Forgotten women of Paleontology: Ava C. Ellisor


Alva C. Ellisor at her laboratory in 1946

At the beginning of 20th century, many universities started admitting women, with different motivations, including the lack of men following WWI and the Soviet Revolution. The oil and gas industry, long stereotyped as a male profession, also openned to women. In 1919, E. T. Dumble, vice-president and general manager of the Rio Bravo Oil Company, called the geology department at the University of California at Berkeley and asked Professor Bruce Clark for the name of his best man in paleontology. Clark said, “I haven’t a man, would a woman do?”. A year later, Dumble hired Esther Applin (née Richards). Almost at the same time, Wallace Prat, hired Alva Ellisor to build a paleontology department for Humble Oil Company. In 1921, Raymond Baker, from The Texas Company, hired Hedwig Kniker. As part of the consortium agreement between these companies, Richards, Ellisor and Kniker started working on macropaleontologic solutions to solve the Gulf Coast stratigraphy. Dumble believed that macrofossils were the key to understand the chaotic Tertiary stratigraphy of the Gulf Coast. But macrofossils were too badly broken to be identifiable as to species, so Ellisor and Richards turned their attention to the microfossils, especially foraminifera. It was the beggining of the micropaleontological revolution.

Hedwig Kniker, Esther Applin and Alva Ellisor. From Wikipedia

Alva Christine Ellisor was one of the first female stratigrapher in U.S. She was born on April 26, 1892, in Galveston, Texas. In 1915, she received her Bachelor of Arts Degree in Geology from The University of Texas. After graduating, she taught science at Ball High School in Galveston. In 1918 Ellisor began working as a professor at the University of Texas. A year later, she worked as a geologist for the Kansas Geological Survey. In 1920 she began to work for Humble Oil & Refining Company, until her retirement in 1947.

Hedwig Kniker, Esther Richards and Alva Ellisor. From Gries, 2020

In 1924, at a meeting of the American Association of Petroleum Geologists (AAPG), Alva Ellisor, Esther Applin née Richards, and Hedwig Kniker presented their seminal paper: Subsurface stratigraphy o f the Coastal Plain of Texas and Louisiana. Since then, Micropaleontology was quickly embraced by industry. However, the role of these women was downplayed over time, and by 1975 the credit for this technology was shifted to four men.

From left to right: Esther Richards, John Suman, James L. Ballard, Alva Ellisor at Hidalgo Bluff near the Brazos River Botton. (From R.R.Gries. 2018)

Ellisor was one of the founders of the Houston Geological Society in 1923. She was also one of the founding member of the Society of Economic Paleontology and Mineralogy (SEPM) in 1927, along with Richards and Kniker. In 1929 she was elected a Fellow of the Geological Society of America. In 1962, she received the Distinguished Alumni Award from the Geology Department of The University of Texas at Austin. Thomas Barrow, senior vice president at Exxon Corporation, remarked her contributions to the field: “Miss Ellisor not only had to prove to her company, but also to convince the rest of the geologic profession that micropaleontology was important”.

She died on September 22, 1964 in Galveston, Texas.


Richards Applin, Esther; Ellisor, Alva E.; Kniker, Hedwig T. (1925). “Subsurface Stratigraphy of the Coastal Plain of Texas and Louisiana”. American Association of Petroleum Geologists Bulletin. 9 (1): 79–122.

Gries, Robbie Rice (2018). “How female geologists were written out of history: The micropaleontology breakthrough”. Women and Geology: Who Are We, Where Have We Come from, and Where Are We Going?. doi:10.1130/2018.1214(02).

Gries, R. R. (2020). Buried Discoveries of Early Female Petroleum Geologists. Geological Society, London, Special Publications, SP506–2019–216. doi:10.1144/sp506-2019-216 

The ‘ghost’ fossils of the future past

Fossil Coccolithophores. Image Credit: S.M. Slater, P. Bown et al / Science journal

Calcareous nannoplankton represent a major component of oceanic phytoplankton, ranging in size from 0.25 to 30 μm. The first records are from the Late Triassic. Their calcareous skeletons can be found in fine-grained pelagic sediments in high concentrations and the biomineralization of coccoliths is a globally significant rock-forming process. This heterogeneous group includes coccoliths, discoasters and nannoconids. They are crucial elements for our understanding of past and present oceans. Their skeletons take up chemical signals from the sea water, in particular isotopes of oxygen and carbon.  In the Jurassic and Cretaceous oceans, the calcareous nannoplankton was the most efficient rock-forming group, for that reason the characterization of calcareous nannofloras in OAE (Oceanic Anoxic Events) intervals are used to improve our understanding of the marine ecosystem and biological processes such as photosynthesis (biological pump) and biomineralisation (carbonate pump) that affect the organic and inorganic carbon cycle, as well as adsorption of atmospheric CO2 in the oceans (Erba, 2013). 

Schematic representation of a generic coccolithophore cell. From Flores & Sierro, 2013.

Coccolithophores are unicellular marine golden-brown algae differing from other Chrysophyta in having two flagella and a third flagella-like appendage called a haptonema. They also posses calcified scales, called coccoliths, at some stage in their life as a protective armour that eventually falls to the ocean floor to build deep-sea ooze and fossil chalks. Declines in the abundance of nannofossils through several past global warming events were linked to biocalcification crises caused by climate change and ocean acidification. Now, a new study presents a global record of ‘ghost’ nannofossils that reveals that  nannoplankton were more resilient to past warming events than traditional fossil evidence would suggest.

Scanning electron microscope images of calcareous nannofossil imprints preserved on the surface of organic matter; Toarcian (including T-OAE interval), Yorkshire, UK. From Slater et al., 2022

The new study focused on the Toarcian Oceanic Anoxic Event (T-OAE ∼183 mya) considered as one of the most severe of the Mesozoic era. This event is associated with a major negative carbon isotope excursion, mass extinction, marine transgression and global warming caused by massive volcanism in the Southern Hemisphere. Previous studies had indicated that during the peak of this event calcareous nannofossils collapsed due to ocean acidification. But the new research have found that these fossils had been overlooked due to their tiny size and their mode of preservation.

Ghost nannofossils  in rocks from the T-OAE, OAE1a and OAE2. Credit: S.M. Slater et al., 2022

After their death, nannofossils were buried in soft sediment at the bottom of the sea, while their imprints were preserved in the surfaces of other organic matter, such as pollen or spores. These imprints—or “ghost”—nannofossils were found in sediments through the TOAE in the UK, Germany, Japan and New Zealand, but also from two similar global warming events in the Cretaceous: the early Aptian Oceanic Anoxic Event (OAE1a, 120 Ma) from Sweden, and the Oceanic Anoxic Event 2 (OAE2, 94 Ma) from Italy. These findings provide new tools to understand how the calcareous nannoplankton respond to warming events.



S. M. Slater, P. Bown, R. J. Twitchett, S. Danise, V. Vajda, Global record of ‘ghost’ nannofossils reveals plankton resilience to high-CO2 and warming, Science (2022).

Elisabetta Erba, Calcareous nannofossils and Mesozoic oceanic anoxic events, Marine Micropaleontology 52 (2004) 85 – 106

Doney, D. S. Busch, S. R. Cooley, K. J. Kroeker, The impacts of ocean acidification on
marine ecosystems and reliant Human communities. Annu. Rev. Environ. Resour. 45, 83
112 (2020).

J.-A. Flores, F.J. Sierro, Flores, PALEOCEANOGRAPHY, BIOLOGICAL PROXIES| Coccolithophores. (2013): 783-794,


The Middle Eocene Climatic Optimum and the Patagonian floras.

Spore-pollen species from the Eocene of southern South America. From Fernandez et al., 2021

The geological records show that large and rapid global warming events occurred repeatedly during the course of Earth history. Ecological models can predict how biodiversity is affected by those events, but only the fossil record provides empirical evidence about the impact of rising temperatures and atmospheric CO2 on species diversity.

The Middle Eocene Climatic Optimum (MECO, ~40 Ma) was a transient period of global warming that interrupted the general cooling trend initiated at the end of the early Eocene climate optimum (EECO, ~49 Ma). The MECO is related to major oceanographic and climatic changes in the Neo-Tethys and also in other oceanic basins, and lasted about 500–600 Kyr. The MECO altered the pelagic ecosystem with repercussions on the food web structure. The lack of nutrients in the surface waters led to a significant decrease in planktonic foraminiferal accumulation rates, while autotroph nannoplankton accumulation rates remained stable.

Relative frequency of the most common plant groups across the MECO and Late Eocene. From Fernández et al., 2021

The MECO also influenced terrestrial biotas. A new study quantify the response of the floras of southern Patagonia to this warming event. The samples were collected from the Río Turbio Formation in southern Patagonia. The terrestrial palynological assemblage revealed a clear inverse relationship between the abundance of ferns and angiosperms. At the beginning of the MECO, ferns highly increase in abundance (with Cyatheaceae, Dicksoniaceae, and Osmundaceae as the most frequent families), while the abundance of angiosperms decreases dramatically. Podocarpaceae also increases from ~5 % to ~20%. At the core of MECO, ferns drop to a minimum, and angiosperms become dominant. Finally, at the end of the MECO ferns rise again to maximum values and angiosperms decrease.

Palynological analysis also revealed that floras in southern Patagonia were in average ~40% more diverse during the MECO than pre-MECO and post-MECO intervals. The penetration of neotropical migrant species to the highest latitudes along with the persistence of southern Gondwanan natives may have triggered the gradual increasing diversity that can be observed across the MECO.



Fernández, D.A., Palazzesi, L., González Estebenet, M.S. et al. Impact of mid Eocene greenhouse warming on America’s southernmost floras. Commun Biol 4, 176 (2021).

Giorgioni, M., Jovane, L., Rego, E.S. et al. Carbon cycle instability and orbital forcing during the Middle Eocene Climatic Optimum. Sci Rep 9, 9357 (2019).

Sonal Khanolkar, Pratul Kumar Saraswati & Karyne Rogers (2017) Ecology of foraminifera during the middle Eocene climatic optimum in Kutch, India, Geodinamica Acta, 29:2,181-193, DOI: 10.1080/09853111.2017.130084


Learning from Past Climate Changes

In the last 540 million years, five mass extinction events shaped the history of the Earth. Those events were related to extreme climatic changes and were mainly caused by asteroid impacts, massive volcanic eruption, or the combination of both.  On a global scale the main forces behind climatic change are: solar forcing, atmospheric composition, plate tectonics, Earth’s biota, and of course, us. Human activity is a major driver of the dynamics of Earth system. From hunter-gatherer and agricultural communities to the highly technological societies of the 21st century, humans have driven the climate Earth system towards new, hotter climatic conditions. Until the Industrial Revolution, the average global CO2 levels fluctuated between about 170 ppm and 280 ppm. But with the beginning of the Industrial Era, that number risen above 300 ppm, currently averaging an increase of more than 2 ppm per year. The average monthly level of CO2 in the atmosphere in last April exceeded the 410 ppm for first time in history. Thus we could hit an average of 500 ppm within the next 45 years, a number that has been unprecedented for the past 50–100+ million years according to fossil plant-based CO2 estimates. This current human-driven change far exceed the rates of change driven by geophysical or biosphere forces that have altered the Earth System trajectory in the past, and it poses severe risks for health, economies and political stability. Learning from past climatic changes is critical to our future.

Planktonic foraminifera from the Sargasso Sea in the North Atlantic Ocean. (Photograph courtesy Colomban de Vargas, EPPO/SBRoscoff.)

Microfossils from deep-sea are crucial elements for the understanding of our past and present oceans. Their skeletons take up chemical signals from the sea water, in particular isotopes of oxygen and carbon. Over millions of years, these skeletons accumulate in the deep ocean to become a major component of biogenic deep-sea sediments. The importance of microfossils as tool for paleoclimate reconstruction was recognized early in the history of oceanography. John Murray, naturalist of the CHALLENGER Expedition (1872-1876) found that differences in species composition of planktonic foraminifera from ocean sediments contain clues about the temperatures in which they lived. The ratio of heavy and light Oxygen in foraminifera shells can reveal how cold the ocean was and how much ice existed at the time the shell formed. Another tool to reconstruct paleotemperatures is the ratio of magnesium to calcium (Mg/Ca) in foraminiferal shells. Mg2+ incorporation into foraminiferal calcite  is influenced by the temperature of the surrounding seawater, and the Mg/Ca ratios increase with increasing temperature.

Diatoms and radiolarians are susceptible to different set of dissolution parameters than calcareous fossils, resulting in a different distribution pattern at the sea floor and have been used for temperature estimates in the Pacific and in the Antarctic Oceans, especially where calcareous fossils are less abundant. Diatom assemblages are also used in reconstructions of paleoproductivity.

Scanning electron microscope image of different types of pollen grains. Image from Wikipedia.

Pollen and other palynomorphs proved to be an extraordinary tool to paleoenvironmental reconstruction too. Pollen analysis involves the quantitative examination of spores and pollen at successive horizons through a core, specially in lake, marsh or delta sediments, especially in Quaternary sediments where the parent plants are well known. This provide information on regional changes in vegetation through time, and it’s also a valuable tool for archaeologists because it gives clues about man’s early environment and his effect upon it.

Stomatal frequency of land plants, which has been shown in some species to vary inversely with atmospheric pCO2, has been used to estimate paleo-pCO2 for multiple geological time periods. Stomata are the controlled pores through which plants exchange gases with their environments, and play a key role in regulating the balance between photosynthetic productivity and water loss through transpiration.

Temple I on The Great Plaza and North Acropolis seen from Temple II in Tikal, Guatemala. From Wikimedia Commons

Paleoecological records indicate that the transition to agriculture was a fundamental turning point in the environmental history of Mesoamerica. Tropical forests were reduced by agricultural expansion associated with growing human populations. Also soil loss associated with deforestation and erosion was one of the most consequential environmental impacts associated with population expansion in the Maya lowlands. This environmental crisis ended with the collapse of the Classic Maya society.

Human activity has significantly altered the climate in less than a century. Since 1970 the global average temperature has been rising at a rate of 1.7°C per century, and the rise in global CO2 concentration since 2000 is 10 times faster than any sustained rise in CO2 during the past 800,000 years. Today the most politically unstable countries are also places where environmental degradation affected food production and water supply. Other human societies have succumbed to climate change – like the Akkadians – while others have survived by changing their behavior in response to environmental change. We have the opportunity to protect the future of our own society by learning from the mistakes of our ancestors.


David Evans, Navjit Sagoo, Willem Renema, Laura J. Cotton, Wolfgang Müller, Jonathan A. Todd, Pratul Kumar Saraswati, Peter Stassen, Martin Ziegler, Paul N. Pearson, Paul J. Valdes, Hagit P. Affek. Eocene greenhouse climate revealed by coupled clumped isotope-Mg/Ca thermometry. Proceedings of the National Academy of Sciences, 2018; 201714744 DOI: 10.1073/pnas.1714744115

Nicholas P. Evans et al., Quantification of drought during the collapse of the classic Maya civilization, Science (2018); DOI: 10.1126/science.aas9871 

Will Steffen, et al.; Trajectories of the Earth System in the Anthropocene; PNAS (2018) DOI: 10.1073/pnas.1810141115

The EECO, the warmest interval of the past 65 million years.

Cenozoic strata on Seymour Island, Antarctica (© 2016 University of Leeds)

Cenozoic strata on Seymour Island, Antarctica (© 2016 University of Leeds)

During the last 540 million years, Earth’s climate has oscillated between three basic states: Icehouse, Greenhouse (subdivided into Cool and Warm states), and Hothouse (Kidder & Worsley, 2010). The “Hothouse” condition is relatively short-lived and is consequence from the release of anomalously large inputs of CO2 into the atmosphere during the formation of Large Igneous Provinces (LIPs), when atmospheric CO2 concentrations may rise above 16 times (4,800 ppmv), while the “Icehouse” is characterized by polar ice, with alternating glacial–interglacial episodes in response to orbital forcing. The ‘Cool Greenhouse” displays  some polar ice and alpine glaciers,  with global average temperatures between 21° and 24°C. Finally, the ‘Warm Greenhouse’ lacked of any polar ice, and global average temperatures might have ranged from 24° to 30°C.

Reconstructions of Earth’s history have considerably improved our knowledge of episodes of rapid emissions of greenhouse gases and abrupt warming. Consequently, the development of different proxy measures of paleoenvironmental parameters has received growing attention in recent years.

A) Scanning electron microphotographs of fossil Ginkgo adiantoides cuticle showing stomata (arrows) and epidermal cells. B) Scanning electron microphotographs of modern Ginkgo biloba cuticle.

A) Scanning electron microphotographs of fossil Ginkgo adiantoides cuticle showing stomata (arrows) and epidermal cells. B) Scanning electron microphotographs of modern Ginkgo biloba cuticle (From Smith et al. 2010)

The early Eocene was characterized by a series of short-lived episode  of global warming, superimposed on a long-term early Cenozoic warming trend. Atmospheric CO2 was the major driver of the overall warmth of the Eocene. For  the  Paleocene-Eocene  Thermal  Maximum (PETM; 55.8 million years ago), and the Early Eocene Climate Optimum (EECO; 51 to 53 million years ago) the transient rise of global temperatures has been estimated to be 4 to 8° (Hoffman et al., 2012).

Reconstructions using multiple climate proxy records, identified the EECO as the warmest interval of the past 65 million years. One such proxy measure is the stomatal frequency of land plants, which has been shown in some species to vary inversely with atmospheric pCO2 and has been used to estimate paleo-pCO2 for multiple geological time periods. Stomata are the controlled pores through which plants exchange gases with their environments, and play a key role in regulating the balance between photosynthetic productivity and water loss through transpiration. (Smith et al., 2010).

Sin título

Foraminiferal assemblage of the EECO (From KHANOLKAR and SARASWATI, 2015)

Pollen and other palynomorphs proved to be an extraordinary tool to palaeoenvironmental reconstruction. Terrestrial  microflora from the EECO indicates a  time  period  with  warm  and  humid  climatic  conditions and displays a higher  degree  of tropicality  than the microflora of  the PETM.

A new high-fidelity record of CO2 can be obtained by using the boron isotope of well preserved planktonic foraminifera. The boron isotopic composition of seawater is also recquiered to estimate the pH. The global mean surface temperature change for the EECO is thought to be ~14 ± 3 °C warmer than the pre-industrial period, and ~5 °C warmer than the late Eocene.

Evolution of atmospheric CO2 levels and global climate over the past 65 million years

Evolution of atmospheric CO2 levels and global climate over
the past 65 million years (From Zachos et al., 2008)

Since the start of the Industrial Revolution the anthropogenic release of CO2 into the Earth’s atmosphere has increased a 40%. Glaciers  from the Greenland and Antarctic Ice Sheets are fading away, dumping 260 billion metric tons of water into the ocean every year. The ocean acidification is occurring at a rate faster than at any time in the last 300 million years, and  the patterns of rainfall and drought are changing and undermining food security which have major implications for human health, welfare and social infrastructure. These atmospheric changes follow an upward trend in anthropogenically induced CO2 and CH4. If  fossil-fuel emissions continue unstoppable, in less than 300 years pCO2 will reach a level not present on Earth for roughly 50 million years.



Eleni Anagnostou, Eleanor H. John, Kirsty M. Edgar, Gavin L. Foster, Andy Ridgwell, Gordon N. Inglis, Richard D. Pancost, Daniel J. Lunt, Paul N. Pearson. Changing atmospheric CO2 concentration was the primary driver of early Cenozoic climate. Nature, 2016; DOI: 10.1038/nature17423

Zachos, J. C., Dickens, G. R. &  Zeebe, R. E. An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics. Nature 451, 279283(2008)

Loptson, C. A., Lunt, D. J. & Francis, J. E. Investigating vegetation-climate feedbacks during the early Eocene. Clim. Past 10, 419436 (2014)

Robin Y. Smith, David R. Greenwood, James F. Basinger; Estimating paleoatmospheric pCO2 during the Early Eocene Climatic Optimum from stomatal frequency of Ginkgo, Okanagan Highlands, British Columbia, Canada; Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 293, Issues 1–2, 1 (2010).

Application of diatoms to tsunami studies.

Lisbon earthquake and tsunami in 1755 (From Wikipedia Commons)

Lisbon earthquake and tsunami in 1755 (From Wikipedia Commons)

Diatoms are unicellular algae with golden-brown photosynthetic pigments with a fossil record that extends back to Early Jurassic. The most distinctive feature of diatoms is their siliceous skeleton known as frustule that comprise two valves. They live in aquatic environments, soils, ice, attached to trees or anywhere with humidity and their remains accumulates forming diatomite, a type of soft sedimentary rock. Diatoms are the dominant marine primary producers in the oceans and play a key role in the carbon cycle and in the removal of biogenic silica from surface waters. But diatoms are also a valuable tool in reconstructing paleoenvironmental changes because of their sensitivity to environmental factors including salinity, tidal exposure, substrate, vegetation, pH, nutrient supply, and temperature found in specific coastal wetland environments. Through years, diatoms become part of the coastal sediments, resulting in buried assemblages that represent an environmental history that can span thousands of years. Diatoms alone cannot differentiate tsunami deposits from other kinds of coastal deposits, but they can provide valuable evidence for the validity of proposed tsunami deposits (Dura et al., 2015).

Electron microscope image of Diatoms from high altitude aquatic environments of Catamarca Province, Argentina (From Maidana and Seeligmann, 2006)

Electron microscope image of Diatoms from high altitude aquatic environments of Catamarca Province, Argentina (From Maidana and Seeligmann, 2006)

Tsunami deposits can be identify by finding anomalous sand deposits in low-energy environments such as coastal ponds, lakes, and marshes. Those anomalous deposits are diagnosed using several criteria such as floral (e.g. diatoms) and faunal fossils within the deposits. The delicate valves of numerous diatom species may be unusually well preserved when removed from surface deposits and rapidly buried by a tsunami.

Diatoms within the tsunami deposits are generally composed of mixed assemblages, because tsunamis inundated coastal and inland areas, eroding, transporting, and depositing brackish and freshwater taxa. Nonetheless, problems differentiating autochthonous (in situ) and allochthonous (transported) diatoms complicates reconstructions. In general, planktonic diatoms are considered allochthonous components in modern and fossil coastal wetland assemblages, while benthic taxa can be considered as autochthonous. Diatoms can also be used to estimate tsunami run-up  by mapping the landward limit of diatom taxa transported by the tsunami.



Hemphill-Haley, E., 1996. Diatoms as an aid in identifying late Holocene tsunami deposits. The Holocene 6, 439–448.

Tina Dura, Eileen Hemphill-Haley, Yuki Sawai, Benjamin P. Horton, The application of diatoms to reconstruct the history of subduction zone earthquakes and tsunamis, Earth-Science Reviews 152 (2016) 181–197. DOI: 10.1016/j.earscirev.2015.11.017

Armstrong, H. A., Brasier, M. D., 2005. Microfossils (2nd Ed). Blackwell, Oxford.

Barron, J.A. (2003). Appearance and extinction of planktonic diatoms during the past 18 m.y. in the Pacific and Southern oceans. “Diatom Research” 18, 203-224