70 



M.K. HOWARTH 



1984 Aspidoceras (Physodoceras) circumspinosum (Oppel); 

 Verma & Westermann: 71, pi. 17, fig. 3. 



1985 Schaireriaavellana(Z\n.t\)\C\\&cdi: 197, pi. 40, figs 1, 2: pi. 

 41, fig. 1. 



Material. 18 specimens: CA 1204-05 from the upper marly part, 

 and 16 (including CA1206-19) from the lower marly part of the 

 Kilya Member in Wadi Kilya. 



Description. The 16 examples from the lower marly part of the 

 Kilya Member are from a shell bed; all are crushed flat laterally, and 

 they are up to 88 mm diameter. The two from the upper marly part 

 are better preserved and are not crushed; one (pi. 10, fig. 4) is 

 probably septate up to 115 mm diameter, then has a quarter of a 

 whorl of body-chamber up to its aperture at 1 3 1 mm diameter; the 

 other is 59 mm diameter, and apparently is an immature, being 

 septate up to 55 mm diameter, then has a short length of body- 

 chamber. The whorl section is nearly circular, being only slightly 

 depressed, and the umbilical edge is rounded except where accentu- 

 ated by the tubercles. The inner whorls bear large spiny umbilical 

 tubercles directed inwards over the umbilicus, and there are occa- 

 sional lateral tubercles in some specimens. The density of the 

 umbilical tubercles is 10-12 per whorl at 30-50 mm diameter 

 decreasing to 8-1 at 60-80 mm diameter, while the lateral tubercles 

 are very irregular at only 2^ per whorl. In the largest ammonite all 

 tubercles disappear by 80 mm diameter and the specimen becomes 

 smooth. There are no ribs. 



Measurements 



D Wh 



CA1024 126.0 56.0(0.45) 



CA1024 107.5 44.2(0.21) 



CA1025 58.7 26.3 (0.45) 



Remarks. This is a species of Orthaspidoceras that occurs in the 

 Beckeri and Hybonotum Zones in several areas of east Africa. Verma 

 & Westermann ( 1984) described 1 specimens from the Hybonotum 

 Zone of Mombasa, and Scott (1943) had three specimens from 

 Ethiopia that probably belong to this species. The smoothly rounded, 

 moderately involute whorls, in which whorl height and thickness are 

 almost equal, and the reduction or disappearance of the umbilical 

 tubercles at larger sizes are distinctive. Of the similar Aspidoceratids 

 in the same horizons in Yemen, O. gortanii is more evolute and has 

 large umbilical tubercles on the body-chamber, Aspidoceras 

 longispinum is bituberculate throughout growth and has more mas- 

 sive, more depressed whorls, and A. apenninicum is much more 

 evolute. 



Occurrence. Upper and lower marly parts of the Kilya Member, 

 Naifa Formation, Wadi Kilya; Beckeri Zone, Upper Kimmeridgian, 

 and Hybonotum Zone, Lower Tithonian. 



Aspidoceras or Orthaspidoceras spp. indet. 



Material. Nine specimens: CA840-42 from the shell bed 26 m 

 above the base of the Arus Member in eastern Jebel Billum; one 

 large specimen (CA1200) from the upper part of the middle lime- 

 stone part, and three small specimens (CA 120 1-03) from the base of 

 the same part of the Kilya Member, in Wadi Kilya; one large 







1905 







1943 



Wb 



U 





59.5 (0.47) 



36.3(0.29) 



1959 



46.5 (0.43) 



25.7(0.24) 





28.3 (0.48) 



15.9(0.27) 



1985 



fragment (C A 1044) from the base of the middle limestone part of the 

 Kilya Member in Naifa Cliff; and one very large (300-350 mm 

 diameter) ammonite was photographed 16 m below the top of the 

 Mintaq Member, in the road gorge leading eastwards out of the 

 southern end ofWadi Arus. All are Aspidoceras or Orthaspidoceras, 

 but are too fragmentary to be identified. 



Genus SIMASPIDOCERAS Spath, 1925 



Type species. Aspidoceras argobbae Dacque, 1905. 



Remarks. Simaspidoceras is a genus of Aspidoceratinae that has 

 massive whorls and radial ribs, and is characterisfic of the Ethiopia 

 - Somalia- south Yemen area. Early records by Dacque (1905) and 

 Scott ( 1 943 ) were poorly dated, and Venzo ( 1 959 ) did not have good 

 evidence of dating. Zeiss (1971: 537, 540) placed the genus in the 

 Lower Kimmeridgian in Ethiopia, but the associated ammonites are 

 not strongly indicative of such a low horizon. Other ammonites 

 accompanying the present specimens provide good evidence that the 

 age of Simaspidoceras in southern Yemen is Beckeri Zone, Upper 

 Kimmeridgian and Hybonotum Zone, Lower Tithonian. 



Simaspidoceras argobbae (Dacque, 1905) 



PI. 11, fig. 3; PI. 12, fig. 3; PI. 13, fig. 1; 

 PI. 14, fig. 1;P1. 15, fig. 6 



Aspidoceras argobbae Dacque: 151, pi. 18 (5), fig. 1. 



Simaspidoceras ganamense Scott: 86, fig. 22; pi. 20, figs 1 , 



2.5. 



Simaspidoceras argobbae (Dacque); Venzo: 175, pi. 11, 



fig. 4; pi. 12, figs la, lb; pi. 13, figs la-lc. 



Simaspidoceras argobbae (Dacque); Checa: 177. 



Material. 14 specimens: eight, CA737^4, from the Breadloaf 

 Concretions in the lower marly part of the Kilya Member in the east 

 cliff of Wadi Arus; SM E12209 from the base of the middle 

 limestone part of the Kilya Member, and four, CA 1045^8, from the 

 lower marly part of that member, all in Naifa Cliff; and CA 1 220 

 from the upper marly part of the Kilya Member in Wadi Kilya. 



Description. Three of the specimens from Wadi Arus have in- 

 complete body-chambers of 200-220 mm diameter, and the best 

 preserved of them (PI. 13, fig. 1) has a complete phragmocone 

 ending half a whorl before the aperture; the other five consist of an 

 incomplete fragment 70 mm diameter, and four well-preserved 

 immatures, all with short portions of body-chambers ending at 20- 

 32 mm diameter. SM El 2209 from Naifa Cliff is septate up to 190 

 mm diameter, then has a quarter of a whorl of body-chamber up to its 

 aperture at 215 mm diameter. The four specimens from the slightly 

 lower horizon in Naifa Cliff (see Howarth & Morris, 1998: fig. 3) are 

 larger, but partly crushed. They have apertures in the range 270-300 

 mm diameter, one of which appears to contract and might be the 

 adult aperture (PI. 15, fig. 6). The best preserved of them has two- 

 thirds of a whorl of body-chamber, and has an especially 

 well-preserved inner whorl, now separated (PI. 14, figs Ic, Id). The 

 specimen from Wadi Kilya is a poorly preserved fragment of a body- 

 chamber of about 1 80 mm diameter. 



PLATE 13 



Fig. 1 Simaspidoceras argobbae (Dacque), Breadloaf Concretions. Kilya Member (fauna 7), east cliff, Wadi Arus. la, lb, CA737, xO.65. 

 Fig. 2 Aulacosphinctes natricoides (Uhlig), microbialite boulders, Arus Member (fauna 9), west cliff, Wadi Arus. 2a, 2b, CA769. 

 Fig. 3 Aulacosphinctes spitiensis (Uhlig), microbialite boulders, Arus Member (fauna 9), east cliff. Wadi Arus. 3a, 3a, CA768. 



Fig. 4 Simaspidoceras irregulare (Dacque), 16.5 m above base of lower marly part of Kilya Member (fauna 7), Naifa Cliff. 4a, 4b, CA1049, the outer 

 whorl is body-chamber, xO.57. 



