Saturday 19 August 2017

Nothodichocarpum lingyuanensis: A new species of Angiosperm from the Jehol Biota.

The Early Cretaceous Yixian Formation of Lioaning Province, China, has produced a wide range of well preserved animals and plants, including many Vertebrates and Insects, collectively known as the Jehol Biota. The plants from these deposits include the earliest known diverse community of Angiosperms (Flowering Plants), providing a valuable insight into the earliest members and initial diversification of this group.

In a paper published in the journal Acta Geologica Sinica on 1 February 2017, Han Gang of the Hainan Tropical Ocean University and the Palaeontological Center at Bohai University, Liu Zhongjiang, also of the Palaeontological Center at Bohai University, and of the Shenzhen Key Laboratory for Orchid Conservation and Utilization at the National Orchid Conservation Center of China and Orchid Conservation & Research Center of Shenzhen, and Wang Xin of the State Key Laboratory of Palaeobiology and Stratigraphy at the Nanjing Institute of Geology and Palaeontology, describe a new species of Angiosperm from the Jehol Biota.

The new species is named Nothodichocarpum lingyuanensis, where 'Nothodichocarpum' means 'false-Dichocarpum', in reference to a modern genus of herbaceous Flowering Plants in the Ranunculaceae (Buttercup Family) that it superficially resembles, and 'lingyuanensis' means 'from Lingyuan', the locality where the fossils were discovered. 

 Nothodichocarpum and its details. (a) Holotype including branches, leaves, and flowers. Scale bar is 1 cm. (b) One of the leaves with a midrib (arrow). Scale bar is 1 mm. (c) Another narrow obovate leaf with attenuated tip and several teeth (black arrows), overlapped by a young flower including two carpels (1, 2) and at least one male part (white arrow). Scale bar is 1 mm. (d) Pinnate venation in one of the leaves. Scale bar is 0.5 mm. (e) Opposite branching. Note the main branch (2), axillary branch (1), subtending leaf (3), and leaves (l). Scale bar is 1 mm. (f) Three flowers (1-3) of different stages overlapping leaves. Note several male parts (arrows) beside carpels. Scale bar is 1 mm. (g) Two divergent, basally coalescent follicles and two male parts (arrows). Note the spatial relationship between the follicles and male parts. Scale bar is 2 mm. (h), Two young basally coalescent carpels (1-2). Scale bar is 1 mm. (i) Detailed view of the right male part in (g). Scale bar is 0.5 mm. (j) The right fruit in (g) showing abutting seeds (black arrows) inserted along the dorsal vein (white arrow). Scale bar is 1 mm. (k) Four abutting seeds (1-4) in the fruit shown in (g). Note seed 4 is apparently connected to the dorsal of the fruit (black arrow). Scale bar is 1 mm. Han et al. (2017).

Nothodichocarpum lingyuanensis shows a number of unusual features, including the arrangement of the leaves, and the structure of the flower. 

The leaves of Nothodichocarpum are opposite, that is to say arranged in pairs which are opposite one-another on the stem. This is a very common arrangement in modern Flowering Plants, and is also found in a variety of other, non-flowering, Plants, but has not previously been recorded in an early Angiosperm. However re-examination of Archaefructus, another Flowering Plant from the Jehol Biota, and a candidate for the most primitive Angiosperm known, suggests that this plant may also have had an opposite arrangement of leaves, suggesting that this may have been the condition in the very first Flowering Plants.

Sketches of Nothodichocarpum. (a) Sketch showing the physical connection among various parts. Green: leaf; gray: leaf vein; red: follicle/carpel; black: branch; blue: male part. Scale bar is 10 mm. (b) The fruit shown in (g) and (j) above. Note the seeds inserted onto the dorsal vein (right) and male part (blue). Scale bar is 1 mm. (c), Semi-idealised sketch of the leaf shown in (c) above. Han et al. (2017).

Nothodichocarpum lingyuanensis is preserved with both fruit and flowers, enabling examination of both these structures, an unusual opportunity in such an early Plant. The seeds are enclosed within an ovule, which considered to be one of the defining features of an Angiosperm, but the 'flowers' are arranged in a way quite unlike that seen in modern Plant, suggesting that Nothodichocarpum lingyuanensis represents an early stage in the development of the Angiosperms, with a floral structure on the way to developing into a flower, but which we would not necessarily recognise as such.

Details of the flowers of Nothodichocarpum under Scanning Electron Microscope. (a) Basal portion of the fruit shown in top (g). Note the scars left by fallen off male parts (arrows), and their spatial relationship with the carpel (c) and filament (f). Scale bar is 1 mm. (b) A filament (f, white arrow) subtended by a bract (b, black arrow). This male part corresponds to the one marked by left white arrow in above (a). Scale bar is 0.1 mm. (c) Detailed view of rectangle in (a) above. Note spatial relationship among the bract (b) filament (f) in its axil, and carpel (c). Scale bar is 0.1 mm. (d) Detailed view of the right follicle of fruit 2 in top (f). Note the relationship between the carpel (c, white arrow) and male part (black arrows). Scale bar is 1 mm. Han et al. (2017).

The flower of Nothodichocarpum lingyuanensis appears to have had four male parts, two of which were opposite the female folicles and two located between; this is represented by two surviving parts, one in each of these positions and two scars, where the remaining two would be predicted to be. Each of these has a bract beneath which appears to support it. This places the male and female reproductive parts of the plant together at the end of a stem, but does not integrate them into a single flower; forming a structure different to that seen in any living or fossil plant (though a number of early fossil Angiosperms are identified as such from preserved fruiting bodies, without the flowers being known, so they could presumably have supported structures similar to those seen in Nothodichocarpum.

 Reconstruction of Nothodichocarpum in its flower (a) and fruit (b) stages. Note the dorsal vascular bundle connected with seeds in an opened fruit (b). Han et al. (2017).

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