Observations

-- 379-

OBSERVATIONS to the note by E. Vorobjeva : ~ First discovery of Palaeozoic fishes on the territory of the Mongolian Republic ~.

Palaeontology and Biostratigraphy of Mongolia, Moscow, 3, 134-136 (in Russian).

by EMILIA I. VOROBJEVA * & PHILIPPE JANVIER **

In 1973, a Hungarian geological team discovered a vertebrate dermal bone fragment in an Eifelian limestone series on the northern slope of the Oulan Obo Hill, Barun Urt region, S0chbaatar, Mongolia (fig. 1A). Later, in 1975, a Soviet-Mongolian expedition including one of us (E.V.) worked in the Barun Urt region, and collected more material in the vertebratebearing site of the Oushan Obo - Oulan Obo area, 3235 kms South-Southeast of Barun Urt (fig. IA). In this region, the Devonian vertebrate-bearing layer consits of a 1 m. thick limestone horizon which can be followed over nearly 6 kms, and lies on top of a thick series of yellow calcareous beds containing Middle Devonian (? Eifelian) brachiopods, crinoids and bryozoans. The new material collected in 1975, yet undescribed, consists mainly of large arthrodiran plates (up to 8 m m . thick),acanthodian fin spines, teeth and dermal bones fragments of onychodontids and osteolepids. The specimen under discussion here is the first Devonian fishremain found at that time in Mongolie. It was described by one of us (Vorobjeva, 1976), and doubtfully referred to the lower jaw of an undetermined rhipidistian,yet its ornamentation was said to bear some similaritieswith that of the rhenanid placoderm Ohioaspis.

of the U.S.S.R., n ° 3548/1) leads to the conclusion that it has to be referred to a large brachythoracid arthrodire. Firstly, a thin section through a tubercle (fig. 1C) shows typical unipolar cell cavities for odontocystes (c. sp.), which indicate that it consists of semidentine, a type of hard tissue unique to placoderms. Second, one of the margins of the specimen is slightly thickened and everted (m., fig. 1B and section 2), a condition which is met with along the margin of the non-overlapping plates of large brachythoracid arthrodires. Finally, the external surface of the specimen shows a blunt ridge (v.l.r., fig. 1B) which has the same aspect as the ventrolateral ridge of the posterior ventrolateral plate of mist arthrodires (p.v.i., fig. 1D). This ridge occurs in <~dolichothoracid >~arthrodires and in primitive brachythoracids, but becomes reduced and even disappears in most pachyosteomorph brachythoracids. Consequently, we refer now this specimen to a large primitive brachythoracid arthrodire, possibly a large <~coccosteid ~ more or less similar to Eidenosteus (fig. ID, E ; Miles, 1964), Plourdosteus or Livosteus, from the Middle and Upper Devonian of Europe and North America.

A reexamination of this specimen (fig. IB, C; Palaeontological Institutof the Academy of Sciences

* U.S.S.R. Academy of Sciences, Severtzov Institute of Evolutionary Animal Morphology and Ecology, 33, Leninsky Pst., Moscow V-71. ** LA 12 du CNRS, Institut de Paleontologic, 8 rue de Buffon, 75505 Paris, France.

Geobios, n ° 17, fasc. 3

p. 379-380, 1 fig.

Lyon, juin 1984

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A, Locallon map. B, Bntchythoraci gen. et sp. lndet. (P.I.A.S., Moscow, n o 3548/1), Eifelian, Oushan Obo - Oulan Obo, Eastern Mongolia ; fragment of the lateral margin of a right posterior ventrolateral plate, lateral view (from Vorobjeva, 1976) ; 1,2, transverse sections. C, section of a tubercle of the specimen in B, showing the unipolar cell spaces from Vorobjeva, 1976. D,E, Eldenosteus arizonensls (HUSSAKOF),Middle Devonian, Arizona (after Miles, 1964, modified) ; D, trunk-shield in lateral view, showing the probable position (a-b) of the fragment from Mongolia ; E, isolated posterior ventrolateral plate in ventral view. Scale 1 cm. c.sp., celle space ; m., natural (unbroken) margin ; v.l.r, ventrolateral ridge of the posterior ventrolateral plate. A, Locailntion du gllement. B, Brachythomei 8an. et sp. indet. (I.P.A.S., Moscou, n ° 3548/1), Eif~lien, Oushan Obo - Oulan Obo, Mongolie orientale ; fragment du bord lateral d'une plaque ventro-lat~rale post~rieure droite, rue lat6rale (d'apr~s Vorobjava, 1976) ; 1,2, sections transversales. C, coupe d'un tubercule du Sl~ciment en B, montrant les cavit~s cellulaires unipolalres. D,E Eldenosteus artzonensts (HussAKOF), D~vonien moyen, Arizona (d'apr~s Miles, 1964, modifi~) ; D, cuirasse thoracique en rue lat~rale, montrant l'emplacement probable (a-b) du fragment de Mongolie ; E, plaque ventro-lat~rale postgrieure en rue ventrale. Echelle : 1 cm. c.sp., cavit6 cellulaire ; m,, bord naturel ; v.l.r., cr~te ventro-lat6rale de la plaque ventro-lat6rale post6rieure.

REFERENCES

MILES R.S. (1964) - On some coccosteomorph arthrodires from the Devonian of Arizona. Ark. Zool., Stockholm, 2, 16(22), 42%460.

VOROBJEVA E.I. (1976) - First discovery of Paleozoic fishes on the territory of the Mongolian Republic. In Palaeontology and Biostratigraphy o f Mongolia, Moscow, 3, 134-136. (in Russian).