Reinvestigating an enigmatic Late Cretaceous monocot: morphology, taxonomy, and biogeography of Viracarpon

Abstract

Angiosperm-dominated floras of the Late Cretaceous are essential for understanding the evolutionary, ecological, and geographic radiation of flowering plants. The Late Cretaceous–early Paleogene Deccan Intertrappean Beds of India contain angiosperm-dominated plant fossil assemblages known from multiple localities in central India. Numerous monocots have been documented from these assemblages, providing a window into an important but poorly understood time in their diversification. One component of the Deccan monocot diversity is the genus Viracarpon, known from anatomically preserved infructescences. Viracarpon was first collected over a century ago and has been the subject of numerous studies. However, resolution of its three-dimensional (3D) morphology and anatomy, as well as its taxonomic affinities, has remained elusive. In this study we investigated the morphology and taxonomy of genus Viracarpon, combining traditional paleobotanical techniques and X-ray micro-computed tomography (μCT). Re-examination of type and figured specimens, 3D reconstructions of fruits, and characterization of structures in multiple planes of section using μCT data allowed us to resolve conflicting interpretations of fruit morphology and identify additional characters useful in refining potential taxonomic affinities. Among the four Viracarpon species previously recognized, we consider two to be valid (Viracarpon hexaspermum and Viracarpon elongatum), and the other two to be synonyms of these. Furthermore, we found that permineralized infructescences of Coahuilocarpon phytolaccoides from the late Campanian of Mexico correspond closely in morphology to V. hexaspermum. We argue that Viracarpon and Coahuilocarpon are congeneric and provide the new combination, Viracarpon phytolaccoides (Cevallos-Ferriz, Estrada-Ruiz & Perez-Hernandez) Matsunaga, S.Y. Smith, & Manchester comb. nov. The significant geographic disjunction between these two occurrences indicates that the genus Viracarpon was widespread and may be present in other Late Cretaceous assemblages. Viracarpon exhibits character combinations not present in any extant taxa and its affinities remain unresolved, possibly representing an extinct member of Alismatales. The character mosaic observed in Viracarpon and the broad distribution of the genus provide new data relevant to understanding early monocot evolution and suggest that the (thus far) largely invisible Late Cretaceous monocot diversification was characterized by enigmatic and/or stem taxa.

Faunal elements from the Deccan volcano‐sedimentary sequences of India: A reappraisal of biostratigraphic, palaeoecologic, and palaeobiogeographic aspects

“…however, subsequent studies do not attest to the presence of a continuous, mappable intertrappean lithounit within the Nagpur and adjoining areas. The green clay horizon of the Takli section has been well known for both floral and faunal remains (Bhatia, Prasad, & Rana, 1990;Matsunaga et al, 2018;Sahni et al, 1984;Vianey-Liaud, Jain, & Sahni, 1987). The vertebrate assemblage recorded within the Takli section consists of fishes, frogs, lizards, snakes, and non-avian dinosaurs Rana & Sahni, 1989)…”

“…Sahni, Rana, Kumar, and Loyal () formally designated the term “Takli Formation” for the Takli intertrappeans; however, subsequent studies do not attest to the presence of a continuous, mappable intertrappean lithounit within the Nagpur and adjoining areas. The green clay horizon of the Takli section has been well known for both floral and faunal remains (Bhatia, Prasad, & Rana, ; Matsunaga et al, ; Sahni et al, ; Vianey‐Liaud, Jain, & Sahni, ). The vertebrate assemblage recorded within the Takli section consists of fishes, frogs, lizards, snakes, and non‐avian dinosaurs (Khosla & Verma, ; Rana & Sahni, ) in association with numerous molluscan remains, including that of Physa prinsepii .…”

“…Although a Maastrichtian age for the Takli intertrappeans isn't in doubt; however, for obvious reasons, Sahni et al () put forth the view that the long‐ranging (Cretaceous to Eocene) P. prinsepii should not be considered an index fossil for the Deccan volcano‐sedimentary sequences. Interestingly, the Takli intertrappeans are the designated type section for the plant genus Viracarpon , and the palaeobotanical evidence for the monocot Viracarpon suggests its occurrence to be restricted within the late Maastrichtian sequences of the DVP (Matsunaga et al, , and references therein).…”

“…Interestingly, the Takli intertrappeans are the designated type section for the plant genus Viracarpon, and the palaeobotanical evidence for the monocot Viracarpon suggests its occurrence to be restricted within the late Maastrichtian sequences of the DVP(Matsunaga et al, 2018, and references therein).…”

Building the monocot tree of death: Progress and challenges emerging from the macrofossil‐rich Zingiberales

“…For paleobotanists, μCT also enables us to understand what morphological interpretations might result merely from different planes of sections or variation in the preservation of tissues (e.g., Smith et al., ; Adams et al., ). For example, fossils are sometimes obliquely sectioned relative to the standard transverse and longitudinal sections in modern plants, which can affect a researcher's ability to accurately and confidently make comparisons with other taxa that have been preserved differently or may be three‐dimensionally complex (e.g., Matsunaga et al., ; morphology of Viracarpon fruits was disputed for more than 50 years, but resolved with the application of μCT). Because accuracy of morphological interpretation can affect the scoring of characters in a data matrix, using 3D data can play a positive role in assigning homologies and reconciling relationships.…”