Eomiltha Cossmann 1912
Creators: name: Taylor, John D.
name: Glover, Emily A.
Publisher: name: Zenodo
Year: 2018
Subjects: Biodiversity
Taxonomy
Animalia
Mollusca
Bivalvia
Lucinoida
Lucinidae
Eomiltha

description: Genus Eomiltha Cossmann, 1912
Eomiltha Cossmann in Cossmann & Peyrot, 1912: 269.
TYPE SPECIES. — Lucina contorta Defrance, 1825, Paleocene, Thanetian, Abbecourt, France (original designation).
DIAGNOSIS. — Slender, flat shelled, ovoid, anteriorly rounded, posteriorly truncate with shallow sulcus. Sculpture of low commarginal lamellae, fine radial threads. Two cardinal teeth in each valve, lateral teeth absent. Anterior adductor muscle scar, long, narrow, arcuate with posterior termination beyond mid point of shell. Pallial line with sharp angle posteriorly to adductor scar (Fig. 7E, F). Inner shell margin smooth.
GEOLOGICAL RANGE. — Paleocene (Danian) to?Miocene
INCLUDED SPECIES — Paleocene. Late Danian: Eomiltha alburgensis (Vincent, 1930), Calcaire de Mons, Belgium RBINS I.G. 6496 (Fig. 5A). Thanetian: Eomiltha contorta (Defrance, 1825) (1825: 99-100, pl. 16, figs 1, 2). Abbecourt, Paris Basin (Figs 5 B-G; 7E, F).
Eocene. Ypresian: Eomiltha contortula (Deshayes, 1857) (1857: pl. 40, figs 19-22), Cossmann & Pissarro (1904 -1906: pl. 24, fig. 82-9), Paris Basin.
Eomiltha pandata (Conrad, 1833), Middle Eocene, Claiborne Group, Alabama, USA (Figs 5H, I; 7G). See Bretsky (1976: 290- 291, pl. 33, figs 4-7).
Miocene. Eomiltha scolaroi Vokes, 1969, early-mid Miocene Chipola Formation (Fig. 5J, K), is similar in shape to Eomiltha pandata. Eomiltha xustris (Gardner, 1926), Early Miocene, Alum Bluff, Florida. Although claimed as an Eomiltha species by Vokes (1969b) Phacoides (Miltha) xustris Gardner, 1926 (holotype USNM 352496) has the shape and adductor musculature more similar to Miltha.
REMARKS
Fossils identified as Eomiltha are known from deposits in the eastern USA (Vokes 1969b; Bretsky 1976). These are Eomiltha pandata (Conrad, 1833) from the mid-Eocene, Claiborne Group, Alabama (Figs 5H, I; 7G) and E. scolaroi Vokes, 1969 from the early-mid Miocene Chipola Formation of Florida (Fig. 5J, K). The two species are similar to each other and have a subcircular, discoidal shape rather than the elongate shape of Retrolucina n. gen. or Eomiltha with distinct lines of secondary pallial attachment scars located within the pallial line and extending from the anterior to posterior. Eomiltha pandata has a widely bifid posterior cardinal tooth in the right valve. These north American species probably represent a distinct clade from the European lineages and separable at generic level.
Chavan (1938: 98) suggested placement in Eomiltha of ‘ Miltha’ callipteryx (Tournouer, 1874) figured by Cossmann & Peyrot (1912: pl. 27, figs 18-21) from the Miocene of Aquitaine, France. This has distinctive widely spaced commarginal lamellae and in shape is similar to Eomiltha species but has a much shorter anterior adductor scar and is very similar in shape and sculpture to the living Falsolucinoma leloeuffi Cosel, 2006 from West Africa. The latter species is now placed using molecular data near to Lucina in the Lucininae (our unpublished data). Another putative Eomiltha species that can be discounted is Miltha (Eomiltha) multilamellata (Deshayes, 1830) described and figured by Cossmann & Peyrot (1912: pl. 27, figs 3-7) from the Miocene of Aquitaine, but this has the characters of Armimiltha Olsson & Harbison, 1953) (type species Lucina disciformis Heilprin, 1886 from Plio-Pleistocene of Florida). Phacoides (Miltha) woodi Olsson, 1930 from the Eocene of Peru was referred to Eomiltha by Vokes (1969b: 113) but the syntype (PRI 24173) has characters of Miltha. Moore (1988) refers two species from the Paleocene and Eocene of California to Eomiltha; of these Gibbolucina (Eomiltha) packi (Dickerson 1916) is most likely a Miltha species (see Vokes 1939, 1969a) and G. (Eomiltha) gyrata (Gabb, 1864) is a compressed shell of uncertain placement.
In Eocene faunas there were several lucinid genera similar to Eomiltha in possession of long, narrow anterior adductor muscle scars extending to around the mid-line of the shells. A number of species lacking hinge teeth have been assigned to Pseudomiltha (type species Lucina gigantea Deshayes, 1825) but there is considerable morphological disparity among the species (e.g. P.mutabilis (Lamarck, 1807) suggesting probable different clades. Another form with a long anterior adductor scar is the largest known lucinid, Superlucina megameris (Dall, 1901), described from the Eocene of Jamaica (Taylor & Glover 2009), but previously placed in Eomiltha. Lucinids with very long, thin, anterior adductor muscles are absent from modern faunas except for Retrolucina voorhoevei n. comb.
There is much confusion concerning the relationships of Eomiltha: it has been placed as a subgenus of Gibbolucina (e.g. Chavan 1938, 1969; Kilburn 1974) but there is little similarity of shells. Alternatively, it has been classified as subgenus of Miltha (e.g. Bretsky 1976) but the shells differ in shape and musculature.
description: Published as part of Taylor, John D. & Glover, Emily A., 2018, Hanging on - lucinid bivalve survivors from the Paleocene and Eocene in the western Indian Ocean (Bivalvia: Lucinidae), pp. 123-142 in Zoosystema 40 (7) on pages 130-132, DOI: 10.5252/zoosystema2018v40a7, http://zenodo.org/record/3738245