AMMONITES AND NAUTILOIDS OF WADI HAJAR 



93 



ribs are flexuous, radial, curve forwards slightly at the ventro-lateral 

 angle, and pass radially over the venter without interruption; they 

 bifurcate irregularly at various levels on the side of the whorl, and 

 some bifurcate 2, 3 or 4 times. At 50-75 mm diameter the ribs are 

 reduced to striae, though the umbilical ends remain more prominent 

 and there are occasional irregularly spaced stronger ribs. Most 

 specimens were obtained from the shell bed 3 1 m below the top of 

 the Mintaq Member exposed on the north side of the road gorge 

 leading south-eastwards out of Wadi Arus. The majority are crushed 

 flat, though some relief remains in the umbilical area in a few 

 specimens. The holotype is from near the base of the thin representa- 

 tive of the Mintaq Member in eastern Jebel Billum. and is the only 

 one to retain substantial relief, although it also is partly crushed 

 laterally. The other specimens from eastern Jebel Billum are more 

 fragmentary than the holotype, though they show the fine ribs that 

 are progressively lost beyond 50 mm diameter; they were found 

 loose, having fallen from the Mintaq Member. The Mintaq Salt 

 Dome specimen (PI. 20, fig. 3) consists of inner whorls 32 mm 

 diameter that are similar to the Wadi Arus specimens. Dimorphic: 

 the holotype is a macroconch, and is possibly adult and complete at 

 1 1 8 mm diameter; several others are microconchs that retain ribs to 

 the end of growth, and CA787 has a large lappet at 42 mm diameter. 



Remarks. Other species of Substeueroceras have stronger ribs 

 remaining to larger diameters. Two specimens from Mexico figured 

 by Burckhardt (1912, pi. 41, figs 3, 4) as Substeueroceras sp. indet. 

 are as finely ribbed, but more involute, than the Yemeni species, and 

 a very similar specimen from Somalia was figured by Spath ( 1925; 

 146, pi. 15, fig. 3) as Substeueroceras sp. indet. 



Occurrence. Mintaq Member, Hajar Formation, eastern Jebel 

 Billum, Wadi Arus and the Mintaq Salt Dome; Durangites Zone, 

 Upper Tithoni an, ?and Euxinus Zone, Berriasian (see p. 103). 



Genus RIASANITES Spath, 1923a 



Type species. Ammonites rjasanensis Lahusen, 1883. 



Synonym. Tauricoceras Kvantaliani & Lysenko, 1979 (type spe- 

 cies, T. crassicostatum Kvantaliani & Lysenko, 1979). 



Remarks. Riasanites is a Boreal genus, so the discovery of two 

 examples in the Tethyan Province near the southern border of the 

 Arabian Peninsula is remarkable. Apart from some probable occur- 

 rences in Argentina, these are the first Riasanites found outside the 

 Boreal Province, and their date at the bottom of the Upper Tithonian 

 is more remarkable. The age of Riasanites has been the subject of 

 some debate: after its original description by Nikitin (1888) and 

 Bogoslovsky (1897), it has been generally accepted as a wholly 

 Berriasian, and mainly upper Berriasian, genus, especially by 

 Sazonova (1977: 84) and Kvantaliani & Lysenko (1979). It certainly 

 characterizes a zone (or subzone) in the middle to upper part of the 

 Berriasian in the Volga Basin, the Crimea, the Caucasus and the 



Caspian areas. However, Arkell (1956: 492-93) thought that the 

 lowest occurrences of Riasanites in the Volga Basin might be top 

 Tithonian in age, and in a review of all the occurrences Jeletzky 

 (1984: 236-241) argued that in its type locality (the Central Russian 

 Plain, ie. the Volga Basin) it first occurs at the base of the Riasanian, 

 which is to be correlated with the Upper Tithonian, Transitorius 

 Zone (=Durangites Zone), while the earliest occurrences in Crimea, 

 north Caucasus and Mangyshlak (Caspian) are all younger, in the 

 upper Occitanica and Boissieri Zones, of mid to upper Berriasian 

 age. In these areas, therefore, the total age range of Riasanites might 

 be from the top of the Upper Tithonian, Transitorius Zone, to the 

 upper Berriasian, Boissieri Zone. So the discovery of examples of 

 the type species of Riasanites at the bottom of the Microcanthum 

 Zone (the bottom of the Upper Tithonian) in Yemen is more than one 

 zone older than the oldest of the occurrences in the Boreal Province, 

 and questions the accepted correlation between Boreal and Tethyan 

 Provinces. 



Support for such a date for Riasanites is to be found in the only 

 other Tethyan Province occurrences of the genus: in Argentina, H. 

 Leanza ( 1 98 1 a: 78) referred the 22 specimens ofRiasanites described 

 by Krantz (1928: 25-27, pi. 4, figs 7. 8) to the Koeneni Zone 

 (probably equivalent to the Upper Tithonian Durangites Zone as 

 used here), and A.F. Leanza (1945: 40) described one (unfigured) 

 Riasanites from his Altemans Zone (the lower to middle part of the 

 Upper Tithonian). These records in Yemen and Argentina suggest 

 that Riasanites was present during much of the Upper Tithonian in 

 the Tethyan Province, an horizon that is older than that of its better 

 known occurrences in the Boreal Province in Russia. 



Riasanites rjasanensis (Lahusen, 1883) PI. 21, fig. 6 



1883 Ammonites rjasanensis Lahusen [Wenetzky MS]: 69. 



1 888 Hoplites rjasanensis (Lahusen); Nikitin; 9 1 , 1 88, pi. 1 , figs 



1-3. 

 1897 Hoplites rjasanensis (Lahusen); Bogoslovsky: 83, pi. 5, 



figs 3-5. 

 1977 Riasanites rjasanensis (Wenetzky); Sazonova: 85, pi. 18, 



figs 1-3; pi. 19, figs 1, 2; pi. 20, figs 2, 6; pi. 21, fig. 13. 

 1979 Tauricoceras crassicostatum Kvantaliano & Lysenko: 630, 



pi. Lfigs 1,2. 



Material. Two specimens, CA783-84, from the microbialite 

 boulders near the base of the Arus Member in the cliff on the west 

 side of Wadi Arus. 



Description. The larger specimen (CA783) consists of half a 

 whorl of body-chamber, 122 mm diameter at its larger end. It has no 

 septa and appears to be incomplete at both ends. All the inner whorls 

 are missing except for some small fragments of the venter of the next 

 inner whorl attached to the dorsum of the body-chamber. The whori 

 section is slightly compressed, and has flat whorl sides that converge 

 to a narrow, almost flat venter. The whorl measurements are: at 1 19.5 

 mm diameter: 33.0 (0.28), 26.5 (0.22), 62.0 (0.52). Widely spaced. 



PLATE 21 



Fig. 1 Substeueroceras striatum sp. nov., holotype, 4 m above base of Mintaq Member (fauna 1 1 ), eastern Jebel Billum. CA963 most of outer whorl is 



body-chamber. 

 Fig. 2 Malbosiceras sp. indet., bed 36, Mintaq Member (fauna 13), Mintaq Salt Dome, CA686, xO.69, body-chamber 

 Fig. 3 Cheloniceras (C.) cornuelianus (d'Orbigny), upper half of Qishn Formation (fauna 16), Wadi Masila, 220 km ENE ofMukalla; Upper Aplian. 3a, 



3b, C. 86982, septate up to just before the aperture. 

 Fig. 4 Cheloniceras (C.) sp. indet., upper half of Qishn Formation (fauna 16), Wadi Masila, 220 km ENE of Mukalla; Upper Aptian. 4a, 4b, C.86983, 



possibly a microconch. 

 Fig. 5 Blanfordiceras wallichi (Gray), 4 m above base of Mintaq Member (fauna 1 1 ), eastern Jebel Billum. 5a, 5b, CA969. ?body-chamber 

 Fig. 6 Riasanites rjasanensis (Lahusen), microbialite boulders, Arus Member (fauna 9), west cliff, Wadi Arus. 6a, 6b, CA783, body-chamber 



