Quaternary Period

Abstract

Keywords: Quaternary, Quaternary biota, fossil record, evolution

Introduction

The Quaternary Period comprises roughly the last two and a half million years of Earth history, up to and including the present. Globally, it is the most widely used unit in field mapping (Ogg et al. 2008) and is almost certainly the most intensively researched. Despite this, the definition and rank are highly controversial (see below) and many authorities do not recognise the Quaternary as a distinct geochronological unit at all, preferring to subsume the corresponding time interval within the Neogene.

Stratigraphy

Historical Development

The following excellent account is quoted from Ogg et al. 2008, p. 149: ‘Despite being the most widely used unit in field mapping and having the greatest number of active researchers, the interval known as Quaternary is unique among chronostratigraphic subdivisions in having the most controversial definition and rank. The convoluted history and divergent concepts of Quaternary usage is fraught with opinionated debate, beginning with the early International Geological Congresses which considered relegating Quaternary to be an unranked synonym for a vaguely defined Pleistocene epoch (1894) or “Modern” period (1900). The association of Quaternary with the “Ice Ages” created another problem after the investigation of new regions, improved dating methods and deep-sea oxygen isotope records. The onset of these continental glaciations was discovered to begin much earlier in the Neogene than the ratified base of the Pleistocene Series.’

Lower (Neogene–Quaternary) Boundary

‘In 1983, the base Pleistocene GSSP was ratified at Vrica, Italy, near the top of the Olduvai magnetic Subchron [2.58 Ma; Cohen et al. 2015], but the decision “was isolated from other more or less related problems, such as … status of the Quaternary.” The Gelasian Stage was later created (1996) to fill the “gap” between this GSSP and the “traditional” span of the Piacenzian Stage of the Pliocene Series’ (Ogg et al. 2008).

Upper Boundary

The Quaternary Period is unique in having no upper boundary as such; i.e., the upper boundary of the Quaternary is the present.

Chronology

The Quaternary is presently defined as the interval from 2.58 Ma to the present.

Paleogeography

Major Tectonic Events

One probable exception is the formation of the land bridge between North and South America, created by the emergence of the Isthmus of Panama. “Formation of the Isthmus of Panama involved subduction of the Pacific-Farallon Plate beneath the Caribbean and South American plates, ultimately driving the development of a volcanic arc on the trailing edge of the Caribbean Plate. This initial Panama Arc began to form approximately 73 Ma ... as the Caribbean Platemoved eastward, arriving at its current position by ~50 Ma. The North and South American plates continued to move westward past the Caribbean Plate after this time. In addition to their east-west (strike-slip) motion, the South American and Caribbean plates also acquired a north-south component of convergence, leading to the collision of the Panama Arc with South America” (O’Dea et al. 2016, p. 1).

The isthmus is conventionally believed to have formed in the late Neogene, about 3 million years ago. An alternative and widely challenged theory argues for a land bridge as old as 15 Ma. However, a comprehensive, multidisciplinary analysis of geological, paleontological, and molecular records (O’Dea et al. 2016) confirmed the conventional wisdom and concluded that their “independent lines of evidence converge upon a cohesive narrative of gradually emerging land and constricting seaways, with formation of the Isthmus of Panama sensu stricto around 2.8 Ma. The evidence used to support an older isthmus is inconclusive, and we caution against the uncritical acceptance of an isthmus before the Pliocene” (abstract).

The ecological effects from the the migration of plants and animals between the two continents, and the climatic changes arising from changed ocean currents, would profoundly shape the Quaternary world.

Land and Sea

(Also see NASA Panama Isthmus page.)

Paleontology

Introduction

Major Evolutionary Events

The Great American Biotic Interchange

Some dispersal since at least the Miocene; probably dispersal by usual rafting etc. across narrow sea.

“The Great American Biotic Interchange ... is characterized by a surge in successful dispersals in both directions beginning around 2.6 Ma, traditionally defined as beginning with the arrival of the South American porcupine Erethizon in North America..., and various members of the North American families Mustelidae, Canidae, and possibly Gomphotheriidae, along with the extinct horse Hippidion, successfully colonizing South America at the same time.... This wave of successful dispersals by many large mammals is widely considered convincing evidence that animals could, at this time, have walked dry-shod across a fully formed land bridge” (O’Dea et al. 2016, p. 6).

Major Taxa

Mammut americanum (mastodon)

Mammuthus primigenius (mammoth)

Smilodon spp. (sabre-tooth cats)

Coelodonta antiquitatis (woolly rhinoceros)

Hominin Evolution

The earliest species assigned to Homo is the unnamed species appearing at the beginning of the Quaternary, approximately 2.5 million years ago. This species was rapidly succeeded by Homo rudolfensis. Then, two further species, Homo habilis and H. ergaster overlap through much of the interval 2 to 1.5 Ma; the latter seems more likely to have been the direct human ancestor.

Homo erectus is the only recognised representative of the genus between about 1.2 and 0.7 Ma; some interpretations place it on the direct ancestral line to modern humans, others consider H. ergaster to have been directly ancestral to H. heidelbergensis, which first appears about 0.6 Ma, and from there to modern H. sapiens. However, if H. erectus is not directly ancestral to H. heidelbergensis and modern man, we are left with a ~900,000 year gap in which the true intermediaries are unknown.

“The first hominin dispersal out of Africa is thought to have been when members of the species Homo erectus exited some 2 million years ago. The second wave of departures occurred when the ancestral species that eventually gave rise to Neanderthals moved into Europe around 800,000–600,000 years ago” (Delson 2019, p. 488).

This commentary may now be dated – needs checking: Anatomically modern H. sapiens first appears close to 200,000 years ago. The oldest well-dated fossils are the Omo I and Omo II fossils from Kibish, Ethiopia (McDougall et al. 2005).

“The origin and early dispersal of Homo sapiens has long been a subject of both popular and scholarly interest. It is almost universally agreed that H. sapiens (modern humans) evolved in Africa, with the earliest known fossil representatives of our species dated to around 315,000 years ago in Morocco (at a site called Jebel Irhoud) and approximately 260,000 years ago in South Africa (at Florisbad). Stone tools comparable to those found with both of these fossils have been excavated in Kenya (at Olorgesailie) and dated to about 320,000 years ago…. Many key fossil discoveries from Israel document early examples of [human] dispersals. A fossil that includes the forehead region of a skull found there, at a site called Zuttiyeh, is dated to between 500,000 and 200,000 years ago, and analysis of the fossil’s shape indicates that it is either an early Neanderthal or from a population ancestral to both Neanderthals and H. sapiens” (Delson 2019, p. 487, 488).

Lagerstätten

Extinctions

Pleistocene megafauna extinction

The iconic Pleistocene megafauna – including woolly mammoths, sabre-tooth tigers, etc. – became extinct at the end of the last glacial interval, the Würm glaciation. It is called the megafauna extinction because, at least in the Americas and northern Eurasia, it tended to be large animals that were most affected. Human impact (hunting) and climate change (global warming following the the Würm glaciation) have been implicated as major causes, although the relative importance of each is still being debated vigorously, and other contributing agents cannot be categorically ruled out either. “On continents worldwide, about 90 genera of mammals weighing ≥44 kg disappeared…. Timing varied across the globe, but by 10,000 years ago, these animals had vanished except at very high latitudes, on islands (where the extinction of large and small animals was more recent), and in Africa (where many large animals survive today)” (Koch & Barnosky 2006, p. 216).

“In North America south of Alaska, 34 Pleistocene genera of megafaunal mammals did not survive into the Holocene; two mammalian orders (Perissodactyla, Proboscidea) were eliminated completely…. At the species level, the extinction was total for mammals larger than 1000 kg, greater than 50% for size classes between 1000 and 32 kg, and 20% for those between 32 and 10 kg” (Koch & Barnosky 2006, p. 217).

South America was profoundly affected by this extinction: some 50 genera and three orders (Notoungulata, Proboscidea, Litopterna) disappeared, as did all large xenarthrans. Humans did not coexist with megafauna throughout South America, and were therefore not the only driver of megafauna extinctions. However, the Argentine Pampas extinctions do appear to be more common following the arrival of humans and during the intensified climatic change between 11.2 and 13.5 ka BP (Prado et al. 2015).

“Northern Eurasia (Europe and northern Asia) lost 9 genera (35%)…. Comprehensive dating campaigns in Eurasia and the extension of Eurasian steppe biomes in Alaska and the Yukon reveal that the extinction occurred in two pulses (Guthrie 2003, 2004; MacPhee et al. 2002; Stuart et al. 2002, 2004). In Eurasia, warm-adapted megafauna that were abundant during prior interglacials (straight-tusked elephants, hippos) became extinct between 48.5 and 23.5 kyr BP. In Alaska and the Yukon, hemionid horses and short-faced bears disappeared at 35.4 and 24.8 kyr BP, respectively. ... The second pulse of extinctions began in the latest Pleistocene and hit cold-adapted animals” (Koch & Barnosky 2006, p. 219).

Most Australian megafauna appear to have survived until 51 to 40 thousand years ago, with human impact by hunting or vegetation change proposed as the extinction drivers. Yet, persistent claims have been made that the Cuddie Springs site, an ephemeral lake in interior New South Wales, is anomalous: Here, it is claimed, megafauna fossils are associated with stone tools in sediments deposited 40 to 30 thousand years ago, thus indicating prolonged overlap between people and megafauna. Sidestepping the question of whether the fossils are associated with the human proxies, Grun et al. (2010) directly dated the teeth of several species of megafauna using electron spin resonance and uranium-series dating methods. Their results provide no evidence for the late survival of megafauna at the site; none of the dated megafauna in SU6 were younger than 50 thousand years. (Adapted from Roberts & Brook 2010.)

“Africa is a fortunate anomaly in this story. Although 10 genera of Pleistocene megafauna (21%) disappeared, extinction of 7 of these cannot be bracketed tighter than in the last 100 kyr BP, and three went extinct in the Holocene.... Yet even in Africa, the species-level extinction was most intense for larger megafauna (40% to 50% for mammals between 10,000 and 1000 kg and moderate for those between 1000 and 320 kg)” (Koch & Barnosky 2006, p. 221).

Overall, before the extinction, “body size distributions were bimodal and similar in North America, South America, and Africa, whereas Australia had a right-skewed distribution (Lyons et al. 2004). On all four continents, however, extinct species were significantly larger than surviving species, and for the Americas and Australia, almost the entire large mode or tail of the distribution was removed” (Koch & Barnosky 2006, p. 221).

Modern extinction

What’s it like to be in the middle of a mass extinction? Well, nobody knows for sure, but some people believe we are experiencing one right now.

Whole books have been written about the “sixth extinction” which, if it is real, is largely a consequence of habitat destruction by humans. The difficulty with recognising the modern world as a mass extinction in progress is that there is still relatively little detailed quantitative data to establish a reliable background level, or baseline, of what is “normal” species turnover of the millions of small plants and animals which, often, do not fossilise well. However, there can be little doubt that – again – large animals are going extinct at a horrifying rate. It will be truly remarkable if people living in the next century will ever see a living black rhinoceros or an Amur leopard.

It is less obvious that human depredations are having the same effects on smaller organisms, although, intuitively, it seems inevitable that they must be. (There are many people, who simply don’t care, of course. This attitude is understandable for those living below the poverty line, where mere survival is a struggle, but can only be attributed to anthropocentrism for the rest of us.)

The phrase “sixth extinction” is an implied reference to the so-called “big five” Cenozoic mass extinctions, which is unfortunate for many reasons, chief among which are because there is no objective cut-off between the five largest Cenozoic mass extinctions and the rest of them (their magnitudes form an essentially smooth power curve), and because the modern extinction is in no way comparable to any of them. As my colleague, Mike Hannah (watch out for his forthcoming book) points out, making this comparison risks the very real severity of the modern extinction being dismissed along with the inappropriate comparison.

New Zealand Occurrences

Present day New Zealand is shaped by a tectonic regime comprising subduction to the north east (Hikurangi Trough), back arc extension through the Taupo Volcanic Zone, and crustal collision-driven mountain building in the southern axial range. Land area has generally increased over the past 5 million years, supplying “vast quantities of sediment to low-lying areas, the prograding inner continental shelf and offshore sedimentary basins” (Edbrooke 2017, p. 32).

References

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