A new early Cretaceous relative of Gnetales: Siphonospermum simplex gen. et sp. nov. from the Yixian formation of northeast China

Abstract

Background: Knowledge on fossil and evolutionary history of the Gnetales has expanded rapidly; Ephedra and ephedroids as well as the Gnetum-Welwitschia clade are now well documented in the Early Cretaceous. However, hypotheses on evolutionary relationships among living and fossil species are hampered by restricted knowledge of morphological variation in living groups and recent studies indicate that gnetalean diversity and character evolution may be more complex than previously assumed and involve additional extinct groups (Bennettitales, Erdtmanithecales and unassigned fossil taxa).

Results: Here we describe a new fossil related to Gnetales, Siphonospermum simplex from the Early Cretaceous Yixian Formation, an impression/compression of a reproductive shoot. The slender main axis bears one pair of opposite and linear leaves with primary parallel venation. The reproductive units are ovoid, without supporting bracts and borne on one median and two lateral branches. The most conspicuous feature of the fossil is the long, thread-like micropylar tube formed by the integument. Each ovule is surrounded by two different layers representing one or two seed envelopes; an inner sclerenchymatous layer and an outer probably parenchymatous layer.

Conclusions: The vegetative and reproductive features of Siphonospermum simplex exclude a relationship to any other group than the Gnetales. A combination of opposite phyllotaxis, linear leaves and ovules surrounded by seed envelope(s) and with a long exposed micropylar tube are known only for extant and extinct Gnetales. Siphonospermum simplex constitutes a new lineage within the Gnetales. Its morphology cannot be directly linked to any previously known plant, but the organization of the reproductive units indicates that it belongs to the Gnetum-Welwitschia clade. Based on the absence of cone bracts and the inferred histology of the seed envelope(s) it could be related to Gnetum, however, there are also affinities with the ephedran lineage, some of which are likely plesiomorphic features, others perhaps not. Phylogeny and character evolution in the Bennettitales, Erdtmanithecales and Gnetales are currently only partly understood and under debate; the exact systematic position of Siphonospermum simplex, i.e., its position within the Gnetales, cannot be resolved with certainty.

Figure 1

Siphonospermum simplex gen. et sp. nov. Overview of the holotype (part and counterpart): (a) specimen 9880A; (b) specimen 9880B. Scale bars: 1 mm.

Figure 2

Siphonospermum simplex gen. et sp. nov. Details of (a, c): specimen 9880A; (b, d): specimen 9880B. (a) Reproductive units of Siphonospermum simplex. (b) Close up of the reproductive unit seen in Figure 1b. (c-d) Node and linear leaf with parallel first order venation. Arrows 1: micropylar tube formed by the integument; 2: second (mostly sclerenchymatous) layer; 3: third (probably mostly parenchymatous) layer; 4: the ovule has been detached and slightly displaced; 5: supportive strands of the (inner) seed envelope. Scale bars: 1 mm.

Figure 3

Reconstruction of Siphonospermum simplex. Reproductive unit; reconstruction of a longitudinal section. Arrows 1: micropylar tube, 2: second (mostly sclerenchymatous) layer, 3: third (probably mostly parenchymatous) layer. Uninterrupted lines indicate morphological surfaces. The dashed line indicates the boundary between the second (mostly sclerenchymatous) layer and the third (probably mostly parenchymatous) layer. Drawing: Pollyanna von Knorring.