Comparison of Scirpus weichangensis X.Q. Liang, sp. nov. with extant and fossil species

The fossil fruits from the early Miocene of Weichang are obovate with a slightly 3-sided lateral section (Fig. 2). They have three stout persistent bristles with downward-directed barbellae (Fig. 2). They share similar characteristics with Cyperaceae with respect to the fruit outline, the lateral section, and bristles. As such, they belong to the family, with more than ninety genera. Among the genera, fruits of Scirpus L., Phylloscirpus C.B. Clarke, and Pseudoschoenus (C.B. Clarke) Oteng-Yeb. have persistent stout bristles with downward-directed barbellae. However, the fruits of Phylloscirpus have six bristles, and the cell walls of their surfaces are minutely reticulate–foveolate. Pseudoschoenus have fruits with 3–5 (6) bristles (Goetghebeur 1998). Fruits of Schoenoplectus (Reichenbach) Palla have the faintly to prominently rugose or with transverse wavy ridges, and are 2.5–3.5 mm in length (Whittemore and Schuyler 2002). Fruits of Bolboschoenus (Ascherson) Palla are 2.3–5.5 mm long (Whittemore and Schuyler 2002). Nutlet surface of Schoenoplectiella Lye is smooth or transversely rugulose to sharply ridged, and epidermal cells are narrowly oblong to linear (Hayasaka 2012). Consequently, fossil fruits are ascribed as being part of Scirpus.

Fig. 2 Sketch of Scirpus weichangensis X.Q. Liang, sp. nov. a - Outline in the lateral view; and, b - Outline in the transverse section Full size image

Within the genus, nineteen species have fruits with four, five or up to six bristles (e.g. S. orientalis, S. radicans, S. polyphyllus) (Oh and Ham 1998; Whittemore and Schuyler 2002; Dai et al. 2010) (Table 1). Fruits of S. atrocinctus Fernald, S. congdonii Britton, S. diffuses Schuyler, S. flaccidifolius (Fernald) Schuyler, S. lineatus Michaux, and S. longii Fernald have six slender bristles (Whittemore and Schuyler 2002). The fruits of S. polyphyllus Muell. and S. wichureae Boechkeler have filiform and silk-like bristles, respectively (Healy and Edgar 1980; Oh and Ham 1998; Wilson 1994; Dai et al. 2010) (Table 1).

Table 1 Comparison of Scirpus weichangensis X.Q. Liang, sp. nov. with extant species Full size table

Fruits of S. microcarpus Presl & Presl have bristles with barbellae arranged almost to the base (Whittemore and Schuyler 2002) (Table 1). The bristles of S. georgianus Harper are much shorter than the fruit (Yu 2003) (Table 1). The fruit barbellae of S. chunianus Tang & Wang are near the apex of the bristles (Dai et al. 2010) (Table 1). Fruits of the S. rosthornii Diels are elliptic to subspherical in outline, and are smaller than the fossil fruits (Dai et al. 2010) (Table 1). The fruits of S. ternatanus Reinwardt ex Miquel are biconvex (Dai et al. 2010) (Table 1).

Many fossil fruits of Scirpus are obovate, e.g. S. foveolatus Nikitin from the early Oligocene Tyumen, S. clavatus Nikitin from the Oligocene Novosibirsk, S. chandlerae Nikitin from the late Oligocene, S. nagorskyi Dorofeev from the Miocene Tomsk, S. atroviroides Wieliczk from the Pliocene Czamucha and Pleistocene Kathmandu (Dorofeev 1963; Stachowicz-Rybka 2005; Nikitin 2006; Bhandari et al. 2009). Moreover, fossil fruits of these species are commonly larger than fossil fruits from Weichang, e.g. S. foveolatus Nikitin from the early Oligocene Tyumen, S. clavatus Nikitin from the Oligocene Novosibirsk, and S. nagorskyi Dorofeev from the Miocene Tosmk (Dorofeev 1963; Nikitin 2006) (Table 2). Fruits of S. novorossicus Doweld from the late Miocene Odessa, Ukraine are ovate but also larger (Doweld 2018). S. nagorskyi Dorofeev from the Miocene Tomsk and S. atroviroides Wieliczk from the Pliocene Czamucha have fruits without bristles (Dorofeev 1963; Stachowicz-Rybka 2005). Scirpus sp. from the Pleitocene Kathmandu has fruits with the longitudinal epidermal cells and six to eight bristles (Bhandari et al. 2009).

Table 2 Comparison of Scirpus weichangensis X.Q. Liang, sp. nov. with other fossil species Full size table

No further more information about fossil fruits without bristles can be compared with materials from Weichang, e.g. S. microcarpus Presl from the Pliocene Ellesmere, and Scirpus sp. from the Holocene Niederweningen (Switzerland), Rapa Nuiis (Polynesia), Connecticut (USA), Colorado (USA), and Jiangsu (China) (Pierce and Tiffney 1986; Matthews and Ovenden 1990; Drescher-Schneider et al. 2007; Li et al. 2008; Horrocks et al. 2012; Strickland et al. 2014). Thus, we ascribed fossil fruits from the early Miocene of Weichang to a new species, Scirpus weichangensis X.Q. Liang, sp. nov.

Palaeoecological and palaeobiogeographical implication of Scirpus weichangensis

Extant plants of Scirpus usually grow in wetlands, swamps, and marshes (Goetghebeur 1998). Scirpus weichangensis appeared in the early Miocene deposit in Weichang, with Weigela (Liang et al. 2013), indicating that Guangfayong was a wetland in the early Miocene, and then the wetland gradually disappeared as a result of land uplift.

The fruit fossils of Scirpus were firstly found in the early Oligocene of Tyumen (Nikitin 2006). And, a noticeable point here is that we inclined to keep Nikitin’s fossil species of Scirpus based on the fossil morphological information, although many fossil species of the genus have been ascribed to other genera (Doweld 2018). They appeared in the Miocene of Tomsk, Odessa and Weichang (Dorofeev 1963; Doweld 2018; and this study). They were collected in the Pliocene of Ellesmere Island, Echigawa, Prince Patrick Island, and Czarnucha (Matthews and Ovenden 1990; Stachowicz-Rybka 2005; Yamakawa et al. 2008). They were found in the Pleistocene of Kathmandu, and Łuków (Bhandari et al. 2009; Stachowicz-Rybka 2015); and in the Holocene of Niederweningen, Rapa Nuiis, Hamden, Ziegler Reservoir, Sacramento-San Joaquin Delta, and Tangnan (James-West 1977; Pierce and Tiffney 1986; Drescher-Schneider et al. 2007; Li et al. 2008; Horrocks et al. 2012; Strickland et al. 2014).

Based on genus fossil data, Scirpus originated in the Oligocene of Siberia, and then migrated widely into Europe and Eastern Asia during the Miocene. It occurred in North America through transoceanic dispersal in the Pliocene. Scirpus then spread widely around the world in the Holocene (Fig. 3).