Review: Marine Paleophysiology – A glimpse into prehistory

The term paleophysiology refers to the scientific study of fossilised biota, in an attempt to gain an insight into the physiological processes that the organism possessed. This specific field of research falls under the broader term, Paleobiology which encompasses a vast array of subdivisions including paleoecology (the study of past ecosystems, climate and geographies in relation to prehistoric organisms) and Evolutionary Developmental Paleobiology which examines the evolutionary aspects of the modes and trajectories of growth and development in the evolution of life. The vast majority of our knowledge of prehistoric organisms arises from this study of their morphology, often through comparisons with the morphologies of extant species to attempt to determine the ancestral relationship between them, and to discover how developmental processes evolved. In comparison to the study of prehistoric organism morphologies, the field of paleophysiology has so far contributed significantly less to our understanding of prehistoric organisms, primarily due to the lack of a physiological fossil record. Not all physiological aspects of an organism can be captured in the fossil record. For example, aspects such as basal metabolic rate and an organism’s optimum temperature cannot be determined through the study of fossilised remains. However, some basic physiological attributes can be inferred from fossils because they are associated with preservable morphological structures and conserved within higher taxa. An example of this is deducing the gas exchange mechanisms of marine invertebrates, through the presence or absence of gills that can be inferred from skeletal fossilised remains.

Thus the study of paleophysiology and our understanding of the field, although somewhat difficult and often impossible to investigate in prehistoric fossilised remains, has shown some success in gaining an insight into these organism’s physiology, often through comparison with the physiology of extant and evolutionary related organisms. For example, perhaps the most significant paper influencing our understanding of paleophysiology was conducted by Knoll et al, (2007) who investigated the physiological reasons behind the Permo-Triassic mass extinction (the earth’s most severe known extinction event) that resulted in the extinction of up to 96% of all marine species and 70% of terrestrial vertebrate species becoming extinct. The conclusions of this study theorised that mass extinction resulted from the synergistic effects of several environmentally-linked stresses on organismic physiology, including global warming, anoxia, and toxic sulphide abundance, with hypercapnia (the physiological consequences of increased PCO2) imparting the principal selectivity observed in the Permian–Triassic fossil record. This theory was inferred through the investigation of the vulnerability of extant species and their subsequent physiological responses to increased levels of PCO2. Such physiological responses include a decrease in the capacity of respiratory pigments to oxygenate tissues and disruption of internal pH balance in animals, which affects the precipitation of carbonate skeletons and, at high concentrations, induces narcosis. Studies have shown that an increase in PCO2 as little as 200ppm results in decreased growth rate, survival, and reproduction when animals are exposed chronically (for weeks or more) in modern oceans (Pörtner et al, 2004). Another example cited by Knoll et al was the exposure of modern day copepod and sea urchin populations to PCO2 above 1000 ppm, that did not immediately die but exhibited both reduced fertilization rates and skeletal pathologies. This lack of immediate death in response to elevated PC02 levels however, does not rule out causes for extinction, as a decrease of only 1% per generation will reduce animal populations to unsustainable sizes in little more than a century (Knoll et al, 2007). It is also noted in the paper that geologists state Ordovician invertebrates precipitated carbonate skeletons beneath an atmosphere thought to contain 10–15 times as much carbon dioxide as today, however it is the rate of change in atmospheric conditions that will cause a detrimental impact on organisms. A rapid increase in PCO2 (as in extinction events) is what has shown to important associated changes such as reduced [CO32−], pH, and carbonate saturation of seawater that will impact organisms physiology, as opposed to gradual increase over millions of years where organisms will adapt their physiologies accordingly.  Knoll’s paper also made comparisons in the effect of hydrogen sulphide poisoning, oxygen depletion, global climate change (all of which occurred during the Permo-Triassic mass extinction)  between modern day organisms to attempt to better understand how these factors may have impacted prehistoric organisms physiology and the ultimate reasoning in their extinction. This paper overall is regarded as significant in furthering our understanding of paleophysiology due to it being scientifically relevant in regards to current environmental change occurring in the modern day (global warming) and furthering our understanding of how prehistoric organisms physiologically responded to global change that could be used to predict future survivorship in a world where anthropogenic induced environmental changes are continuously occurring. However there has been very little subsequent research in regards to Knoll et al’s paper for reasons that are not understood, but further understanding of the field could be increased substantially if research into this subject was continued.

Other papers showcasing our current understanding of paleophysiology include a study conducted by Ruben et al, (2003) entitled ‘Respiratory and reproductive paleophysiology of dinosaurs and early birds’. A similar inference of the physiological characteristics of prehistoric animals (in this case respiratory and metabolic capacities) is used through the identification of shared anatomical structures whose presence is causally linked to specialised functions and comparing these structures in living reptiles, birds, and mammals e.g. although dinosaurs and early birds were likely to have been homeothermic, the absence of nasal respiratory turbinates in these animals indicated that they were likely to have maintained reptile-like (ectothermic) metabolic rates during periods of rest or routine activity.

A study conducted by Bernard et al (2010) entitled ‘Regulation of Body Temperature by Some Mesozoic Marine Reptiles’ investigated the thermoregulatory processes of ichthyosaurs, plesiosaurs, and mosasaurs, through comparison of the oxygen isotope compositions (δ18O) in their tooth phosphate to that in coexisting fish that inhabited the same waters (as opposed to comparisons with extant species as in the previous examples).  The δ18O value of vertebrate phosphate depends on both body temperature and the composition of ingested water, so assuming that both these marine reptiles and fish lived in the same water mass, differences in their δ18O values would reflect differences in body temperature. The δ18O values of Mesozoic ichthyosaurs and plesiosaurs showed that these large predators were able to regulate their body temperature independently of the surrounding water temperature even when it was as low as about 12° ± 2°C. Estimated body temperatures for these prehistoric marine reptiles range from 35° ± 2°C to 39° ± 2°C and encompass those of modern cetaceans suggesting these reptiles possessed a high metabolic rate required for predation and fast swimming over large distances, especially in cold waters.

Paleophysiological study of marine organisms specifically seems to have proved more successful than in the study of terrestrial life, due to a number of factors. Marine invertebrates in particular may be the fossils for which paleophysiology is least developed but most promising. Fossil invertebrates chronicle evolution through more than 500 million years of Earth history, and the fates of faunas are commonly interpreted in the context of changing ocean circulation and chemistry (Knoll et al, 2007). The fossil record of marine life is significantly more extensive than that of terrestrial organisms, due to the ocean providing better fossil forming conditions (i.e sedimentation in the water column covering dead organisms at the benthos), with the majority of fossils being found in sedimentary rocks which were formed from the sediments of rivers, lakes and oceans. From here, Stratigraphy is used to determine the date at which the fossilised organism lived (through deciphering the ages of different sediment layers) as with in the terrestrial environment. Hard shelled molluscs are some of the most preservable marine organisms. These shells are very durable and outlast the otherwise soft-bodied animals that produce them by a very long time (sometimes thousands of years even without being fossilized). Most shells of marine molluscs fossilize rather easily, and fossil mollusc shells date all the way back to the Cambrian period. Most of the fossil record of molluscs consists of their shells, since the shell is often the only mineralised part of a mollusc and are usually preserved as calcium carbonate. Due to the excellent preservation of these shells, they can be used (as mention above) in the comparison with morphologically similar modern day molluscs, the physiologies of which can be tested and thus gain an insight into the physiology of these ancient organisms e.g decreases in the capacity for biomineralization under elevated PCO2 levels (Knoll et al, 2007). A study conducted by Richard Fortey (2000) investigating Late Cambrian to early Ordovician trilobites of the family Olenidae and their evolution of features best understood as evidence of sulfur chemoautotrophic mode of metabolism. These trilobites were shown to possess a specialized adaptation to a low-oxygen, high-sulfur benthic marine habitats, based on the morphology and the areas of rock in which their fossils have been found. The study of their morphological features such as a wide thoracic pleurae, weak musculature, and multiplication of thoracic segments suggest that they may have developed a symbiotic relationship with sulfur bacteria (primarily degeneration of the hypostome suggests they did not need to feed via the mouth).

Overall, our understanding of paleophysiology it would seem is primarily gained through comparisons with the physiology of extant and evolutionary related organisms due to a lack of a physiological fossil record.  This method of studying physiology is not ideal due to it being based on assumptions rather than solid facts, but it is the most reliable method that is currently available to scientists.

– JK

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