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May 16, 2022

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May 16, 2022

Plant Life during the Jurassic

Figure 1 - Jurassic Diorama, Royal Ontario Museum
Credit:Keith Schengili-Roberts, Creative Commons
Attribution-Share Alike 3.0 Unported license
 

If you could be transported to the Jurassic Period, there are a few things that would look familiar, and others that would be strange.  The generally warmer climate of the Jurassic favoured the growth of forests.  Also, the breakup of Pangaea into Gondwana and Laurasia separated previously unified habitats and created new ones, this led to the diversification of plant life in the Jurassic. 

The beginning of the Jurassic Period was marked by the Triassic–Jurassic Extinction Event.  The effects of this extinction event on plant life is still under investigation.  While there appears to have been a turnover in plant biota there was not a widespread mass extinction.  Other research  backs up this observation by showing that the change in plant communities could be explained by local ecological succession. 

The fossil record tells us that plant life during the Jurassic included Gymnosperms such as the ancestors of modern Pinophyta (conifers).  Other seed bearing plant life included Spermatophytes such as Czekanowskiales and Pentoxylales

One thing that you wouldn't see during the Jurassic are angiosperms, i.e. flowering plants.  Although some researchers have claimed to have found the progenitors of flowering plants during the Jurassic, there is, at present , no such evidence.  With that in mind, Let's look at some plants from the Jurassic.

Conifers

Fossils from the end of the Triassic and beginning of the Jurassic indicate that there was a major diversification of conifers at that time with evolution of voltzialeans.  New lineages that unambiguously began or diversified during the included: Araucariaceae, Cheirolepidiaceae, Cupressaceae (cypress trees), Pinaceae (pine trees), Podozamites, Podocarpaceae and Taxaceae (yew trees).  Here are some examples:

Araucariaceae

Figure 2 - Petrified Araucaria mirabilis Cone
Credit: Brocken Inaglory,  Creative Commons Attribution-Share Alike
3.0 Unported, 2.5 Generic, 2.0 Generic and 1.0 Generic license
 

While there are examples of Araucariaceae in Triassic deposits such as the Petrified Forest of Arizona, the oldest definitive records of Araucariaceae are from the Early Jurassic.   By the Middle Jurassic, fossils of Araucariaceae included Araucaria mirabilis, from Argentina, and Araucaria sphaerocarpa, from England.   In Argentina, there is an entire petrified forest of Araucaria mirabilis  at the Cerro Cuadrado Petrified Forest .

Wild examples of Araucariaceae are today confined to the Southern Hemisphere and include trees such as the Norfolk Pine and the Monkey Puzzle Tree.  The fossil record does not record any examples of Araucariaceae in the Northern Hemisphere following the Cretaceous–Paleogene Extinction Event.

Cheirolepidiaceae

Cheirolepidiaceae were a variety of conifer that first arose during the Triassic and went extinct at the end of the Mesozoic Era.   One of the Cheirolepidiaceae is Frenelopsis hohenegger.

Figure 3 - Frenelopsis hohenegger
Credit: Peabody Museum of Natural History
 

A distinct pollen type, assigned to the genus Classopollis, defines Cheirolepidiaceae in the fossil record.  Several members of the family appear to have lived in semi-arid and coastal settings with a high tolerance of saline conditions.   An interesting feature from the fossil evidence is  that the plants of this family apparently required scorpionflies for pollination. 

Cypress Trees

Figure 4 - Reconstruction of Austrohamia minuta
Credit: Rubèn Cunéo, Figure 2 in Escapa, Cunéo, & Axsmith, 2008

Cupressaceae or cypress trees are another family of conifers that first arose during the Jurassic.  Modern cypress trees include junipers, cypress and giant sequoia.   Austrohamia minuta is known from fossils in the Jurassic formations of Argentina.  Research published in 2019 identified another species of the genus, Austrohamia asfaltensis, also in fossils from Argentina.

Pinaceae - Schizolepidopsis and Eathiestrobus

Figure 5 - Fossil Cones of Schizolepidopsisediae
Credit
:Kelly K.S. Matsunaga,Matsunaga et al 2021

Schizolepidopsis is among the earliest genera of Pinaceae (pine trees) in the fossil record.  It appears to be the root genus of all subsequent pine trees.   Schizolepidopsis fossils are first found in the Early Jurassic and flourished throughout Laurasia during the Lower Cretaceous but apparently dying out before the Upper Cretaceous.

Figure 6 - Eathiestrobus mackenziei
Credit: Figure 20 in Rothwell et al, 2012

Another early pine tree was Eathiestrobus is known from fossil pine cones found in the Upper Jurassic Kimmeridge Clay Formation of Scotland. One species of Eathiestrobus has been identified, Eathiestrobus mackenziei.  It was found in Eathie, Scotland by a  Mr. W. Mackenzie, who collected the specimen and donated it to the Hunterian Museum in 1896.

Podozamites

Podozamites fossils are the leaves of an extinct conifer.  First appearing in the Permian, Podozamites became common in what is now East Asia during the Jurassic.  The last Podozamites fossils are from the Late Cretaceous. 

Figure 7 - Podozamites distans
Credit: Ghedoghedo, Creative Commons Attribution-Share Alike 4.0 International license
 

There are about seven identified species of Podozamites.  As well, Podozamites fossils are associated with conifer cones of the genera Swedenborgia, Cycadocarpidium, and Krassilovia.  Podozamites fossils have been found in rocks associated with fluvial flood plains and lagoon environments.

Podocarpophyllum

Figure 8 - Fragmentary Leaves of Podocarpophyllum
Credit: Figure 1 in Nosova & Kiritchkova,2008
 

Podocarpophyllum were a genus within Podocarpaceae family.  Fossils of Podocarpophyllum are known from Middle Jurassic rocks in Uzbekistan and Kazakhstan.  Three species have been identified: P. singulare, P. dorofeevii, and P. mesozoicum.

Marskea

Figure 9 - Marskea heeriana
Credit: Plate 16 in Nosova & Kiritchkova, 2018
 

An early type of yew tree, Marskea is known from Jurassic rocks in Europe and Siberia.  Four species of Marskea have been identified:  M.  jurassica, M. thomasiana, M. latifolia and M. heeriana.  Marskea fossilsare generally found in shales and coal deposited in fluvial and deltaic environments.

Marskea fossils have been found in the Sorthat Formation of Denmark, from Middle Jurassic formations in Yorkshire, England, and from the Middle Jurassic of the Irkutsk Coal Basin.  Fossils of M. latifolia are known from Early Cretaceous rocks of the Lena Basin in Siberia. 

Czekanowskiales

Figure 10 - Leaves of Czekanowskia
Credit: Hongshan Wang Figure 3 in Sun et al, 2015
 

Czekanowskiales are an extinct family of seed bearing trees and shrubs that first arose in the Late Permian and apparently thrived in the Jurassic and Early Cretaceous only to go extinct at the end of the Mesozoic.  Their fossils are found mostly in the Northern Hemisphere in rocks that were deposited in warm-temperate and temperate climates under humid conditions.  

Pentoxylales

Figure 11 - Reconstruction of the Male Shoots of the Pentoxylales
Credit: ucmp.berkeley.edu Figure 1 in Agarwal, 2018
 

Pentoxylales is an extinct order of seed plants known from Jurassic and Cretaceous aged fossils.  Researchers have found fossils of in Pentoxylales in India, New Zealand, Australia and Antarctica, all of which were part of ancient Gondwana.  

Pentoxylales plants were probably small shrubs that grew beside water.  It appears to have grown branched leafy shoots.  After a few seasons of growth, the plants formed a thicket after settling onto the ground or on other stems.

Winding it Up

There are many more fossil plants from the Jurassic than  those I've discussed here.   If this interests you, follow up on some of the links and enjoy.

Standard Caveat

The purpose of my weblog postings is to spark people's curiosity in geology.  Don't entirely believe me until you've done your own research and checked the evidence.  If I have sparked your curiosity in the subject of this posting, follow up with some of the links provided here.  If you want to, go out into the field and examine some rocks on your own with the help of a good field guide.  Follow the evidence and make up your own mind.

In science, the only authority is the evidence.