Trochodendron postnastae, Manchester & Pigg & Devore, 2018

Manchester, Steven R., Pigg, Kathleen B. & Devore, Melanie L., 2018, Trochodendraceous Fruits And Foliage In The Miocene Of Western North America, Fossil Imprint 74 (1 - 2), pp. 45-54 : 50

Trochodendron postnastae sp. nov.

Text-fig. 4 a–f, h–j

H o l o t y p e. UCMP 201235 ( Text-fig. 4a ).

P l a n t F o s s i l N a m e s R e g i s t r y N u m b e r.

PFN000103 (for new species).

E t y m o l o g y. This species is named in reference to the geologically older species, Trochodendron nastae , which it resembles except in having basally acrodromous rather than actinodromous veins.

T y p e l o c a l i t y. Moose Mountain flora near

Cascadia, Oregon, USA.

T y p e h o r i z o n a n d a g e. Little Butte Volcanics

Series, middle Miocene.

D i a g n o s i s. Leaves with stout petioles and elliptical laminae, with length/width ratio 1.9 to 2.7:1. Base of lamina acute to almost right-angle and apex acute. Margin finely serrate in apical 2/3 of lamina, entire with a fimbrial vein in the lower 1/3. Venation pinnate with moderately thick midvein; basally acrodromous with a pair of strongly ascendant basal secondary veins plus 3 to 5 additional pairs of secondaries arising in the upper 2/3 of the lamina. Secondaries arising at narrow angles from midvein, slightly curved, craspedodromous. Occasional agrophic/pectinal veins arising from the basal secondary veins. Intersecondary veins common. Tertiary and quaternary veins forming a quadrangular to polygonal mesh and with areoles having branched freely ending veinlets. Teeth simple, rounded to acute, glandular, principal vein entering upper side of tooth. Sinuses angular.

D i m e n s i o n s. Petioles 1.8–2.3 mm thick, 12 mm to more than 15 mm long. Laminae 7.0– 10.2 cm long, 2.6– 5.3 cm wide, length/width ratio 1.9 to 2.7:1 with acute to almost right-angle base and acute apex. Secondaries arising at angles of 15–25° from the midvein.

R e m a r k s. These leaves resemble those of T. nastae from Republic but this new species has a pinnate acrodromous rather than actinodromous pattern. In other words, this species has only three strong veins radiating from near the base – the midvein and a pair of strong basal secondaries ( Text-fig. 4 a–e), whereas in T. nastae , three to five veins radiate from the base of the lamina giving an actinodromous/palmate aspect ( Pigg et al. 2001: fig. 2A, 2B). Extant Trochodendron aralioides is characterized by rather coriaceous, evergreen leaves of variable morphology with long petioles, pinnate venation and a slightly crenulate margin with glandular teeth ( Text-fig. 4g, k ; Bailey and Nast 1945). Previously described Miocene leaves known from Kamchatka ( T. evenense of Chelebaeva and Chigaeva 1988) are quite similar to extant leaves.

Although we cannot prove it, we deduce that T. postnastae leaves were likely borne on the plant which produced the infructescences of Trochodendron rosayi . This builds on a similar hypothesis that T. nastae leaves were produced by the same trees that yielded Trochodendron drachukii infructescences of the McAbee flora ( Pigg et al. 2007). In both instances the leaves differ from those of extant Trochodendron , which has no tendency for actinodromous primaries or strong basal secondaries, but it is easy to imagine a transformational series from T. nastae (actinodromous) through T. postnastae (pinnate, basally acrodromous), to T. aralioides (simple pinnate). Aside from the difference in pattern of primary and basal secondary veins, the leaves of T. postnastae appear identical in architecture to the other fossil and modern Trochodendron leaves, as do leaves of T. nastae ( Pigg et al. 2001) . As yet, we have not confirmed Trochodendron foliage from the Emerald Creek and Sucker Creek localities. It would be nice to know whether they bore secondary veins in a normal pinnate manner or with an acrodromous or actinodromous pattern.

The observation that some geologically older Trochodendron leaves have more palmate, rather than pinnate venation, is consistent with this being a plesiomorphic feature within the clade that also includes Tetracentron with exclusively actinodromous venation. Tetracentron leaves are readily distinguished, in other characters; the leaves are deciduous and typically with a prominent cordate base and closely and evenly-spaced, distinctive “chloranthoid” teeth (sensu Hickey and Wolfe 1975). This proposed transition from actinodromous to pinnate venation is complicated, however, by the occurrence of both pinnate, and actinodromous trochodendraceous leaves in the Eocene Republic flora.

A single rather poorly preserved Trochodendron like leaf with a midvein and pinnate venation was found in the Moose Mountain assemblage ( Text-fig. 4h ). This is the kind of leaf we expect to associate with Concavistylon because of the physical attachment of such leaves on a twig bearing fruits of this genus from Republic, Washington (Manchester et al. in press; work in progress). This leaf type is also known from McAbee, where it was initially identified as Trochodendron ( Pigg et al. 2007: fig. 7A–F); infructescences of both Trochodendron drachukii and Concavistylon sp. occur at McAbee.

Figures

Text-fig. 4. Leaves of Trochodendraceae. a–f) Trochodendron postnastae from Moose Mountain flora, Oregon, hypothesized to be produced by the same species as fruits of T. rosayi. a) Leaf showing thick peduncle, basally acrodromous venation, holotype UCMP 201235. b) Narrow leaf with intact apex, UF 18110-70160. c) UF 18110-70156. d) UCMP 201237. e) UF 18110-70158. f) UF 18110-70157. g, k) Extant Trochodendron aralioides. h) Possible trochodendraceous leaf with simple pinnate venation, UCMP 201236. i, j) Details of margin and venation of T. postnastae enlarged from (b), (f). Scale bars calibrated in mm.