Holocene ostracods (Crustacea) from a whale-fall excavation site from the Chao Phraya delta, Central Thailand Author Chitnarin, Anisong 804F4F3D-F368-4EA7-9D7C-7E0530A5FE42 School of Geotechnology, Institute of Engineering, Suranaree University of Technology, Mueang District, Nakhon Ratchasima, 30000, Thailand. anisong@sut.ac.th Author Forel, Marie-Béatrice 063C1F7E-6D26-48F9-A8B1-2AFA496B5FB8 Muséum national d’histoire naturelle, CR 2 P, MNHN-CNRS-SU, 75005 Paris, France. marie-beatrice.forel@mnhn.fr Author Tepnarong, Prachya 3ABB571B-791D-480C-B053-89B59FEB21CA School of Geotechnology, Institute of Engineering, Suranaree University of Technology, Mueang District, Nakhon Ratchasima, 30000, Thailand. prachya@sut-ac.th text European Journal of Taxonomy 2023 2023-01-31 856 1 120 151 http://dx.doi.org/10.5852/ejt.2023.856.2033 journal article 54141 10.5852/ejt.2023.856.2033 2b1ba956-986f-404d-94f6-d933f8393ad4 2118-9773 7616240 A519A06A-6C20-45A7-9120-B0C795E41E75 Keijella multisulcus Whatley & Zhao, 1988 Figs 7J–O , 8A–I , 9 Keijella multisulcus Whatley & Zao, 1988: 15 , fig. d. Dimensions L = 0.325 –0.875 millimeters; H = 0.175 –0.444 millimeters; H/L = 0.45–0.59 ( Fig. 9 ). Distribution Modern distribution: Malacca Strait ( Whatley & Zhao 1988b ); Malaysia ( Omar et al . 2017 ); Mae Khlong River mouth, north west Gulf of Thailand ( Montenegro et al . 2004 ); southwestern coast of Peninsular Thaiand, Ao Nun, Satun Province, Andaman Sea ( Forel 2021 ). Fossil distribution: Bangkok Clay (samples 20SS01B, 02A, 02B, 03A, 04A, 04B), whale excavation site, Samut Sakhon Province , Thailand , Late Holocene (this work, Fig. 4 ). Remark Dimorphism of Keijella multisulcus can be recognized by the subrectangular shape with H/L ratio less than 0.50 in the male ( Fig. 7L ) and the subovate shape with a higher H/L ratio in the female ( Fig. 7M–O ). The juvenile carapaces are sub-triangular in lateral view with Hmax located anteriorly at one third of L, Lmax is located below midH in both valves. The ventral longitudinal carina is prominent in young juveniles ( Fig. 8G–H ) and extends into a small spine (posteroventral spine). The ventral longitudinal carina is faint and becomes obscured in the larger juvenile stage ( Fig. 8A–F ). Small conical posterior marginal denticles and a large terminal posteroventral spine are observed in juveniles. Figure 9 shows H and L plot of the species which clearly demonstrates that sexual dimorphism can be differentiated in adult specimens.