Mesozoic
Databases


Late Jurassic climate, vegetation and dinosaur distribution
Paleoecology, middle Cretaceous Grebenka flora, Siberia

Introduction

The Mesozoic era encompasses the rise of angiosperms (flowering plants), as well as the rise and demise of dinosaurs. Geographically, the Pangean supercontinent of the Paleozoic had fragmented into the southern and northern landmasses of Gondwana and Laurasia. In terms of global climate and vegetation, there were no polar ice sheets and no equatorial rainforests. This 'hothouse' world began in the Permian and persisted for ~200 million years, to at least the late Mesozoic. The effects of this were most pronounced in the Jurassic and Cretaceous, which have been the focus of my Mesozoic research.


Paleogeography


A knowledge of the paleogeographic context is essential to properly understand global patterns of climate change, faunal and floral distributions, ecology, evolution and extinctions. For my Mesozoic work, I've used maps produced by Fred Ziegler and David Rowley (PGAP), and Chris Scotese (PALEOMAP). Two PGAP maps are shown here - click either map to enlarge. Contact David Rowley for details.

Tithonian (Late Jurassic)

Maastrichtian (Late Cretaceous)


Paleoclimate


I've used fossil plants, specifically leaf remains, to interpret global patterns of climate for different geologic intervals. I've examined the Late Jurassic in detail, using sedimentary as well as floral data. I chose this interval because it represents the epitome of an ice-free world. The lithological indicators are fairly straightforward to interpret (e.g. coals, precipitation > evaporation; evaporites, evaporation > precipitation), but the floral data require a greater level of understanding in terms of fossil plant taxonomy, biological and ecological adaptations, as well as more sophisticated techniques in order to unravel overall biogeographic patterns and climate signals.

In the Jurassic, five main biomes are recognised from the data: seasonally dry (summerwet or subtropical), desert, seasonally dry (winterwet), warm temperate and cool temperate. The boundaries between them remained at near-constant palaeolatitudes while the continents moved through them (south, in the case of Asia, and north, in the case of North America). Net global climate change throughout the Jurassic appears to have been minimal. The data-derived results can be compared with a climate simulation for the Late Jurassic (see below). The use of more detailed palaeogeography and palaeotopography has improved the overall data/model comparisons. Major discrepancies persist at high latitudes, however, where the model predicts cold temperate conditions far beyond the tolerance limits indicated by the plants.


Late Jurassic (Tithonian) climate-sensitive sediment distributions. This is one of eleven stage-length intervals of the Jurassic Period for which PGAP compiled this level of global data. Based on the global distributions of lithological climate indicators such as coals (precipitation > evaporation) and oil source rocks, as well as evaporites (evaporation > precipitation) and carbonates, a general symmetry of climate zones about the (paleo)equator is apparent throughout the Jurassic.


Late Jurassic (Tithonian) plant localities and terrestrial climate-sensitive sediment distributions. An important biogeographic and climatic data source comes from fossil plants. Paleobotanical data provide the best means of interpreting terrestrial palaeoclimates, often revealing important information in the continuum between 'dry' and 'wet' end-member lithological indicators such as evaporites and coals. Leaves are a plant's means of interacting directly with the atmosphere, and their morphology is often attuned to and reflects prevailing environmental conditions.

The combined floral and lithological data were used to determine terrestrial climate zones, or biomes. I used a classification in which the 'macroclimate' of the present-day land surface is expressed in terms of ten major biomes (Walter, 1985). The classification is simple and therefore more readily applicable in the geologic past, for which detailed knowledge of biomes is limited. The result is a series of maps showing biomes, or climate zones, for the Jurassic. These provide a direct means of evaluating the corresponding climate general circulation models. Moreover, these biomes provide the basis for a more complete understanding of the distributional patterns of other organisms, including Jurassic dinosaurs.

Late Jurassic biomes are shown below. View the pdf files of my publication for other Jurassic biomes, and learn more about my methods.


Collaboration with Paul Valdes enabled us to make detailed comparisons between the data- and model-derived biomes for the Late Jurassic. Overall comparison between the data and model is encouraging, maintaining the broad pattern of summerwet equatorial regions, succeeded polewards by desert then warm and cool temperate biomes. In the tropics, there are a few grid points predicting tropical rainforest type biomes, but there are no data near any of these grid points and so the model could be correct. The rest of the tropics are predicted to be summerwet, which is generally in good agreement with the data.

Predicted biomes for the Late Jurassic, based on model results for temperature, precipitation and soil moisture patterns
 

In mid- to high latitudes, the most striking feature of the Northern Hemisphere is that the model predicts more extensive regions of winterwet climates and more restricted regions of warm temperate climates. In some senses, the distinction between these climates is relatively subtle and so perhaps the model error is correspondingly small here. Nonetheless, the model is somewhat too cool at these latitudes. The tendency of the model to be too cool is more striking in the Southern Hemisphere, where overall data and model agreement at high latitudes is poor. On the coast itself, the agreement is reasonable and it can be argued that this suggests that our choice of sea surface temperatures is acceptable. However, in the southern interior of the continent the model is predicting a mixture of cool and cold temperate climates and very few areas of warm temperate climates. As with northern high latitudes, the model is clearly predicting temperatures which are too cold.

The data/model comparison clearly suggests that the model is too cold at high latitudes in both hemispheres. Changing CO2 levels and sea surface temperatures could make some difference, but is unlikely to completely resolve the problem. It appears to be another example of the 'equable climates' issue noted for the Cretaceous and Eocene. Recent work has suggested that some of the disagreement can be reconciled by including feedbacks between the climate and vegetation. This effect is also likely to be important for the Jurassic and we are in the process of incorporating this in our simulations.



Late Jurassic Dinosaur Distributions

I applied my knowledge of the global record of Jurassic climate-sensitive sediments and plants to address broadscale patterns of dinosaur distributions during the Late Jurassic, in collaboration with Chris Noto, Mike Parrish, and Judy Parrish. The global patterns provided a context for our more detailed accounts of the Morrison and Tendaguru Formations in North America and East Africa. View a pdf of our publication for details.

Most Late Jurassic dinosaur remains are known from low-latitude to marginally midlatitude regions where plant fossils are generally sparse and evaporites common. Conversely, few dinosaur remains are known from mid- to high latitudes, which have higher floral diversities and abundant coals. Hence, there is an obvious geographic mismatch between known dinosaur distributions and their primary food source. This may be due to taphonomic bias, indicating that most dinosaur discoveries provide only a small window on the diversity and lifestyles of this group.

Late Jurassic (150 Ma) paleogeographic maps (Mollweide projection with 30 degree latitude and longitude lines; courtesy of Chris Scotese). A, Plant localities, scaled according to the number of constituent genera. B, Evaporite and coal deposits. C, Dinosaur localities, scaled according to the number of constituent taxa and showing the location of the Morrison (M) and Tendaguru (T) Formations.




Distributions of dinosaur taxa (A) and plant genera (B) in 10 degree paleolatitudinal bins

On the basis of our global- and local-scale studies, we suggest that dinosaur preservation was favored in environments toward the drier end of the climate spectrum, where savannas rather than forests predominated. A holistic approach, incorporating climate and vegetation as well as geography, is required to better understand patterns of dinosaur ecology and evolution.


Plant Taxonomy and Paleoecology


In many ways this is where it all began for me, with my PhD studies on "Palaeobotanical contributions to the Mesozoic geology of the northern Antarctic Peninsula region" (Rees, 1990). I was funded by the UK Natural Environment Research Council and supervised by Bill Chaloner (University of London) and Mike Thomson (British Antarctic Survey). I studied the taxonomy, age and environmental setting of Jurassic and Cretaceous floras from the northern Antarctic Peninsula region. These provided the sole basis for dating terrestrial sequences in the region and understanding its paleogeography and volcanic arc evolution. Results necessitated the revision of previous paleogeographic models as well as gondwanan biostratigraphy, phytogeography and climates. You can find out more here.

I spent three months camping in Antarctica (at Hope Bay and Botany Bay) with my field assistant Paul Wood, collecting fossils and measuring sedimentary sections, as part of the BAS 1986/1987 field program. Results provided the basis for extensive taxonomic revision of the Hope Bay flora and documentation of the new flora from Botany Bay. They also led to a significant age revison (from Early Cretaceous to Early Jurassic) of the floras.


Map of Hope Bay and Botany Bay. Inset shows the Early Jurassic paleogeographic location of these sites.

(1) Botany Bay, looking west. The plant beds are in the foreground. (2) Mount Flora, Hope Bay.



The Hope Bay and Botany Bay plants are preserved as leaf impressions and coalified compressions. No cuticle or palynomorphs have been recovered. Some of the fern and cycadophyte fossils are shown here.

The Hope Bay and Botany Bay floras comprise stems of sphenophytes (Equisetum) as well as fern and gymnosperm foliage (Sagenopteris, pteridosperms, cycadophytes and conifers).  No unequivocal ginkgophytes or macrophyllous conifers have been found, despite the intensive collecting of many specimens from these localities. The lithologies in which these plants are preserved represent floodplain deposits, with the silts and fine sands settling out, along with the plant remains, from slow-moving bodies of water.  Their deposition was often punctuated by fining-upward sequences of coarse and medium grained sands which represent crevasse splay deposits, along with mudstones representing lacustrine deposits.

The figure below shows changes in plant associations at the different horizons I sampled systematically at Botany Bay.  Rather than reproduce the entire 115 m measured section in detail, only the main plant-bearing horizons are shown; intervening mudstone, coarse sandstone and conglomerate lithologies, which were either barren or contained only unidentifiable stems and other fragments, have been excluded.  There is no discernible difference in lithology between the horizons shown, most of them being siltstones, with rare very fine or fine-grained sandstones. Consequently, the figure is not to scale; several horizons may be within a metre of each other and there may be several metres between individual horizons elsewhere.

Relative abundances of field-identified specimens were recorded for each horizon, based on field frequency estimates. These were refined, where possible, to species-level estimates based on laboratory identifications.  Species were then grouped into five categories, based broadly on a combination of their taxonomic and morphological characters: (1) microphyllous cycadophytes and microphyllous conifers, as well as Pachypteris indica (with its apparently coriaceous lamina and sunken stomata), (2) 'ferns', including Archangelskya furcata, (3) macrophyllous cycadophytes, (4) Caytoniales, and (5) Equisetales:


The available evidence shows that the 'microphylls' (category (1), possibly adapted to drier conditions, with foliar adaptations for minimising water loss), and those such as the macrophyllous cycadophytes and Equisetum (categories (3) and (5), which probably grew in wetter environments) tend to be mutually exclusive. This could be interpreted as reflecting fluctuations in the regional climate between dry and ever-wet conditions.  Alternatively, the assemblages apparently adapted to drier conditions may represent the vegetation of well-drained areas distal to the ever-wet depositional sites, with periodically higher rainfall or storms resulting in their transport to the depositional sites.  Indeed, there is no lithological evidence for periodically dry climate conditions in the region.  The presence of braided stream, debris flow and sheet-flood sedimentary deposits at both Botany Bay and Hope Bay suggests at least seasonally high rainfall with periodic flooding. The near-continuous presence of ferns (which usually need moist conditions for growth and reproduction, as well as being relatively fragile and thus unlikely to survive significant transportation) also indicates that the depositional sites were everwet.  Furthermore, the variation in horizon abundance of Equisetum from 0-100 per cent indicates absence, variable mixing with other taxa, or in situ dominance, again most probably as a result of variations in flow patterns.

I therefore envisage environments at Botany Bay (and Hope Bay) in which Equisetum, ferns, Sagenopteris and macrophyllous cycadophytes were growing fairly near to where they were buried.  Major flood events would have occasionally carried distal microphyllous forms down to these sites.  Although further fieldwork is needed to confirm this model, I believe that the changing plant associations reflect complex variations in floodplain deposition rather than regional climate change.

Shown below is my taxonomic revision of the Hope Bay flora, and the first comprehensive taxonomic list for the Botany Bay flora. Previously described species accepted here, but not present in the new material, are shown in parentheses. Thirty seven species are now recognised in the Hope Bay flora and 32 from Botany Bay. The floras are closely similar; 80% of the Botany Bay species also occur at Hope Bay.



View PDF files of some of my other Mesozoic publications:


Mesozoic
Databases