The last terror birds

Skeleton of the terror bird Titanis walleri at the Florida Museum of Natural History.

In 1887, Florentino Ameghino, the “father of Argentinian Palaeontology”, described a large, toothless jaw from the Miocene of the Province of Santa Cruz, naming it Phorusrhacos longissimus and assigning it to a new family of edentulous mammal. He used this finding as a critical evidence for his contention that modern mammalian lineages originated in Argentina and later spread across the globe. Four years later, Moreno and Mercerat recognized for the first time that the mandible described by Ameghino was really that of a bird.

The Phorusrhacidae, the so-called “terror birds”, were a group of medium-to large sized extinct ground-dwelling birds, which lived during the Cenozoic, and became the dominant carnivores of South America while it was an isolated continent. They are characterized by their elongated hindlimbs, narrow pelvis, reduced forelimbs, and their huge skull with their tall, long, narrow, and hollow beaks ended in a hook. Kelenken guillermoi, is the largest known phorusrhacid and lived in the Miocene of Argentina. The skull reaches a length of 71.6 cm and the whole animal would reach 3 m high. Kelenken is also represented by a tarsometatarsus and a broken phalanx and proceeds from the locality of Comallo (Río Negro Province, Argentina). Titanis walleri, lived during the Pliocene and Pleistocene of North America. It was 2.5 metres tall and weighed approximately 150 kilograms. This giant bird is interpreted as an early immigrant during the Great American Interchange.

Proximal portion of the left humerus of Psilopterus sp. Caudal, b ventral, c cranial and d dorsal views (From Jones et. al., 2017)

At the end of the Pliocene, Phorusrhacids decline in diversity. Two new specimens support the hypothesis that the latest geologic occurrence of the Phorusrhacidae comes from late Pleistocene sediments of Uruguay. The remains comprise the distal portion of right tarsometatarsus and a left humerus; the latter is assigned to the genus Psilopterus. The first material (MPAB-520) comes from Soriano, Uruguay, and the sediments belong to the Dolores Formation (Lujanian Stage-Age, late Pleistocene/early Holocene). The following features identify the specimen as a phorusrhacid bird. (1) a large and distally expanded trochlea metatarsi III; (2) a very narrow trochlea metatarsi II with the articular surface transversally convex and without any longitudinal sulcus (in dorsal and distal views); (3) in dorsal view the trochlea metatarsi II is almost parallel and much shorter than the middle trochlea, and forming a narrow notch between trochleae II and III. The second material consists of a left humerus without distal epiphysis belonged to Museo Paleontológico Alejandro Berro (MPAB-2024).

There are two explanatory hypotheses proposed for the decline of the terror birds: environmental reasons or direct competition (at least for the larger specimens) with placental carnivore’s immigrants to South America after the setting of the Panamanian bridge. 

 

References:

Jones, W., Rinderknecht, A., Alvarenga, H. et al. PalZ (2017), The last terror birds (Aves, Phorusrhacidae): new evidence from the late Pleistocene of Uruguay, https://doi.org/10.1007/s12542-017-0388-y

ALVARENGA, Herculano M.F.  and  HOFLING, Elizabeth. Systematic revision of the Phorusrhacidae (Aves: Ralliformes). Pap. Avulsos Zool. (São Paulo) [online]. 2003, vol.43, n.4 [cited  2015-03-24], pp. 55-91 .

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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.

 

References:

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).