Mecaster turonensis ( Fourtau, 1921 )

Mohamed Abdelhamid, Marouf Abdel-Aty & Moustafa Azab, Mahmoud, 2012, Turonian-Santonian echinoids from Egypt, Geodiversitas 34 (3), pp. 575-615 : 604-606

Mecaster turonensis ( Fourtau, 1921) ( Figs 15 I-K; 17B, C)

Hemiaster heberti mutatio turonensis Fourtau, 1921: 89 , pl. 11, figs 1-10.

Hemiaster heberti turonensis – Abdelhamid & El Qot 2001: 26, fig. 7G, H.

Hemiaster (Mecaster) heberti Coquand turonensis – El Qot 2006: 153, pl. 34, fig. 6a, b.

Mecaster turonensis – El Qot 2010: 282, pl. VI, figs 4a, b, 5, text-fig. 5f.

MATERIAL EXAMINED. — 130 specimens from the Turonian of Wadi Dakhl (Wata Formation), ASUDLE53-182, Wadi Dakhl echinoid horizon 1 ( DEH 1) and eight from the same stage of Wadi Abu Qaada, ASUAQE149-156, Wadi Abu Qaada echinoid horizon 1 ( QEH 1).

MEASUREMENTS. — See Table 13.

DESCRIPTION

Test size variable. Outline polygonal, slightly narrow at posterior. Maximum width lies to the anterior of apical disc. Posterior margin straight with feeble sulcus below periproct.Adoral surface convex, swollen at the plastron. Posterior surface with oblique truncation.Apical disc semi-ethmolytic, large, transverse, centric to slightly eccentric anteriorly.Frontal ambulacrum nonpetaloid, as deep as and slightly narrower than the paired petals, feebly notching the ambitus. Paired ambulacra petaloid, very wide, depressed, and long. Poriferous zones wide, equal. Pore pairs slit-shaped, equal, conjugate. Anterior paired petals long, reaching the ambitus. Posterior paired petals slightly shorter and less divergent than the anterior paired petals. Fascioles are composed of a narrow band of peripetalous parafasciole (sensu Néraudeau et al. 1998), slightly wider at the distal end of the paired petals.

REMARKS

The present species is distinguished from Mecaster heberti ( Coquand, 1862) in having higher test, and wider and shallower paired petals. It is common in the basal part of the Turonian in many localities of Egypt ( Fourtau 1921; El-Sheikh et al. 1998; Abdelhamid& El Qot 2001; Abdel-Gawad et al. 2004; El Qot 2006, 2010). Studying of the ocular and genital plates of the small-sized individuals of the present species revealed that individuals less than 11.3mm in length are juveniles and have no genital pores whereas individuals more than 14.2mm in length are adults and have well developed genital pores. Between these two test sizes, majority of individuals are having slightly distinguished genital pores; however there are many individuals still juveniles and have no genital pores at all; others have only three genital pores (genital pore no. 1 is absent); furthermore, there are individuals having only genital pore no. 4 is open. The juveniles are distinguished from the adults in having shallower ambulacra. The posterior surface is more oblique. The periproct is relatively larger and lies higher than in the adults. The posterior margin is straight in comparison to the concave margin of the adults. The plastronal area is flattened (swollen in the adults). The primary tubercles have a similar distribution as in the adults but they are more sparsely spaced. The two sub-anal sets of tubercles are less developed, widely spaced and slightly distinguished from the surrounding tubercles. The fascioles (parafascioles sensu Néraudeau et al. 1998) are weakly developed especially on the anterior half of test. These morphological variations might help in understanding of the ontogenetic changes of the present species.