78 



M.K. HOWARTH 



Genus BAETICOCERAS GeyssmU 1979 

 Type species. Baeticoceras baeticum Geyssant, 1979. 



Baeticoceras morrisi sp. nov. 



PI. 15, figs 3, 5 



Etymology. After Dr Noel Morris who accompanied the author 

 to Yemen and collected many ammonites. 



HOLOTYPE. CA765 from the microbialite boulders, Arus Member, 

 west side of Wadi Arus. 



Paratype. CA766 from the same horizon and locality as the 

 holotype. 



Diagnosis. Characterized by rounded-quadrate whorls of ap- 

 proximately equal whorl height and breadth, and a broad, slightly 

 arched venter. Ribs are single up to 75 mm diameter, fine on inner 

 whorls, rapidly becoming more widely spaced from 40 mm diameter. 

 Ventro-lateral tubercles become obliquely clavate at larger sizes, 

 and there are 3-5 narrow constrictions per whorl. 



Description. The holotype is complete and moderately well- 

 preserved up to its maximum size of 74 mm diameter. The 

 paratype is a quarter-whorl fragment at about 64 mm diameter, 

 with part of its next inner whorl attached, and is similarly pre- 

 served. Traces of suture-lines can be seen in a few places on the 

 inner whorls of the holotype, but neither suture-lines nor septa can 

 be seen on the outer whorl, or on the paratype. Measurements of 

 the holotype are: at 60.5 mm diameter: 16.5 (0.27), 16.6 (0.27), 

 31.3 (0.52). The paratype has a whorl height of 15.6 mm and 

 whorl breadth of 16.4 mm, at a diameter of approximately 61 mm. 

 The whorls are very evolute, and the whorl section is rounded- 

 quadrate, with a broad, but slightly arched venter. All the ribs are 

 single, and are fine, straight, sharp and slightly prorsiradiate up to 

 40 mm diameter, then become progressively more widely spaced 

 at larger sizes. The holotype has 36 ribs per whorl at 35 mm 

 diameter, 32 ribs at 54 mm, and 26 ribs at 65 mm. Prominent 

 ventro-lateral tubercles appear on each rib from 35-40 mm 

 diameter, and from 60 mm diameter they become slightly clavate 

 in an oblique direction. There are 3-5 narrow contrictions per 

 whorl between prominent gently curved ribs in front and behind. 



Remarks. Baeticoceras is the youngest genus of the Simo- 

 ceratidae. It is confined to the Microcanthum Zone, and the earliest 

 species, B. principale Geyssant, was probably derived from the last 

 species of Simocems. S. volanense (Oppel), at the top of the Ponti 

 Zone, LowerTithonian. Five species of Baeticoceras were described 

 by Geyssant (1979) from the Microcanthum Zone of the Betic 

 Cordillera in SE Spain, and there are undescribed records from 

 Algeria, Sicily and northern Italy (Geyssant, 1979: 34). Baeticoceras 

 has the ribs, ventro-lateral tubercles and constrictions of the family 

 Simoceratidae, and is characterized by the progressively earlier 

 development of primary ribs looped in pairs to ventro-lateral tuber- 

 cles. In the oldest species, B. principale, looped ribs do not appear 

 until the final whorls from a diameter of about 98 mm, and the inner 

 whorls retain the dense, sharp, single ribs of the ancestral Simoceras. 

 The new Yemeni species differs in having thicker whorls, where the 

 whorl height and breadth are approximately equal, compared to the 

 much more compressed whorls of B. principale, and the ribs on the 

 inner whorls are denser and sharper than in the latter species. The 

 two specimens found so far are smaller than the size at which looped 

 ribs commence in B. principale. It appears to be morphologically 

 more primitive than the latter species. Its inner whorls are finely 

 ribbed and lack umbilical tubercles, and are reminiscent of the inner 

 whorls of Simoceras volanense (eg. in specimens figured by 



Santantonio, 1986: 19, fig. 4, pi. 3, fig. 2). As apossible intermediate 

 between S. volanense and Baeticeras principale. it is good evidence 

 for the basal Microcanthum Zone age of the microbialite boulders in 

 the Arus Formation. Some species of Tithopeltoceras Arkell, 1953 

 (?Himalayifidae), which occurs mainly in the middle part of the 

 Microcanthum Zone, are similar to Baeticocas. The nearest is 

 Tithopeltoceras primus Oloriz &Tavera (1979: 141, pi. 2, fig. 2), but 

 it and all other species of Tithopeltoceras have more depressed 

 whorls (WhAVb always <1, usually 0.8-0.5) and larger ventro- 

 lateral tubercles. 



Occurrence. Microbialite boulders, Arus Member, Hajar For- 

 mation, Wadi Arus; basal Microcanthum Zone, Upper Tithonian. 



Genus VIRGATOSIMOCERAS Spath. 1925 



Type species. Simoceras rothpletzi Schneid. 1915. 



Remarks. Two examples of Virgatosimoceras have been found in 

 the Hajar Formation in Wadi Arus. One is described below as V. 

 broilii. The other is a poorly preserved external mould of an ammo- 

 nite (SM F12160, Beydoun's field no. ZB703) found in Wadi Arus 

 loose in the scree below the Mintaq Member. It is preserved on a 

 piece of heavy grey limestone typical of that member, but its exact 

 horizon is not known. It shows remains of evolute whorls up to about 

 1 30 mm diameter, and has straight primary ribs that are very widely 

 spaced from the smallest size visible of about 20 mm diameter. In 

 this respect its ribbing is very similar to that of the type species, V. 

 rothpletzi, as figured by Schneid (1915: pi. 7, fig. 3). The outer part 

 of the whorl and the venter are not seen, however, and the ammonite, 

 though almost certainly a Virgatosimoceras, is not determinable 

 beyond Virgatosimoceras sp. indet. 



Virgatosimoceras broilii (Schneid, 1915) PI. 16, fig. 2 



1915 Simoceras broilii Schneid: 90, pi. 6, fig. 4: pi. 7, fig. 1. 



Material. CA767 from the microbialite boulders, Arus Member, 

 in the west cliff in Wadi Arus. 



Description. This is a fairly well-preserved specimen, 83 mm 

 diameter maximum size; it has evolute whorls, a quadrilateral 

 whorl section, nearly flat whorl sides and an arched venter; no 

 clear ventral groove can be seen, but it is not well preserved at mid 

 venter. Fine, single, straight prorsiradiate ribs on inner whorls, 

 quickly become widely spaced between 50-65 mm diameter; some 

 ribs divide into 2 or 3 secondaries from 65 mm diameter, and at 

 larger sizes the venter appears to become smooth. No suture-lines 

 are visible. 



Remarks. In whorl proportions and type and density of ribs this 

 ammonite compares well with Schneid's (1915: pi. 7, fig. 1) type 

 specimen, Virgatosimoceras rothpletzi (Schneid, 1915: 88, pi. 4, fig. 

 1; pi. 7, figs 2, 3) is much more coarsely ribbed from 30 mm 

 diameter, and has a ventral interruption of the ribs up to 60 mm 

 diameter, which is lost at larger sizes, while S. broilii has 

 Perisphinctid-like ribbing to a much larger size. Jeletzky ( 1 989; 157) 

 recorded V. broilii in the Lower Tithonian up to the top of the 

 Semiforme Zone, but in the microbialite boulders of the Arus 

 Member the Yemeni ammonite has to be basal Microcanthum Zone, 

 Upper Tithonian, in age. 



Occurrence. Microbialite boulders, Arus Member, Hajar For- 

 mation, Wadi Arus; basal Microcanthum Zone, Upper Tithonian. 



