Cupules and the Angiosperm Second Integument
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Nature, May 2021
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Inner Mongolia, China
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The fossil chert locality at the Zhahanaoer open-cast coal mine, Jarud Banner, eastern Inner Mongolia, China. At this locality fossil plants are preserved in silica, which infiltrated and petrified the tissues of a mass of jumbled plant parts. The preservation of individual cells within the plant tissues is exquisite, which allows the fossils to be described in great detail and compared to living plants. The most difficult task is to understand which of the many dispersed plant parts were produced by the same kind of fossil plant. These photographs show the field team soon after the discovery of the locality in 2017.
Details of the right hand photograph: Left to right – Qijia Li (Chang’an University, Xi’an), Gongle Shi (Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences), Patrick Herendeen (Chicago Botanic Garden, Glencoe), Hui Jiang (Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences), Fabiany Herrera (Chicago Botanic Garden, Glencoe), Peter Crane (Oak Spring Garden Foundation, Upperville and Yale University), Bole Zhang (Qingdao Research Institute of Geotechnical Prospecting and Surveying, Qingdao). Zhahanaoer Chert, Inner Mongolia, China, ca. 126 million years before present.
Fossil plants from the Zhahanaoer chert locality.
Details: A) Chert slab showing cupules and short shoots (arrows), and abundant wood. B) Transverse section of lycopod showing actino–plectostelic protostele. C) Transverse section of fern petiole showing vasculature of single leaf trace. D) Transverse section of pinaceous needle showing vasculature and two resin canals. E) Longitudinal section of short shoot showing central axis and leaves. F) Transverse section of gymnosperm wood. Scale bars: A=1cm; B=500μm; C=500μm; D=200μm; E=2mm; F=1mm. Zhahanaoer Chert, Inner Mongolia, China, ca. 126 million years before present.
A variety of plant fossils are preserved within blocks of chert at the Zhahanaoer chert locality as revealed by this peeled section (A). Illustrations to the right show only some of the diversity of fossil plants preserved within the chert, which includes stems of club mosses (B), stalks of fern leaves (C) and several kinds of extinct of seed plants, including conifers such as pines (represented by leaves, D), and many short shoots (E) and stems (F) of extinct seed plants.
Fossil cupules and an associated leaf from the Zhahanaoer chert locality.
Details: A) Median longitudinal section of an empty seed-bearing unit. B) Median transverse section of a cupule. The two lateral flaps are fused to the cupule stalk and the median flap is free, together they form the cupule that tightly encloses two triangular seeds. C) Median longitudinal section of cupule showing seed attachment on the tip of the strongly reflexed cupule stalk resulting in the micropyle being oriented toward the base of the cupule stalk. D) Transverse section of a leaf showing eight longitudinal veins. Scale bars, A=1mm; B=1mm; C=1mm; D=100μm. Zhahanaoer Chert, Inner Mongolia, China, ca. 126 million years before present.
Especially common in the Zhahanaoer chert flora are the seed-bearing cupules of an extinct seed plant that is related to flowering plants, but also shows features similar to living Ginkgo. Many cupules are empty and had shed their seeds before being preserved (A). However, many other cupules still have seeds inside: two in each cupule (B), each suspended from a small pad if tissue at the cupule apex (C). It is this extinct plant and its potential relationship to angiosperms that is the focus of the 2021 paper published in Nature. The leaf (D) may be part of the same plant.
Mongolia
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The Tevshiin Govi fossil locality is a small coal mine, located approximately 220 km south of Ulaanbaatar, Mongolia. Superbly preserved plant fossils occur three dimensionally, little altered from their original shape and size. Individual plant parts can be extracted whole from the soft lignitic matrix that surrounds them. The Tevshiin Govi locality has yielded a variety of fossil plants, some similar to living conifers and filmy ferns, but others that have no close living relatives.
Details: TheTevshiin Govi locality (left); compressed log removed from the sediment (center); cone of Krassilovia compressed in the sediment (upper right); compressed leaf remains preserved as lignite (lower right). Tevshiin Govi lignite, Mongolia, ca. 125 million years before present.
Strap shaped leaf fossils from the Tevshiin Govi locality.
Details: A) Podozamites harrisii; incomplete leafy shoot showing three attached leaves (left). Detail of stomatal band from abaxial leaf cuticle showing cell outlines of transversely oriented, paracytic (monocyclic) stomata (top right). Detail from adaxial leaf cuticle showing rectangular epidermal cell outlines (bottom right). B) Pseudotorellia resinosa; two isolated multiveined leaves (left). Detail of stoma showing lateral subsidiary cells and inner periclinal walls of guard cells (top middle). Detail of outer surface of abaxial cuticle showing stomatal pit with stomatal aperture (bottom middle). C) Pseudotorellia palustris; two isolated multiveined leaves (left). Detail of stoma showing lateral subsidiary cells (top right). Detail of outer surface of abaxial cuticle showing a stoma with papillae (bottom right). Scale bars: A= 1cm, 20μm, 20μm respectively; B= 5mm, 40μm, 40μm respectively; C= 1cm, 20μm, 20μm respectively. Tevshiin Govi lignite, Mongolia, ca. 125 million years before present.
The Tevshiin Govi plant fossils were preserved in a peat that formed in an ancient swamp, where growing conditions were probably poor. There are no large leaves in the fossil deposit. The largest leaves are three strap-shaped forms that we now know belong to three different kinds of extinct plants and that can be distinguished based on their venation and details of their cellular structure. Two of the three leaf forms (B, C) can be assigned to the genus Pseudotorellia, and have similarities to leaves of living Ginkgo. The other leaf (A) is very different and is the leaf of a unusual group of ancient conifers.
Reconstructions of fossil plants from the Tevshiin Govi locality.
Details: A) Doylea (Umkomasia) mongolica (left) with leaves of Pseudotorellia palustris. B) Krassilovia mongolica (center) with leaves of Podozamites harrisii. C) Umaltolepis mongolica (right) with leaves of Pseudotorellia resinosa. Reconstructions by Pollyanna von Knorring. Tevshiin Govi lignite, Mongolia, ca. 125 million years before present.
Three of the fossil plants at Tevshiin Govi locality have been carefully reconstructed from their dispersed parts. Doylea (Umkomasia) mongolica (left) with leaves of Pseudotorellia palustris (left) is a peculiar extinct plant, that shares features with flowering plants, but that also shows some similarities to living Ginkgo. Krassilovia mongolica (center) with leaves of Podozamites harrisii is a plant with interesting unusual features that is probably a strange extinct conifer. Umaltolepis mongolica (right) with leaves of Pseudotorellia resinosa is a strange extinct plant probably related to living Ginkgo.
Umaltolepis mongoliensis – an extinct probable Ginkgo relative from the Tevshiin Govi locality.
Details: A, B) Two seed-bearing structures showing their stalks: closed (left) and open (right). C) Details of winged seeds (top and bottom left) and stalk-column (right) removed from seed-bearing structure in A (left). Note remains of wing tissue (orange) attached to the edges of the column. D) Branched short shoot with two incomplete seed-bearing structures each consisting of a stalk and a broken central column. E) Detail of short shoot showing transition into a long shoot; note pseudo-whorls of persistent bud scales alternating with pseudo-whorls of leaf bases (indicated in gray). F) Stomata from lobe of seed-bearing structure (left) and detail of unicellular trichomes (hairs) from a leaf base (right). G) Detail of tips of four lobes of seed-bearing structure, note prominent stomata on the outer surface. Scale bars: 5mm=A–C; 2mm=E; 500 μm=C (bottom); 100 μm=F (right); 20μm=F (left). Tevshiin Govi lignite, Mongolia, ca. 125 million years before present.
Umaltolepis mongolica has been reconstructed from several of its dispersed plant parts found at the Tevshiin Govi locality that include leaves (Pseudotorellia resinosa), short shoots and seed-bearing structures. Umaltolepis is a very strange kind of extinct seed plant, with four delicate seeds enclosed inside each very tough seed-bearing structure that is closed with a central stalk - like a closed-up umbrella. Among living plants Umaltolepis is probably most closely related to living Ginkgo.
Krassilovia mongolica: an archaic fossil conifer from the Tevshiin Govi locality.
Details: A) Tightly closed seed cone showing spiny scales. B) Isolated cone axis from which scales have been shed; note conspicuous spirally arranged abscission scars. C) Two isolated spiny cone scales (left) and two isolated winged seeds. Reconstruction of scales and seeds below. Scale bars: 5mm=A; 2mm=B, C. Tevshiin Govi lignite, Mongolia, ca. 125 million years before present. Reconstructions by Pollyanna von Knorring.
Krassilova mongolica has been reconstructed from several of its dispersed plant parts found at the Tevshiin Govi locality, including leaves (Podozamites harrisii), cones and dispersed cone scales. Exactly how this unusual extinct seed plant relates to plants living today is not clear, but it is probably a peculiar extinct kind of conifer.
Fossils of the pine-spruce family (Pinaceae) from the Tevshiin Govi locality.
Details: A) Top: Isolated cone scale of Schizolepidopsis canicularis showing bilobed shape and abundant yellow-orange remains of broken resin canals. Bottom: Fragment of female cone of Schizolepidopsis ediae showing bilobed scales with needle-like bracts. B) Two large seed cones of Picea farjonii (spruce plant), note broad, persistent, spirally arranged scales. C) Two seed cones of Pityostrobus stockeyae showing pointed, persistent, spirally arranged scales. Note pair of isolated winged seeds. D) Disarticulated cone scale of undescribed Pinaceae species. E) Processed lignite samples showing abundant pinaceous needle associated with seed cones. Scale bars: 1cm=A (top), B; 5mm=A (bottom), C, D. Tevshiin Govi lignite, Mongolia, ca. 125 million years before present.
Several different plant fossils have been recovered that are related to the living pine-spruce family and that bore needle like leaves, which are among the most common fossils at the Tevshiin Govi locality. Some of these fossil plants appear related to living pines, firs and spruces but others, especially Schizolepidopsis appear to be ancient relatives of this important group of living conifers that survived from earlier geologic times.
Fossil of cypress family (Cupressaceae) from the Tevshiin Govi locality.
Details: A, B, C) Tiny seed cones (A), scale (B) and winged seed (C) of Pentakonos diminutus. D-G) Fossils of Stutzeliastrobus foliates. D) Leafy shoot, note resin canals (orange remains). E) Two seed cones with one attached at the tip of a leafy shoot. F) Neutron tomographic reconstruction of seed cone showing abundant resin canals (orange remains). G) Isolated bract-scale showing three winged seeds (left) and isolated winged seeds (right). Scale bars: A-C= 2mm, 1mm, 500μm respectively; D-G= 5mm, 5mm, 1mm, 500μm respectively. Tevshiin Govi lignite, Mongolia, ca. 125 million years before present.
Several kinds of extinct conifers related to the modern cypress family have been recovered from the Tevshiin Govi locality. They do not appear to be closely related to any living genus, but are probably extinct progenitors of the great variety of living conifers in the family Cupressaceae, which includes redwoods, junipers, cypresses and a variety of other living conifer trees.
Fossils of Gnetales and Bennettitales from the Tevshiin Govi locality.
Details: A) Bennettitalean leaf of Nilssoniopteris tomentosa with line drawing showing base, and stout midrib, note dichotomizing lateral veins. B) Detail of typical Bennettitalean cuticle showing three brachyparacytic (syndetocheilic) stomata. C) Detail of dense covering of branched trichomes from leaf lamina; note isolated trichome showing spine-like tips of lateral branches. D) Probable seed of Problematospermum showing rugulate, transversely ribbed seeds and an elongated apical micropyle. E) Unidentified seed probably related to living Gnetales showing outer seed envelope and an apical micropyle. Scale bars: A= 3mm, B= 50μm, C= 200μm (left), 100 μm (right); D= 1mm (top), 200μm (bottom); E= 1 mm. Tevshiin Govi lignite, Mongolia, ca. 125 million years before present.
The Gnetales are a strange and isolated group of living seed plants that include only three living genera: Ephedra, Welwitschia and Gnetum. Recent research has revealed a more extensive fossil record of extinct plants related to Gnetales than had recognized previously. While there is still some controversy regarding the relationships of Gnetales, there is increasing evidence supporting their close relationship to extinct Bennettitales, a very important group during the Mesozoic.
Fossil cupules from the Tevshiin Govi locality.
Details: A) Doylea mongolica (Umkomasia mongolica) showing seed cone (left), isolated, lateral seed-bearing unit (middle), and three-angled seed with ovate lateral outline and elongated micropyle (right). B) Umkomasia corniculata showing isolated, lateral seed-bearing unit and three-angled seed (right). C) Umkomasia trilobata showing isolated, lateral seed-bearing unit and three-angled seed (right). Line drawings show the bract (green), stalked cupules (blue), and the axis on which they are borne (red brown). Scale bars: 2mm, except in isolated seeds, 500μm, 200μm, 500μm respectively. Tevshiin Govi lignite, Mongolia, ca. 125 million years before present.
Doylea (Umkomasia) mongolica that bore leaves of Pseudotorellia palustris (left) is a peculiar extinct plant, that shares features with flowering plants, but that also shows some similarities to living Ginkgo. The recurved cupules in which the seeds are borne are directly comparable to the recurved of ovules that appear to be basic for all flowering plants.
Fossil filmy fern (Hymenophyllaceae) from the Tevshiin Govi locality.
Details: A) Fragment of sterile frond of Hymenophyllum iwatsukii accompanied by line drawing; note segmented pinnules with anadromous venation and toothed margin. B) Fertile pinna accompanied by line drawing showing marginal sori with some indusial flaps bent downward (grey). C) Detail of sorus showing bivalvate and indusial flaps, note several articulated sporangia. D) Detail of sporangium with approximately 22 thickened annulus cells. E) Tightly curled fiddlehead. Scale bars: 5mm= A,B; 1mm=E; 100μm=C; 50μm=D. Tevshiin Govi lignite, Mongolia, ca. 125 million years before present.
Ferns are a common component of the most fossil floras from the Early Cretaceous, but at the Tevshiin Govi fossil locality they are represented only by these well-preserved fossils of a filmy fern. Today most filmy ferns are epiphytes growing on rocks or the branches of trees in areas where there is abundant moisture.