Dicranodontium minutum HEDENÄS, BOMFLEUR

Bomfleur, Benjamin, Hedenäs, Lars, Friis, Else Marie, Crane, Peter R., Raunsgaard, Kaj, Pedersen, Mendes, Mário Miguel & Kvaček, Jiří, 2023, Fossil Mosses From The Early Cretaceous Catefica Mesofossil Flora, Portugal - A Window Into The Mesozoic History Of Bryophytes, Fossil Imprint 79 (2), pp. 103-125 : 112

Dicranodontium minutum HEDENÄS, BOMFLEUR et E.M.

FRIIS sp. nov.

Text-fig. 7 a–d

H o l o t y p e. S266382 (Catefica sample 49; figured

Text-fig. 7a, c ).

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.

PFN003149.

P a r a t y p e. S266384 (Catefica sample 49; figured

Text-fig. 7b, d ).

R e p o s i t o r y. Palaeobotanical Collections , Department of Palaeobiology, Swedish Museum of Natural History, Stockholm, Sweden .

E t y m o l o g y. The epithet refers to the small size.

T y p e l o c a l i t y. Catefica (39° 03ʹ 30ʺ N, 09°14ʹ 30ʺ W), between the villages of Catefica and Mugideira, about 4 km south of Torres Vedras, Portugal .

T y p e s t r a t u m a n d a g e. Almargem Formation,

Early Cretaceous (Aptian–early Albian).

S p e c i f i c d i a g n o s i s. A member of the genus

Dicranodontium , but with leaves much smaller than those of any extant species.

D i s t i n g u i s h i n g f e a t u r e s. Leaves of

Dicranodontium minutum are up to 1.9 mm long, much smaller than the leaves of any extant species, which are at least 4 mm long and may be up to 12 mm long. Leaves have a very broad costa that occupies about half the leaf width at the base of the leaf and that fills the acumen almost completely.

Leaves have a well-developed epidermis on both the adaxial and abaxial side, with one layer of large guide cells, and multiple layers of both adaxial and abaxial stereids.

D e s c r i p t i o n a n d C o m m e n t s o n t h e

M a t e r i a l. The two shoot fragments (S266382, S266384)

are up to ca. 2.0 mm long, with long and narrow leaves that are up to 1.9 mm long based on estimates of the length of the broken leaf apices ( Text-fig 7a, b ). Costa very broad, occupying about half the leaf width in the basal leaf portion,

most of the acumen above, and the entire acumen near the leaf apex. The costa has a well-differentiated epidermis on both the abaxial and adaxial sides, with enlarged, sometimes partly collapsed, epidermal cells ( Text-fig. 7c, d ). There is one layer of large guide cells, and multiple layers of both adaxial and abaxial stereids ( Text-fig. 7c, d ). Cells of the upper leaf lamina are elongate or (elongate-) rectangular.

S y s t e m a t i c r e l a t i o n s h i p s. The order

Dicranales comprises about 15 families within which the fossil material most closely resembles the extant genus Dicranodontium , one of twelve genera currently recognized in the Leucobryaceae . The fossil material is so closely similar to living Dicranodontium in leaf shape and costa width and anatomy that it can be readily assigned to the extant genus. The fossil material is smaller in size than any extant species of Dicranodontium , where shoots may be 5–10 cm tall and have leaves up to 4–12 mm long. However, the fossil material is otherwise similar to some extant species in the long and narrow leaves with a broad costa, and in details of costa anatomy (Frahm 1997, Bonfim Santos and Stech 2017). Dicranodontium differs from extant Dicranum HEDW.

species with a broad costa by a distinctly differentiated long subula, and elongate cells of the upper leaf lamina. Among genera of Leucobryaceae the well differentiated epidermis on both the adaxial and abaxial sides of the costa distinguishes the fossils from species of Atractylocarpus MITT. , which have only abaxial stereids (Bonfim Santos and Stech 2017),

and from species of Campylopus BRID. , which have only a single layer of adaxial stereids and also a shorter subula in relation to the basal portion of the leaf.

An important consideration for Dicranodontium minutum is whether the minute size of the fossils is considered conspicuous enough to serve as a species-diagnostic criterion, given that only the apical-most portions of the fossil shoots are preserved and that shrinkage may also have occurred during charcoalification. It is possible that only the apical portions of the gametophytes were sufficiently protected from the fire, perhaps by the surrounding leaves,

and hence became charcoalified rather than completely burnt to ash (cf., Li et al. 2022a). Experimental charcoalification has shown to reduce the size of plant organs between 14 and 47 %, depending on the organ type (Lupia 1995). However, this is much less than the difference between the fossils and their morphologically modern counterparts, and thus seems unlikely to account fully for the size differences. In addition, the single spore capsule recovered and the Physcidium shoot fragments, have sizes that are more in agreement with those of modern mosses.

Figures

Text-fig. 7. Synchrotron radiation X-ray tomographic microscopy (SRXTM) images (volume renderings, a, b, e, f; orthoslices, c, d, g, h) from shoots of Dicranodontium minutum HEDENÄS, BOMFLEUR et E.M.FRIIS (a–d) and Campylopus lusitanicus HEDENÄS, BOMFLEUR et E.M.FRIIS (e–h) from the Early Cretaceous Catefica mesofossil flora, Almargem Formation, Portugal. a, b: Shoot apices with several leaves showing the broad costa comprising almost the full width of the leaf. c, d: Transverse sections (orthoslices xy2925, xy1725) of leaves near the shoot apices showing the broad costa. Note the prominently expanded guide cells with several layers of stereids on both their dorsal and ventral sides, especially distinct in (c). e, f: Shoot apex with several leaves showing the broad costa. g, h: Transverse sections (orthoslices xy0867, xy1675) of leaves near the shoot apices showing the expanded guide cells with uniformly unistratose stereids on their ventral side. Specimens, D. minutum, Catefica 49-S266382 (a, c; holotype), Catefica 49-S266384(b, d); C. lusitanicus Catefica 49-S266385 (e–h; holotype). Scale bars = 1 mm (a, b, e, f), 100 µm (c, d, g, h).