Palms are an important part of the flora of most tropical environments, but are notoriously under-represented in Africa, where there are around 65 described species, compared to 437 in South America. Palms appeared during the Cretaceous, and their current diversity and biogeography is thought to be connected to a series of extinction and diversification events, though why this has left Africa with relatively few species of Palm is unclear. The genus Sclerosperma contains three living species, found in the understories of lowland tropical rainforests from Liberia to Rwanda. They typically form bushy Palms growing from a horizontal underground stem (rhizome), though one species can produce erect stems up to 9 m in height. The genus has a poor fossil record, with a few fossil leaves from the Miocene of South Kivu Province in the Democratic Republic of Congo and some pollen from the Miocene of Senegal.
In a paper published in the journal Grana on 25 October 2018, Friðgeir Grímsson of the Department of Palaeontology at the University of Vienna, Bonnie Jacobs of the Roy M. Huffington Department of Earth Sciences at Southern Methodist University, Johan Van Valkenburg and Jan Wieringa of the Naturalis Biodiversity Center, Alexandros Xafis, also of the the Department of Palaeontology at the University of Vienna, Neil Tabor, also of the Roy M. Huffington Department of Earth Sciences at Southern Methodist University, Aaron Pan of the Don Harrington Discovery Center, and Reinhard Zetter, again of the Department of Palaeontology at the University of Vienna, describe two new species of Sclerosperma, based upon pollen obtained from Late Oligocene Deposits exposed in the Guang River Valley at Chilga in the Amhara Region of northwest Ethiopia.
The Guang River exposure comprises a sedimentary sequence about 100 m thick, bounded at the bottom by a basalt layer dated to 32.4 million years ago using potassium-argon dating and at the top by an ash layer dated to 27.36 million years ago using argon-argon dating. The pollen grains described by Grímsson et al. come from a lignite (coal) layer about 30.5 m above the base of this sequence, which is estimated to be between 27 and 28 million years old on the basis of palaeomagnetic data.
Potasium-Argon dating relies on determining the ratio of radioactive Potasium⁴⁰ to Argon⁴⁰ within minerals from igneous or metamorphic rock to determine how long ago the mineral cooled sufficiently to crystallise. Potasium⁴⁰ is often incorporated into cooling volcanic rocks, whereas any inert Argon present will escape as a gas. No further Potasium⁴⁰ or Argon⁴⁰ will enter the mineral from this point, but Argon⁴⁰ is produced by the decay of radioactive Potassium⁴⁰ at a steady rate, enabling scientists to establish a precise date for the crystallisation of the minerals containing the two elements.
Argon-Argon dating relies on determining the ratio of radioactive Argon⁴⁰ to non-radioactive Argon³⁹ within minerals from igneous or metamorphic rock (in this case volcanic ash) to determine how long ago the mineral cooled sufficiently to crystallise. The ratio of Argon⁴⁰ to Argon³⁹ is constant in the atmosphere, and this ratio will be preserved in a mineral at the time of crystallisation. No further Argon³⁹ will enter the mineral from this point, but Argon⁴⁰ is produced by the decay of radioactive Potassium⁴⁰, and increases in the mineral at a steady rate, providing a clock which can be used to date the mineral.
Palaeomagnetic dating relies on the fact that the Earth’s magnetic field undergoes periodic reversals to provide dates for strata. In deposits where iron rich minerals are able to settle slowly in liquids these will settle in alignment with the Earth’s magnetic field. Since this field reverses periodically, but irregularly, and these reversals have been mapped for many well dated deposits, these reversals can be used to date suitable deposits.
Grímsson et al. collected four samples from the lignite layer at closely spaced localities. These were treated with hydrochloric acid (HCl) and hydrofluoric acid (HF) in the laboratory, to remove any carbonates and silicates, then further treated with an oxidising agent to free any pollen.
Pollen is extremely useful to archaeologists and palaeontologists. It is resilient both and distinctive, and plants produce it in large amounts, and scatter it freely in the environment. Scientists who study pollen, called palynologists, are able to use pollen to date ancient sediments and to reconstruct the vegetation, and therefore climate, of ancient sites.
The pollen of modern species of Sclerosperma is roughly triangular, with a pattern of perforations on its surface and an aperture at the apex of one corner. The only previously described fossil material reliably assigned to the genus, from the Miocene of Senegal, conforms to this pattern but is to poorly preserved for further diagnosis.
The first new species is named Sclerosperma protomannii, meaning ‘before mannii’, in reference to the modern species Sclerosperma mannii, which it resembles. This species is described from roughly triangular pollen grains measured as 24–35 μm across with a scanning electron microscope, with up to 20 lumina (openings) per 100 μm², compared to 25 lumina per 100 μm² in the modern Sclerosperma mannii.
The Guang River exposure comprises a sedimentary sequence about 100 m thick, bounded at the bottom by a basalt layer dated to 32.4 million years ago using potassium-argon dating and at the top by an ash layer dated to 27.36 million years ago using argon-argon dating. The pollen grains described by Grímsson et al. come from a lignite (coal) layer about 30.5 m above the base of this sequence, which is estimated to be between 27 and 28 million years old on the basis of palaeomagnetic data.
Stratigraphic section, measured along the Guang River. Stars indicate stratum that produced the Sclerosperma fossil pollen, and the fossil leaf locality, CH41, is labelled and marked by a leaf icon. Radioisotopic dates are shown near the base and top of the section. Grímsson et al. (2018).
Potasium-Argon dating relies on determining the ratio of radioactive Potasium⁴⁰ to Argon⁴⁰ within minerals from igneous or metamorphic rock to determine how long ago the mineral cooled sufficiently to crystallise. Potasium⁴⁰ is often incorporated into cooling volcanic rocks, whereas any inert Argon present will escape as a gas. No further Potasium⁴⁰ or Argon⁴⁰ will enter the mineral from this point, but Argon⁴⁰ is produced by the decay of radioactive Potassium⁴⁰ at a steady rate, enabling scientists to establish a precise date for the crystallisation of the minerals containing the two elements.
Argon-Argon dating relies on determining the ratio of radioactive Argon⁴⁰ to non-radioactive Argon³⁹ within minerals from igneous or metamorphic rock (in this case volcanic ash) to determine how long ago the mineral cooled sufficiently to crystallise. The ratio of Argon⁴⁰ to Argon³⁹ is constant in the atmosphere, and this ratio will be preserved in a mineral at the time of crystallisation. No further Argon³⁹ will enter the mineral from this point, but Argon⁴⁰ is produced by the decay of radioactive Potassium⁴⁰, and increases in the mineral at a steady rate, providing a clock which can be used to date the mineral.
Palaeomagnetic dating relies on the fact that the Earth’s magnetic field undergoes periodic reversals to provide dates for strata. In deposits where iron rich minerals are able to settle slowly in liquids these will settle in alignment with the Earth’s magnetic field. Since this field reverses periodically, but irregularly, and these reversals have been mapped for many well dated deposits, these reversals can be used to date suitable deposits.
Grímsson et al. collected four samples from the lignite layer at closely spaced localities. These were treated with hydrochloric acid (HCl) and hydrofluoric acid (HF) in the laboratory, to remove any carbonates and silicates, then further treated with an oxidising agent to free any pollen.
Pollen is extremely useful to archaeologists and palaeontologists. It is resilient both and distinctive, and plants produce it in large amounts, and scatter it freely in the environment. Scientists who study pollen, called palynologists, are able to use pollen to date ancient sediments and to reconstruct the vegetation, and therefore climate, of ancient sites.
The pollen of modern species of Sclerosperma is roughly triangular, with a pattern of perforations on its surface and an aperture at the apex of one corner. The only previously described fossil material reliably assigned to the genus, from the Miocene of Senegal, conforms to this pattern but is to poorly preserved for further diagnosis.
The first new species is named Sclerosperma protomannii, meaning ‘before mannii’, in reference to the modern species Sclerosperma mannii, which it resembles. This species is described from roughly triangular pollen grains measured as 24–35 μm across with a scanning electron microscope, with up to 20 lumina (openings) per 100 μm², compared to 25 lumina per 100 μm² in the modern Sclerosperma mannii.
Light microscopy (A) and scanning electron microscopy (B)–(E) micrographs of Sclerosperma protomannii. (A) Pollen grain in polar view (upper, high focus) and equatorial view (lower). (B) Pollen grain in polar view, distal side. (C) Pollen grain in polar view, proximal side. (D) Close-up of apex with aperture, distal side. (E) Close-up of central polar area, distal side. Scale bars are 10 μm in (A)–(C), and 1 μm in (D) and (E). Grímsson et al. (2018).
The second new species is named Sclerosperma protoprofizianum, meaning ‘before profizianum’, in reference to the modern species Sclerosperma profizianum, which it resembles. The pollen of this species is triangular in polar view, but bean-shaped in profile, with a convex side and a concave side and measures 21–29 μm across with a scanning electron microscope. The grains have 50-65 lumina (openings) per 100 μm², compared to between 35 and 55 lumina per 100 μm² in the modern Sclerosperma profizianum.
Light microscopy (F) and scanning electron microscopy (G)–(J) micrographs of Sclerosperma protoprofizianum. (F) Pollen grain in polar view (high focus). (G) Pollen grain in polar view, distal side. (H) Pollen grain in polar view, proximal side. (I) Close-up of apex with aperture, distal side. (J) Close-up of central polar area, distal side. Grímsson et al. (2018).
In addition to the pollen grains Grímsson et al. describe a partial Palm leaf, collected from an ash layer 43 m above the lignite layer that produced the pollen samples (i.e. 73 m above the base of the section). This leaf comprises a leaflet (part of a divided leaf that itself resembles a leaf) attached to a rachis (leaf stem), with two other leaflets, presumed to have come from the same leaf in proximity.
In addition to the pollen grains Grímsson et al. describe a partial Palm leaf, collected from an ash layer 43 m above the lignite layer that produced the pollen samples (i.e. 73 m above the base of the section). This leaf comprises a leaflet (part of a divided leaf that itself resembles a leaf) attached to a rachis (leaf stem), with two other leaflets, presumed to have come from the same leaf in proximity.
Fossil leaf fragment collected at 74 m above the base of the measured Guang River section. Grímsson et al. (2018).
The ash layer that produced the leaf fragment, and the one at the top of the sequence that yielded the 27.36 million year argon-argon date, have produced a variety of Plant fossils including Ferns, Horsetails, Palms and other flora consistent with modern African forests found on seasonal floodplains, an environment consistent with that favoured by Sclerosperma today.
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The leaf fragments are not preserved in sufficiently well preserved to be assigned to a species or even genus, but almost certainly come from one of two still extant families of Palms, the Arecoideae or the Ceroxyloideae. The Family Ceroxyloideae is absent from mainland Africa today, but is found in Madagascar, the Comoros Islands and Australia, so a presence in Africa in the past is not implausible. The Family Arecoideae, which includes the genus Sclerosperma, is present in Africa today, though it is not a major part of the African flora; of the four extant genera of Arecoideae on the African mainland and one on the island of Pemba (off the coast of Tanzania), the fossil leaf fragments most closely resemble those of Sclerosperma.
The ash layer that produced the leaf fragment, and the one at the top of the sequence that yielded the 27.36 million year argon-argon date, have produced a variety of Plant fossils including Ferns, Horsetails, Palms and other flora consistent with modern African forests found on seasonal floodplains, an environment consistent with that favoured by Sclerosperma today.
An example of the modern Palm, Sclerosperma mannii, in the Forêt de la Mondah lowland rainforest of coastal Gabon. Thomas Couvreur/Palms of Africa.
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