The end-Cretaceous extinction event wiped out 76% of known species on Earth, but was strangely selective in the way it did so. The non-Avian Dinosaurs were wiped out, as were the Pterosaurs, most Marine Reptiles, Ammonites, Belemnites, and Rudists, amongst other groups. The extinction event is believed to have been caused by a bolide impacting the Gulf of Mexico near the Yucatan Peninsula, creating the Chicxulub Impact Crater. Evidence of the direct effects of this impact, including impact glass fallout, large-scale forest fires and tsunamis, have been found in areas of North America more than 3500 km from the impact site, and the subsequent events are thought to have included a global climatic breakdown which lasted for thousands of years.
The Tanis Event Deposits of North Dakota record a seiche event (tsunami-like wave within an enclosed environment such as a river valley or small lake) including a significant death assemblage of latest Cretaceous fauna, directly overlying the End Cretaceous Hell Creek Formation. This is thought to represent the very end of the Cretaceous, containing the fossils of organisms that died and to have been burried more-or-less instantly in a seiche event brought about by that impact. The reconstruction scenario is that within a few tens of minutes of the impact, a large volume of water and soil was forced upstream from the Tanis Estuary, pushing with it large volumes of marine, freshwater, and terrestrial organisms, whilst also accumulating impact spherules (spherical silica particles formed as melted droplets of rock recrystalise in mid air) which were falling from above. Within this deposit were large numbers of Acipenseriform Fish (Sturgeon and Paddlefish), which were buried alive in alignment with the flow of the seiche, and which had numerous impact spherules trapped within their gills.
In a paper published in the journal Nature on 23 February 2022, Melanie During of the Department of Earth Sciences at the Vrije Universiteit Amsterdam, and the Subdepartment of Evolution and Development at Uppsala University, Jan Smit, also of the Department of Earth Sciences at the Vrije Universiteit Amsterdam, Dennis Voeten, also of the Subdepartment of Evolution and Development at Uppsala University, and of the European Synchrotron Radiation Facility, Camille Berruyer and Paul Tafforeau, also of the European Synchrotron Radiation Facility, Sophie Sanchez, again of the Subdepartment of Evolution and Development at Uppsala University, and the European Synchrotron Radiation Facility, Koen Stein of the Directorate ‘Earth and History of Life’ at the Royal Belgian Institute of Natural Sciences, and of Earth System Science at the Vrije Universiteit Brussel, Suzan Verdegaal-Warmerdam, again of the Department of Earth Sciences at the Vrije Universiteit Amsterdam, and Jeroen van der Lubbe, once again of the Department of Earth Sciences at the Vrije Universiteit Amsterdam, and of the School of Earth and Environmental Sciences at Cardiff University, attempt to determine the season in which the Fish of the Tanis Event Deposits died by examining cyclical bone growth patterns in their skeletons to see at which point growth ceased.
Reconstruction of a Paddlefish with impact spherules in the gill rakers. (a) Three-dimensional rendering of Paddlefish FAU.DGS.ND.161.4559.T in left lateral view with the location of a higher-resolution scan (depicted in (b)) indicated (white outline). (b) Three-dimensional rendering of the subopercular and gills in a with trapped impact spherules (yellow). Scale bars are 2 cm. During et al. (2022).
Tree-ring evidence of the Maastrichtian (latest Cretaceous, 72.1 to 66 million years ago) climate of North Dakota suggests a temperate climate with four distinct seasons. The Tanis site is reconstructed as having had annual temperature fluctuations that varied from an average of roughly 19 °C in summer, down to a winter average of 4–6 °C. As well as the rings in trees, these climate variations are recorded in the bones of Acipenseriform Fish, potentially providning a clock which could have recorded the season in which the Chicxulub Impactor fell. To this ende During et al. analysed three Paddlefish dentaries and three Sturgeon pectoral fin spines from the Tanis deposits.
PPC- SRμCT data of FAU.DGS.ND.161.4559.T, a partial Paddlefish from the Tanis locality. (a) Orthogonal virtual thin sections (100 μm thick, average-value projections) obtained in front, top, and right view. (b) Impact spherules in virtual thin sections of (a), indicated with yellow circles. Scale bars (a) 1 mm. (c) Three-dimensional rendering (in left lateral view) with virtual cross sections of (d) (blue), (e) (green), and (f) (red) indicated. (d) Coronal virtual slice. (e) Sagittal virtual slice. (f) Axial virtual slice, brain-enveloping tissues indicated with red arrows. (g) Three-dimensional rendering in right lateral view with anatomical labels. (h) Three-dimensional rendering in left lateral view with anatomical labels. During et al. (2022).
During et al. traced lines of arrested growth (marks left in growing bone by slowed growth at one time of year, typically winter in a temperate climate) in dermal bones from six Acipenseriform Fish, by preparing slices of bone as microscope slides. They corroborated this by creating a three-dimensional map of the skulls using propagation phase-contrast synchrotron radiation micro-computed tomography at the European Synchrotron Radiation Facility, which enabled optimal projection of the bone deposition pattern across multiple cross-sectional planes, enabling them to determine the relationship between growth lines and seasonality across a wider area of bone. In addition, During et al. carried out an investigation into the isotopic composition of the growth lines in one Paddlefish specimen.
Osteohistological thin sections of five Acipenseriform Fish. (a)–(e) Thin sections in transmitted light of VUA.GG.2017.MDX-3 (a), VUA. GG.2017.X-2743M (b), VUA.GG.2017.X-2744M (c), VUA.GG.2017.X-2733A (d) and VUA.GG.2017.X-2733B (e), showing congruent pacing of bone apposition during the final years of life, terminating in spring. Red arrows indicate lines of arrested growth. Scale bars are 0.5 mm. During et al. (2022).
The tomographic reconstructions also demonstrate that impact spherules are found only in the gill rakers of the Fish, and are absent elsewhere within their bodies, strongly supporting the idea that they were taken in during the last minutes of their lives, and did not have time to penetrate the oral cavity or further down the digestive tract before they died. This also rules out the possibility that they were introduced to the Fish post-mortem, by penetrating decomposing bodies, indicating the Fish were buried rapidly after their death, which strongly supports the idea that the death of the Fish was close to simultaneous with the arrival of the seiche wave, and therefore that these Fish were alive and active in the very last moments of the Cretaceous.
Carbon isotope record alongside the incremental growth profiles. (a) Proportion of carbon¹³ expressed as ‰ on the Vienna Pee Dee Belemnite reference scale. The colour gradient highlights the theoretical range between maximum values during seasonal (summer) trophic increase of carbon¹³ (yellow) and minimum values during trophic decrease of carbon¹³ (winter) (blue). (b) Virtual thick section (average-value projection with 0.1 mm depth) showing growth zones during the favourable growth seasons and annuli and lines of arrested growth outside the favourable growth seasons. (c) Cell density map of a virtual thick section (minimum-value projection with 0.2 mm depth) showing fluctuating osteocyte lacunar densities and sizes, with higher densities and largest sizes recorded during the favourable growth seasons (orange) and lower densities and smaller sizes outside the favourable growth seasons (purple). (d) Microscopic thin section in transmitted light showing lines of arrested growth (red arrows) and a single growth mark indicated (bracket) spanning the distance between two subsequent lines of arrested growth and including a zone and an annulus, Scanning data visualized in (b) and (c) were obtained approximately 10 mm distal from the physically sectioned thin slice of (d), which itself was located directly proximal to the thick section sampled for (a). Scale bars are 1 mm. During et al. (2022).
During et al. used micro-X-ray fluorescence to search for signs of taphonomic alteration within the bones, finding that iron and manganese oxides were present both within the surrounding sediments and the sediments surrounding the bones, and the vascular canals within the bones, but had not penetrated into the bone tissue itself. Potassium and silicon were present in the sediment but not in the bones, while the bones themselves maintained a homogeneous distribution of phosphorus and calcium, which would be expected in unaltered bone. The fine detail of the fossils, including the preservation of non-ossified tissues that surrounded the brains of the Fish, also points towards there being almost no taphonomic alteration of the specimens.
Elemental distribution maps of Acipenseriform elements from the Tanis locality obtained with micro-X-ray fluorescence. (a) Calcium, phosphorus, and manganese distribution in Paddlefish dentary VUA.GG.2017.X-2724. (b) Calcium, phosphorus, and manganese distribution in Sturgeon pectoral fin spine VUA.GG.2017.MDX-3. (c) Calcium, phosphorus, and manganese distribution in Paddlefish dentaries VUA.GG.2017.X-2733A, VUA.GG.2017.X-2733B, and the surrounding sediment matrix. (d) Calcium, phosphorus, and manganese distribution in Sturgeon pectoral fin spine VUA.GG.2017.X-2743M. (e) Calcium, phosphorus, and manganese distribution in Sturgeon pectoral fin spine VUA.GG.2017.X-2744M. (f) Potassium, silicon, and iron distribution in Paddlefish dentary VUA.GG.2017.X-2724. (g) Potassium, silicon, and iron distribution in Sturgeon pectoral fin spine VUA.GG.2017.MDX-3. (h) Potassium, silicon, and iron distribution in Paddlefish dentaries VUA.GG.2017.X-2733A, VUA. GG.2017.X-2733B and the surrounding sediment matrix. (i) Potassium, silicon, and iron distribution in Sturgeon pectoral fin spine VUA.GG.2017.X-2743M. (j) Potassium, silicon, and iron distribution in Sturgeon pectoral fin spine VUA.GG.2017.X-2744M. During et al. (2022).
The dentaries of Paddlefish form by the ossification of tissue around the Meckel’s cartilage, while the pectoral spines of Sturgeon form by the ossification of embryonic mesoderm tissue within the skin. Neither of them form by the direct ossification of cartilage. Instead they grow incrementally, with new bone tissue being secreted by rows of osteoblasts on the growing surface. This generates an annual growth pattern, with each year represented by a thick zone of bone laid down under favourable conditions, followed by a narrower area of bone laid down under less favourable growth conditions, then a line of arrested growth, representing a period when no bone was laid down. During et al.'s examination of Acipenseriform Fish remains from the terminal Cretaceous Tanis Event Deposits showed that in all cases growth had stopped for the final time shortly after the bones had begun to grow again following a line of arrested growth.
Osteohistology of Acipenseriform Fish from the Tanis locality. (a) Thin section of Paddlefish dentary VUA.GG.2017.X-2724 under transmitted light. (b) Detail of VUA.GG.2017.X-2724 thin section (white box in (a)), scale bar 100 μm. (c_ Detail of VUA.GG.2017.X-2724 thin section (white box in (b)), scale bar 100 μm. (d) Thin section of Sturgeon pectoral fin spine VUA.GG.2017. MDX-3 under transmitted light. (e) Detail of VUA.GG.2017.MDX-3 thin section (white box in (d)), scale bar 100 μm. (f) Detail of VUA.GG.2017.MDX-3 thin section (white box in (e)), scale bar 100 μm. (g) Thin section of Paddlefish dentary VUA. GG.2017.X-2733A under transmitted light. (h) Detail of VUA.GG.2017.X-2733A thin section (white box in (g)) with red arrows indicating lines of arrested growth, scale bar 100 μm. (i) Detail of VUA.GG.2017.X-2724 thin section (white box in (h)), scale bar 100 μm. During et al. (2022).
The visible growth lines within the bones were corroborated using stable isotope ratios, which showed seasonal variations in the proportion of carbon¹³, which is linked to diet, and a constant proportion of oxygen¹⁸, which is related to environment, and therefore indicates that the Fish had been living within the river basin their entire lives, rather than migrating to the sea (as some modern do, and which is likely to have been done by some of their Mesozoic forebears). This confirms that the the variation in carbon isotope values and growth rates reflect seasonal variations within that river basin, and not migratory behaviour on behalf of the Fish. This can be seen in both Paddlefish and Sturgeon, despite the different lifestyles of the two types of Fish.
The Paddlefish of the Late Cretaceous Tanis River Basin were filter-feeders (as are Paddlefish today), which are thought to have fed on zooplankton such as Copepods. Such a feeding strategy would have led to seasonal variations in the availability of food, with the maximum availability falling in the summer. This would have led to a summer season when the Fish was both growing faster, and absorbing a higher proportion of carbon¹³. Examination of the distribution of carbon¹³ within fossil Paddlefish bones from the Tanis deposits reveals that at the time of their deaths the proportion of carbon¹³ in their diets was rising, but had not reached its annual peak, supporting the hypothesis that when the Fish died it was spring in the northern hemisphere.
Previous studies on palaeobotanical material from Wyoming has suggested that the Chicxulub Impactor fell in June, although those studies are now considered unreliable for a variety of reasons, leaving us with no standing theory on the time of year when the impact event happened. During et al.'s study appears to show a direct record of the season in which Fish that died directly as a result of the impact died, giving a strong line of support for a springtime extinction event.
If correct, the finding that the Chicxulub Impactor fell in the Northern Hemisphere's spring season may go some way to explaining the selective nature of the extinction it caused. Spring is the breeding season for many Animals, which can potentially make them more vulnerable to environmental perturbations. In the aftermath of the impact the ecosystems of the Southern Hemisphere are known to have recovered much more quickly than those of the north, which could potentially have been linked to an event which hit Northern Hemisphere organisms in their breeding season. The event also impacted larger organisms, with longer breeding cycles, more than it did smaller ones with shorter breeding cycles, which could again be linked to the disruption of a breeding season. An event which happened shortly before the onset of winter in the Southern Hemisphere would have been favourable for the survival of organisms which hibernate in burrows (such as many small Mammals), and which might already have been entering a dormant phase when the bolide impacted, something which could potentially also apply to some Amphibians, Birds, and even Crocodilians.
If this is the case then it goes some way towards decoupling the short- and long-term effects of the Chicxulub Impact, with the initial events being particularly harsh on organisms with a spring breeding cycle in the Northern Hemisphere, and relatively benign to organisms with a winter dormant phase in the Southern Hemisphere, while the longer term climatic and ecosystem breakdowns would favour organisms flexible in their environmental and dietary needs, which would be better able to survive amid collapsing food webs, in either hemisphere.
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