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89. Agnostodd Trilobites in a Devonian Formation in West Malaysia

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89. Agnostodd Trilobites in a Devonian Formation in West Malaysia 396 Proc. Japan Acad., 47 (1971) [Vol. 47, 89. Agnostodd Trilobites in a Devonian Formation in West Malaysia By Teiichi KoBAYASHI M. J. A., and Takashi HAMADA (Comm. April 12, 1971) The order Agnostida which consists exclusively of isopygous and miomeric trilobites is one of four orders which existed already in the early Cambrian period. Because the Redlichiida, Corynexochida and Ptychopariida are three polymeric orders, Kobayashi (1962) has pre- viously accepted two subclasses Polymera and Miomera. The propriety of this bipartation simply by means of the number of thoracic seg- ments, however, became a question because Nanshanaspis Chang and Fan, 1960 is a raphiophorid with a trisegmented thorax (Lu et al., 1965) . Nevertheless the Agnostida constitute a solid order which is readily divisible into more and less specialized suborders, Agnostina and Eodiscina respectively. As shown in his critical review (1962), the latter can now be classified into two superfamilies and four fami- lies, while the classifications of the former proposed by Kobayashi, 1939, Hupe, 1953, Howell, 1959 and Pokrovskaya, 1960, still greatly disagree with one another. It has long been well known, however, that the ranges of the Eodiscina and Agnostina are Lower to Middle Cambrian and Lower Cambrian to Upper Ordovician respectively. Therefore Reed (1915) considered that his Agnostus cf. glabratus i.e. Trinodus reedi Kobayashi, 1939 which was labeled Silurian "Upper Namshim beds, Northern Shan States, Burma" was perhaps derived from the Ordovician Naungkangyi beds. Likewise, Kobayashi has primarily suggested an Ordovician age for a Trinodus-like cephalon collected by Burton and his assistants from silty mudstone near Kroh, Perak, Northwest Malay. It was, however, accompanied by proetoids resembling Upper Devonian-Lower Carboniferous cyrtosymbolids and tentaculites which were close to Lower Devonian Nowakia acuaria according to Boucek (Jones et al., 1966). Because the Kroh fauna is such a heterochronous admixture, Burton (1967) and Jones (1968) elucidated the Ordovician trilobites as remarries f ossiles redeposited with tentaculites in early Devonian time. Because neither the trilobites nor their mother rocks do reveal such reworking of an Ordovician formation, the authors carried out an intensive study on the collection amplified by geologists of the No. 4] Agnostoid Trilobites in Devonian Formation in Malaysia 397 Geological Survey of Malaysia and Drs. Tamura and Igo. At length it was determined that the fauna in question must be Emsio- Couvinian or Coblenzian-Eifelian in age because Plagiolaria poothaii is contained in it as in the tentaculites shale in Peninsular Thailand (Kobayashi and Hamada, 1968) . (See Fig. 1) . This chronology was further confirmed with Orbiculoidea tarda, Plectodonta paci fica, P. burtoni and other brachiopods (Hamada, 1969) . Now it is found that the agnostoids represent a new group of trilobites most probably belonging to the Agnostida. Here Pseudotrinodus aenigma is pro- posed for them because it is isolated from the known groups in the possession of marginal, intramarginal and median sutures and three or more segments in thorax, in spite of conspicuous resemblance with Trinodus. Trinodus and Sphaeragnostus are two agnostidian genera sur- vived until the late Ordovician period. Setting aside the possible Silurian relic species in Burma, Pseudotrinodus is quite isolated from Ashgillian Trinodus for 50 million years or more. Its sporadical recurrence is really so astonishing that it would be unique in the trilobite history, but it is no surprise if one considers that a few relic genera of the Monoplacophora are surviving in the eastern Pacific deep sea, notwithstanding the fact that no fossil record is known in the prolonged geological age since late Devonian (Knight and Yochelson, 1960). As pointed out by the authors (Jones et al., 1966), the habitat of the Kroh fauna was a quiet muddy bottom so deep that it was almost aphotic as indicated by the degeneration of visual organs in new proetoids, blind dalmanitids, harpids and other trilobites all of which will be described in a paper in Geology and Palaeontology of Southeast Asia, Volume 10, in preparation. There were very weak bottom currents by which dismembered carapaces of these trilobites were slightly displaced. They are however, often not overturned and so close-set that they can be easily recognized to be parts of an in- dividual. It is not rare to see that thoracic segments are still anchylosed with a cephalon or a pygidium. Their outlines are scarcely damaged, although they suffered from secondary deformation. The stagnant bottom water would have been especially unfavourable for sessil benthos, in view of the fact that most brachiopods are diminitive or dwarfed, although an ambocoeliid of large size is contained in the Kroh collection. Pseudotrinodus is, therefore, a survivor in such a shelter in deep sea of the Burmese-Malayan geosyncline (Kobayashi, 1964). Here the authors record their sincere thanks to Dr. C. M. Burton, Dr. C. R. Jones and other geologists of the Geological Survey of 398 T. KOBAYASHI and T. HAMADA [Vol. 47, Fig. 1. Plagiolaria poothaii Kobayashi and Hamada. X2.0 Figs. 2-6. Pseudo- trinodus aenigma Kobayashi and Hamada, nov. Figs. 2-3. Cephalon, photograph and restoration. x8.8 Figs. 4-5. Cephalic doublure, three tho- racic segments and pygidium; photograph and restoration. X 10.6 Fig. 6. Idealized ventral view of cephalon showing median and marginal sutures (thick line) and intramarginal suture on dorsal side (broken line). No. 4] Agnostoid Trilobites in Devonian Formation in Malaysia 399 Malaysia, Dr. M. Tamura of the Kumamoto University and Dr. H. Igo of the Tokyo University of Education who collected the fossils. Genus Pseudotrinodus, new genus Dorsal shield similar to Trinodus but having no less than three thoracic segments and marginal cephalic sutures which become intra- marginal laterally and united into a median suture ventrally ; intra- marginal suture long and arcuate. On account of the trisegmented thorax it is more allied to certain eodiscidian genera than the Agnostina. The course of the intra- marginal suture is quite different from that of Pagetia and other sutures in the Eodiscina (Kobayashi, 1944) . Pseudotrinodus aenigma, new gen, and sp. (Figs. 2-6) 1966. Trinodus (?) sp. Kobayashi and Hamada in Jones et al. Geol. and Pal. SE Asia, Vol. 2, p. 313. (Preliminary identification cited.) Cephalon nearly as long as wide, broadest at about one-fourth the length from posterior margin, gently convex ; lateral margins more convergent in anterior than the other and meeting at an obtuse angle in front, while they are abruptly incurved at genal angles and continue to straight posterior margin ; posterior border pointed just inside of rounded genal angle. Glabella outlined by weak dorsal fur- rows, a little shorter than two-thirds the cephalon, subcylindrical, slightly tapering backward and abruptly rounded in front where it is subtruncated, occupying less than one-third of the cephalic breadth, expanding backward gradually, but expansion is somewhat empha- sized at basal lobes which are isolated by diagonal furrows; neither transverse furrow nor median tubercle present on glabella ; occipital furrow comparatively deep ; occipital ring narrow, thickened medially, bent antero-laterally near basal lobes of glabella. Cheeks confluent with each other in front ; preglabellar area large ; axial furrow ab- sent ; posterior border furrow weak ; posterior border narrow, as wide as lateral border; genal spine absent; cephalic sutures marginal anteriorly, but laterally intramarginal, taking an arcuate course near marginal border; median suture divides doublure. The holotype cephalon measures 5.1 mm in length and 5.2 mm in breadth ; its glabella 3.2 mm long and 2 mm wide at neck ring. A pygidium beneath a cephalic doublure on a slab looks similar to the cephalon in outline, but more or less subquadrate and moder- ately inflated ; marginal borders narrow and depressed ; axial lobe outlined by axial furrows elevated above pleural lobes, short cylin- drical, nearly one-third as wide and two-thirds as long as pleura ; anterior two axial rings clearly marked by ring furrows. Behind the 400 T. KOBAYASHI and T. HAMADA [Vol. 47, cephalon are three anchylosed thoracic segments which are similar to one another ; their axial ring apparently broader than pleurae. This paratype enables one to figure a nearly complete shield of this species. References Burton, C. K. (1967) : Trans. Proc. Pal. Soc. Japan, NS, No. 65, 27-46. Hamada, T. (1969) : Geology and Palaeontology of Southeast Asia, 7, 1-13, p1.1. University of Tokyo Press. Howell, B. F. (1959) : in R. C. Moore's Treatise on Invertebrate Paleontology, Part 0. Arthropoda, 1, 172-190. Geol. Soc. Am. and Univ. Kansas. Hupe, P. (1953, 55) : Ann, de Pal., tom. 39 and 41, 325 pp. Jones, C. R. (1968) : Am. Assoc. Petrol. Geol., Bull., 52, no. 7, 1259-1278. Jones, C. R., Gobbett, D. J., and Kobayashi, T. (1966) : Geol. Pal. of SE Asia, 2, 309-359, map. Knight, J. B., and Yochelson, E. L. (1960) : Monoplacophora in R. C. Moore's Treatise on Invert. Paleont, Part I. Mollusca, 1, 177-184. Kobayashi, T. (1939) : Jour. Fac. Sci., Imp. Univ. Tokyo, sec. 2, 5, part 5, 69- 198, 1 pl., 1 table. -- (1944) : ibid. , sec. 2, 7, part 1, 1-74, 4 tables, 2 pls. (1962) : ibid., sec. 2, 14, part 1, 1-152, 8 pls. (1964) : XXII Intern. Geol. Congress, India (1964), Proc. of Section 11, 123-131. Kobayashi, T., and Hamada, T. (1968) : Geol. Pal, of SE Asia, 4, 22-28 , 2 pls. Lu Yenhao, Chang Wutung, Chu Chaoling, Chien Yiyuan , and Hsiang Leewen (1965) : Trilobites of China, 2 vols., 766 pp., 135 pls. Each Group of Fossils in China. Science Publisher, Peking. Pokrovskaya, N. V. (1960) : Order Miomera in P . A. Orlov's Osnoby Paleon- tologii, 8, 5461. Reed, F. R. C. (1915) : Palaeontol. Indica, NS, 6, Mem. no. 2, 122 pp., 12 pls..
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