Mastixia BLUME.

Tiffney, Bruce H. & Manchester, Steven R., 2022, The Early Middle Eocene Wagon Bed Carpoflora Of Central Wyoming, U. S. A., Fossil Imprint 78 (1), pp. 51-79 : 65

Genus Mastixia BLUME.

Text-figs 6f–r , 7a–e

M a t e r i a l. Four chalcedony casts. USNM PAL

772364, 772362, 772363, 772365.

D e s c r i p t i o n. Endocarp ellipsoidal, longitudinally ribbed, unilocular ( Text-figs 6f–k , 7a–c ), with a u-shaped locule ( Text-fig. 6r ). Endocarp elliptic in lateral view, almost circular in cross-section, averaging 23.5 mm in length (range 20.7 mm to 27.2 mm) and 13.2mm in diameter (range 11.4 mm to 16.1 mm) with a clear dorsal infold in some specimens ( Text-fig. 6f, j, k, l, o, q ). The best-preserved specimen (Textfig. 6f–k) possesses low, discontinuous, rugose, longitudinal ridges about 1 mm in width, approximately 10 present at one end, 14 at the other; the eroded specimens possess suggestions of similar ridges, although poorly preserved at one end. The ends of the intact specimen are marked by small protrusions, one with a small, cap-like structure against which the longitudinal ridges end ( Text-fig. 6j ) and the other formed by the coalescence of the longitudinal ridges ( Text-fig. 6k ). One eroded specimen has only one end preserved, this with a protrusion formed by the coalescence of the ridges while a second exhibits an apical cap against which the ridges abut in a manner similar to the intact specimen.

D i s c u s s i o n. The shape, rugose longitudinal ridges and the single prominent longitudinal groove are all consistent with the endocarps of some species of Mastixia , the groove being interpreted as the infold of the dorsal germination valve. Unfortunately, the preservation of these fossils as casts, lacking any permineralized tissues, precludes further validation. The fossils can be distinguished from Mastixicarpum M.CHANDLER , as they do not exhibit any sign of the persistent epicarp distinctive of that genus. Rather, it appears the epicarp was fleshy and thus lost as in extant Mastixia , exposing the endocarp. One of the specimens exhibits a semi-circular pit excavated on one side strongly suggestive of predation ( Text-fig. 6m, n ), perhaps by a rodent-like animal. A lesser gouge is visible on the surface of another specimen ( Text-fig. 6h ).

The occurrence of Mastixia in the Wagon Bed flora is not startling, as fruits conforming to this genus are known from the latest Paleocene to earliest Eocene Sand Draw flora and late early to early middle Eocene Sepulcher floras of Wyoming ( Tiffney and Haggard 1996), as well as from the middle Eocene Clarno flora of Oregon ( Manchester 1994) and late Eocene Auriferous Gravels of California ( Tiffney and Haggard 1996). The genus is widespread in the European Tertiary ( Mai 1993). Cornaceae was not observed in the pollen flora of the Wagon Bed Formation ( Leopold and MacGinitie 1972), but wood with anatomy consistent with Nyssaceae (subsumed by some authorities along with Mastixiaceae into Cornaceae ) and/or Styracaceae DC. et SPRENG. is reported (J. H. Jones, pers. comm.). In the current day, Mastixia is represented by ca. 20 species of trees and shrubs in Indomalaysia and the Philippines through India ( Matthew 1976), although more recent works have suggested a greater diversity ( Liu and Peng 2009).

Figures

Text-fig. 6. Cornaceae. Alangium (a–e), Mastixia (f–r). a–e: Alangium, DMNH EPI.47806. Scale bar = 1cm. b, e: Reflected light, palladium coated. a, c, d: Micro-CT scan surface rendering. a: Locule cast, face view of slightly larger locule. b: Face view of slightly smaller locule. c: Lateral view of the endocarp, the slightly enlarged left carpel separated from the smaller carpel by a longitudinal septal groove; the faint pitting in the groove suggestive of the septal vasculature. d, e: Views of either end of the endocarp, illustrating the size difference between the two carpels and the pitting in the septal groove suggestive of the septal vasculature. f–k: Mastixia USNM PAL772362. Scale bar = 1cm. f, g, j, k: reflected light,palladium coated; h, i: micro-CT scan surface rendering. f: Lateral view of endocarp, inferred dorsal germination valve groove facing the viewer. Note irregular, rugose, longitudinal ridges. g: Lateral view of endocarp, inferred germination valve with median longitudinal groove to left. h: Lateral view of endocarp reoriented with the same longitudinal groove to the right. i: Lateral view, rotated to ventral surface. j: View of one end of the endocarp, germination valve groove up. k: Opposite end view, with prominent radial ridges and intervening grooves, germination valve groove up. l–r: Mastixia USNM PAL 772363. Scale bar = 1cm. l: View of one face of endocarp, displaying a groove that may represent the surficial expression of the dorsal infold of a Mastixia-like germination valve. Surface badly eroded, reflected light, palladium coated. m: Opposite face of endocarp displaying extensive erosion and a central hole interpreted as feeding damage. n: Lateral view; m, n micro-CT scan surface renderings. o: A view of one end, displaying the prominent groove, reflected light, palladium coated. p: Opposite end to (o). q: View as in (o); p, q micro-CT scan surface renderings. r: Virtual transverse section showing curved locule (arrows).

Text-fig. 7. Cornacaeae (a–j), Icacinaceae (k–o). a–e: Mastixia. USNM PAL 772364. Scale bar = 1cm. a: Lateral view of eroded endocarp – the opposite side being missing and the endocarp broken near its mid point, reflected light, palladium coated. b–e: Micro-CT scan surface renderings. b: Rotated 90° from the view in (a). c: Rotated 90° from the view in (b). d: Rotated 90° from (c), exhibiting the damaged “back” face of the endocarp. e: Axillary view of the endocarp; the opposite end missing as apparent in (d). f–j: Cf. Nyssa. DMNH EPI.47808. Scale bar = 1cm. Micro-CT scan surface renderings. f: Intact face of the endocarp; note ridges and “apical” point. g: Eroded (?gnawed; note horizontal grooving) opposite face of the endocarp. h: Lateral view of the endocarp, eroded/gnawed face to left. i: Apical view, eroded/gnawn portion below. j: Basal view of endocarp. k–o: Iodes DMNH-EPI.47807. Micro-CT scan surface renderings. Scale bar = 1cm. k: Face view of endocarp. l: Opposite face of endocarp. m: Lateral view demonstrating the compressed nature of the endocarp, note thickened suture marking the probable track of the primary bundle. n: Apical view, primary bundle trace to right. o: Basal view, primary bundle trace to right.