannia material described here is housed in the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP), Beijing. The specimens were collected from the middle part of the Xitun Formation (late Lochkovian, Early Devonian) at a locality close to Xitun village in the suburb of Qujing, Yunnan, southwestern China (Fig. 1). The material occurs in association with galeaspids Microholonaspis microthyris, Hyperaspis acclivis (Pan 1992), antiarchs Yunnanolepis chii, Y. parvus, Y. porifera, Phymolepis cuifengshanensis, P. guoruii, Chuchinolepis gracilis, C. qujingensis, C. sulcata, C. robusta, 199 D2 0 500 km 1000 m 0 m shale conglomerate mudstone muddy limestone siltstone limestone sandstone dolomite Fig. 1. Locality map and stratigraphic position of the fossil bed yielding Meemannia eos. Zhanjilepis aspratilis (Zhu 1996), arthrodire Szelepis yunnanensis (Liu 1979), and sarcopterygians Youngolepis praecursor, Diabolepis speratus, Psarolepis romeri, Achoania jarviki and Styloichthys changae (Chang & Yu 1981, 1984; Yu 1998; Zhu & Yu 2002; Zhu et al. 2001). The vertebrate microremains from the Xitun Formation include thelodonts Parathelodus scitulus, P. asiatica, P. catalatus, P. trilobatus, P. cornuformis, acanthodians Nostolepis sp., Youngacanthus gracilis, and chondrichthyans Gualepis elegans, Changolepis tricuspidus, Peilepis solida, and Ohiolepis? xitunensis (Wang 1984, 1997). From the Xitun Formation of neighboring localities were described other galeaspids including Polybranchiaspis liaojiaoshanensis, Nanpanaspis microculus, Laxaspis qujingensis, ‘L ’. rostrata, Cyclodiscaspis ctenus (Liu 1965, 1975) and Siyingia altuspinosa (Wang & Wang 1982). All these findings, together with Meemannia as the most basal sarcopterygian, suggest that during the beginning of the Devonian, early vertebrates were highly diversified in the restricted coastal embayment along the coast of the Upper Yangtze Paleocontinent in South China (Wang 1991). Description Skull roof General features. As briefly mentioned in the earlier report (Zhu et al. 2006), the three skull roof speci- mens only preserved the portion behind the rostral region (Figs. 2, 3B), suggesting a loose connection between the unpreserved anterior rostral unit (rostral and nasal bones) and the dermal bones behind it. In this feature, Meemannia resembles Dialipina (Schultze 1992) and many placoderms (Goujet 1984, 2001), suggesting that the ‘loose nose’ condition may be characteristic of ancestral osteichthyans. The 200 pi Pros p.soc n.orb p.soc n.orb ptoc pl.m pl.m n.spir St pl.p AB Fig. 2. Meemannia eos Zhu et al. 2006, reproduced at higher magnification for clarity in morphological details. Dorsal view of skull roof. A, holotype, IVPP V14536.1; B, IVPP V14536.2. Scale bars = 5 mm. Abbreviations: n.orb, orbital notch; n.spir, spiracular notch; pi, pineal opening; pl.m, pl.p, middle and posterior pit-lines; Pros, postrostral; ptoc, postorbital corner; soc, supraorbital canal; St, supratemporal. post-rostral unit of the skull roof is long and narrow, with the orbital notch (n.orb, Fig. 2A,B) relatively large and anteriorly positioned. Behind the postorbital corners (ptoc, Fig. 2A), the lateral margins run in parallel to the level of the spiracular notches (n.spir, Fig. 2A). The skull roof reaches its maximum width in the otico-occipital region, and presents an embayed posterior margin. No dermal intracranial joint exists between the parietal (P, Fig. 3A) and postparietal (Pp, Fig. 3A) shields which can barely be distinguished by vague indications of incomplete sutures. Dermal bone pattern. Meemannia has a large shield-shaped postrostral bone, whose sutures with neigh- boring bones are clearly detectable. The postrostral bone bears a round pineal opening (Fig. 2A) close to its posterior margin. The pineal opening is present in V14536.1 (Fig. 2A) and V14536.3 (Fig. 3B), but absent in V14536.2 (Fig. 2B), suggesting the variable distribution of this feature. Two series of dermal bones can be identified on the skull roof, the parietal-postparietal in the middle and the dermosphenotic-supratemporal along the lateral margin (Fig. 3A). The parietal is very long and narrow, with its length nearly 3.8 times its width. Anteriorly, the parietals are separated by the postros- tral (Figs. 2, 3A,B). The postparietal is also rectangular, about half as long as the parietal. The parietal is flanked by an elongate longitudinal bone which is referred to the dermosphenotic (Dsp, Fig. 3A). Along the lateral margin of the postparietal is a single bone which is attributed to the supratemporal (St, Figs. 2B, 3A). Anteriorly the supratemporal is narrow and constitutes the upper margin of the spiracular opening. Behind the spiracular opening, the supratemporal is postero-laterally expanded, with its posterior margin forming the lateral portion of the embayed posterior margin of skull roof. Sensory canals. On the skull roof, two pairs of sensory canals and two pairs of pit-lines are detectable. The supraorbital canal (soc, Fig. 3A) extends posteromedially and traverses the anterior portion of the parietal without contacting the otic portion of main lateral line canal (lc, Fig. 3A). After passing through the center of the supratemporal, the main lateral line canal proceeds anteriorly along the lateral margin of the elongated dermosphenotic and exits at a point close to the posterior corner of the shallow orbital margin (Figs. 2, 3A). The trajectory of these two canals can be confirmed by the thin sections from V14534.3 (Fig. 3). The middle (horizontal) pit-line (pl.m, Figs. 2, 3A) and curved posterior (oblique) pit-line (pl.p, Figs. 2, 3A) are situated close to the median line of the skull roof, as in Dialipina. In other basal sarcopterygians such as Psarolepis and Achoania, these two pit-lines have more lateral positions. 201 Pros n.orb pi S14 n.orb S15 soc S12 S13 p.soc ptoc S10 S11 S8 S9 P S6 S7 Dsp S4 n.spir S5 pl.m S2 S3 Pp lc pl.p S1 St 5 mm B A soc S15 e lc S12 S9 lc d S6 lc b c f lc S4 a 1 mm S1 lc C g Fig. 3. Meemannia eos Zhu et al. 2006. A, reconstruction of skull roof; B, dorsal view of skull roof, IVPP V14534.3, a speci- men sectioned at positions marked S1-S15; C, sketch drawings of six selected thin sections (S1, S4, S6, S9, S12, S15) of V14534.3. Histological details in boxed areas a-f are shown in Figure 6A-F;