Cheirolepidiaceae
| Cheirolepidiaceae | ||||||||||||
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Brachyphyllum fossil in the Mayor Müller Museum in Solnhofen . |
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| Temporal occurrence | ||||||||||||
| Triassic to Chalk | ||||||||||||
| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Cheirolepidiaceae | ||||||||||||
| Takht. |
The Cheirolepidiaceae are an extinct group of conifers (Coniferales). They are morphologically quite diverse, the common feature is the pollen type , which is referred to as the genus Classopollis . Their representatives were particularly common in the Mesozoic in tropical and subtropical climates.
features
Many representatives, which are grouped together as Frenelopsiden, do not resemble living conifers. They have sprouts that more closely resemble today's trunk succulent flowering seeds. They have reduced leaves, a thick cuticle , sunken stomata, and shoots that photosynthesize . Pseudofrenelopsis varians hardly develops any wood and should have been of a very small habitus. Pseudofrenelopsis parceramosa was also succulent, but it should have been a stately tree. The logs reached a diameter of 70 cm.
Hirmeriella muensteri is reconstructed as a typical large, evergreen conifer tree that grew on sandstone. Also Cupressinocladus valdensis from the late Jurassic was a big forest tree and the dominant tree in what is now southern England. Different sized growth rings indicate a strictly seasonal climate. Associated evaporites indicate the proximity of a hypersaline bay. Pseudofrenelopsis ramosissima was less strongly xeromorphic than the other representatives. On the one hand it is reconstructed as a small shrub similar to today's Crassula or Kalanchoe , on the other hand as a tree over 20 m high.
rung
In 1988, 22 species from seven genera of sprouts could be safely assigned to the family.
- Brachyphyllum (part)
- Cupressinocladus (part)
- Frenelopsis
- Hiermeriella
- Pagiophyllum
- Pseudofrenelopsis
- Tomaxellia
Androvettia , Geinitzia and Glenrosa belong to the Cheirolepidiaceae .
The leaves were either in a spiral arrangement as in Brachyphyllum or in whorls such as in Frenelopsis . The latter arrangement is also known as the frenelopsider type. Here the leaf bases of each segment form a smooth cylinder around the stem axis. In Frenelopsis alata there is a special form of axillary branching of the shoot axes, which is otherwise only known from the Cupressaceae .
Cuticle
The cuticula provides important features for the differentiation of the species. The leaves have small hairs on the edge, the surface is covered with papillae , which range from small humps to pointed outgrowths to long hair. In Frenelopsis harrisii , they look like cobblestones.
In many species the cuticle is extremely thick, it can reach up to 100 µm. It can be three to ten times thicker than other conifers of the same flora.
The stomata have randomly oriented openings. Their arrangement is either scattered or in longitudinal rows, the latter irregular or regular, depending on the species. The stomata usually has a ring of four to six escort cells, in Pseudofrenelopsis varians also up to nine. These cells form a deep pit. Often around this pit there are a row or two of conspicuous papillae and a thickened ring around the pit opening. This will make the opening smaller. In other species, papillae are located in the pit, forming a ring that protrudes from the pit.
In some species this double structure is missing. You have a simple set of papillae around the edge of the pit. Representatives are Brachyphyllum , Pagiophyllum , Pseudofrenelopsis and Cupressinocladus . But there are also types with stomata without papillae.
Internal anatomy
Only five types of wood are known (Watson 1988). This will Protocupressinoxylon and Protopodocarpoxylon asked. These are two Mesozoic Morphotaxa, which are characterized by semiaraucarioid or protopinacioid tracheid pits . There are parenchymal rays and transverse pits. The wood anatomy remains consistent throughout the family.
Male cones
The male cones of 15 species are known (Watson 1988). Cones found in isolation that belong to the family are placed in the Classostrobus form . All cones have the characteristic shape of the conifers: they are egg-shaped to spherical, the microsporophylls are spiraling on the axis of the cones. The microsporangia are located on the underside of the sporophyll . The cone diameter ranges from 3 mm to 23 mm in Pseudofrenelopsis intermedia . In the genus Hirmeriella , the microsporophylls are almost shield-shaped (peltat). There are two (in Hirmeriella muensteri ) to eight (in Classostrobus pseudoexpansum ) sporangia on a sporophyll .
Pollen
The pollen of the Cheirolepidiaceae is summarized under the name Classopollis . The pollen grain is approximately spherical. At the proximal end there is a triple (three-pointed) scar, at the distal end a circular cryptopore: at the cryptopore the exine is thinner, this is where germination probably took place. Near the equator, shifted towards the distal end, there is a furrow around the pollen grain, the rimula. At the equator there is a thickened band that is striped on the inside. The surface of the pollen grain is sculpted in many ways.
The pollen wall consists of clearly defined nexins and sexins . The inner Nexine layer shows around 20 layers of electron-tight lamellae. It follows the inside of the outer layer, the sexine. This consists of four distinguishable layers. Layer S1 is a uniform, conspicuous layer on the inside. The S1 is significantly reduced on the rimula, the cryptopore and the triple scar. The S2 is also thinner and often perforated. S4 on the very outside is the surface sculpture that sits on the thin S3.
There are often small spheres on the pollen grains that have layers S3 and S4 in their structure. Membranes associated with these suggest that the outer sculptured layers were formed by the tapetum , which would explain the globules as well.
Often the pollen also occurs in tetrads.
The oldest finds from Classopollis come from the Upper Triassic , not before the Carnian . They occur worldwide in low and medium paleo latitudes. The variety of sculptures increases in Jura and Chalk. The most recent finds come from the Upper Cretaceous ( Maastrichtian ).
Female cones
Little is known about the female cones. The best known are the cones from Hirmeriella muensteri . The seed scales carry a single ovule covered by a lobed structure. Up to ten layers of cuticula have been found in various fossils , some with pollen in between. It is assumed that the pollination proceeded differently than via the frequent way that the pollen arrives at the micropyle .
The cone scales of Frenelopsis alata are up to 6 cm wide, a size that is otherwise only found in representatives of the Araucariaceae .
From Tomaxiella also insulated cone scales were found, an indication that the cones disintegrate at maturity and dismissed the individual scales.
Temporal spread
Classopollis occurs in the Upper Triassic (Carnium), initially with a smooth surface and hardly developed equatorial stripes. Towards the end of the Triassic, the classic Classopollis type appears; in Jurassic and Chalk it becomes more diverse in sculpting. The first macrofossils from Hirmeriella muensteri come from the Lower Jura .
Spatial distribution and ecology
Classopollis pollen was widespread in the Mesozoic, especially in lower Paleo latitudes, but rarely or absent towards the Poles. The conditions in today's Russia have been particularly well studied. The pollen reaches its highest density in the Upper Jura. At this time, the pollen north of the 50th palaeo-latitude is very rare with 6 to 10% of the pollen finds, often only single pollen. A warm, temperate and humid climate prevailed here. Subsequently to the south, the Classopollis share increases to up to 50% in a zone with more warmth and drought. Even further south, Classopollis takes up to 90% of the pollen, this always coincides with particularly dry periods. Similar relationships between the frequency of Classopollis and drought are also known from Australia, Africa and South Africa. The accumulation of Classopollis in deposits from receding marine areas is particularly striking .
Pseudofrenelopsis parceramosa occurs in the Potomac and Wealden sediments of the Lower Cretaceous is large in numbers and is accompanied by only a few other plant megafossils. The Wealden sediments are river deposits, while the Potomac sediments depict tidal coastal locations. Pseudofrenelopsis ramosissima occurs in the somewhat younger Potomac layers together with a species-rich flora consisting of ferns , horsetail , non- xeromorphic gymnosperms, as well as flowering plants. Pseudofrenelopsis varians may have been a real halophyte in salt marshes.
supporting documents
- Thomas N. Taylor, Edith L. Taylor, Michael Krings: Paleobotany. The Biology and Evolution of Fossil Plants . Second Edition, Academic Press 2009, ISBN 978-0-12-373972-8 , pp. 831-838.
- Joan Watson: The Cheirolepidiaceae . In: Charles B. Beck (Ed.): Origin and Evolution of Gymnosperms . Columbia University Press, New York 1988, ISBN 0-231-06358-X , pp. 382-447.