There are four possible explanations for the unique mix of cold- and heat-loving animals that were common during the Ice Age: 1) seasonal migration, 2) increased climatic tolerances, 3) the mixing of glacial and interglacial fossils, and 4) an equable climate with cool summers and mild winters.1
A few scientists have opted for seasonal migration, but this hypothesis has never made it very far. Scientists have pointed out that the hippopotamus could not have migrated very far during the summer, even if the temperatures were warm enough to lure the animals to England.2 Many of the disharmonious animals are small and are not expected to have migrated. Problems have already been noted for the suggestion that woolly mammoths migrated into Siberia during the summer (see chapter 2). Plants and insects do not migrate but can spread into new territory over time.
One difficulty with using extinct, or even living, animals to infer disharmonious associations is that the climatic tolerances of most animals are rarely known.3 It is recognized that most animals are more climatically elastic than their current habitats would suggest.4 Tigers, for example, can live in cold as well as warm climates. The Siberian tiger lives in east central Asia.5 Vereshchagin and Baryshnikov6 state:
At the beginning of the twentieth century in the USSR the tiger occurred in Transbaikal [south central Siberia], along the courses of central Asiatic rivers, in the Far East, and the Amur Valley. Occasional individuals were encountered in the southern part of western Siberia and Yakutia [north central Siberia].
Some had argued that the hippo, found in northwest Europe during the Ice Age, was cold adapted, but this has been dismissed by most,7 although the hippo is known to handle cool weather for short periods of time in English zoos.8
Despite the wider range of climatic tolerances for animals, one cannot use this unknown factor to dismiss too many disharmonious associations. Disharmonious associations are so common that they cannot all be attributed to a greater tolerance to cold or heat. Furthermore, some of the fossils have living representatives whose climate tolerance is well known, such as holly, ivy, and water chestnuts. Fossils of these plants occurred in the British Isles during the Ice Age, while today they are normally found farther south.9
Mixing of glacial and interglacial deposits is a common explanation for the hippopotamus fossils alongside of cold-adapted animals in northwest Europe.10 Interglacials are the periods between ice ages according to the standard uniformitarian understanding of the Pleistocene epoch. Each ice age is believed to have lasted 100,000 years and repeated cyclically over the past one million years. Between 1 million and 2.4 million years ago, the ice ages are believed to have cycled every 40,000 years. During the 100,000-year cycle, an interglacial is supposed to last only 10,000 years, while the glacial phase is 90,000 years. According to their theory, since the last ice age ended somewhere between 10,000 and 20,000 years ago, the next ice age is due soon. The interglacial climate we now enjoy is supposed to be waning. Previous interglacials can be thought of as having a similar climate to today.11 So, animals that prefer the cold would live in England during glacial time and animals that prefer the warmth would spread up to England during interglacials. Then, because of landsliding or other mixing processes, glacial and interglacial animals would be mixed together. This is what Nilsson12 suggests:
The occurrences of such taxa as hippopotamuses that are closely adapted to warmth, may result from the reworking of older, interglacial deposits.
There are several problems with the mixing hypothesis. First, there should be evidence for the mixing of sediments, not just a mixing of animals with contrary climatic tolerances. There does not appear to be much mixing of sediments. Grayson13 informs us:
In the valley of the Thames [southern England], for instance, woolly mammoth, woolly rhinoceros, musk ox, reindeer (Rangifer tarandus), hippopotamus (Hippopotamus amphibius), and cave lion (Felis leo spelaea) had all been found by 1855 in stratigraphic contexts that seemed to indicate contemporaneity.
Grayson does not believe the hippopotamus fossils were mixed with the cold-tolerant animals. In an example of disharmonious fossils from eastern Washington, Rensberger and Barnosky14 see little evidence of significant mixing:
Although we cannot rule out minor stratigraphic mixing … the observed amounts of bioturbation at most of the collecting sites are insufficient to support a contention that stratigraphic mixing by itself produced the no-analogue [disharmonious] species associations.
Mixing is possible in cases where the stratigraphic units have been poorly distinguished. Graham and Lundelius15 discount mixing because disharmonious associations are much too common to be spurious in all cases.
Second, the mixing hypothesis assumes that warmth-loving animals could have migrated far to the north and, therefore, beyond their climatic limits of today. Old-age theorists believe the earth currently enjoys a “warm interglacial,” called the Holocene. Today’s “interglacial” climate has not motivated the hippo to journey from Africa up to England, nor has the climate inspired any other warmth-loving animal to migrate to the colder climes. It is likely too cold in today’s northern climate, as well as in previous postulated interglacials in northwest Europe, for the hippopotamus.
Third, if mixing is the cause, it should have carried over into the current interglacial, the Holocene epoch. Why would the supposed frequent mixing of sediments suddenly stop at the end of the Ice Age? Disharmonious associations, however, are rare in the Holocene.16
Fourth, disharmonious associations are found in all previous interglacials in the standard sequence as well as in glacial times. Guthrie17 states: “Earlier interglacials and glacials reveal a fauna and flora more like that of a heterogeneous savanna.” Guilday18 reinforces this statement:
Pleistocene local faunas assigned to both pre-Wisconsinan glacial and interglacial periods are equally diverse in composition and their taxonomic makeup, especially of the reptiles and amphibians, suggests climate equability without a hint of Holocene polarity [separation].
Guilday is essentially saying that no matter whether a particular layer is assigned a glacial or an interglacial age, the fauna is still equally diverse and disharmonious. In referring to the last interglacial, called the Sangamon in North America, Alroy19 discovered:
If the Sangamonian faunas are correctly categorized, then disharmony was nearly as common during the Wisconsinan [last Ice Age] as during an earlier interglacial that was every bit as warm as the Holocene. The climate equability hypothesis would be hard pressed to explain such a pattern.
Although Alroy rightly points out that such interglacial disharmonious associations are evidence against an equable climate, they are also evidence against the second option of mixing. Mixing would have to be common in these interglacial sediments — a situation that rarely occurs in the interglacial of today. This brings up the question of why interglacials also show disharmonious associations. Maybe there were no interglacials but all the animals represent life in one Ice Age.
These associations are very common in Pleistocene faunas in all parts of the world where there is adequate data. They have been termed “disharmonious.” … They have been interpreted as indicating more-equable climates in the past.
Grayson22 reinforces this conclusion in regard to the hippopotamuses living alongside musk ox and reindeer:
If the musk ox required cold, and the hippopotamus required warmth, and the stratigraphic evidence implied that they had coexisted, then a straightforward reading of all this information could imply that glacial climates had not, as most felt, been marked by severe winters, but had instead been equable.
It is interesting that as long ago as 1867 Lartet argued that an equable climate can account for the surprising distribution of warm- and cold-tolerant animals:
There must have been cooler summers for the reindeer and musk-ox; and, on the other hand, warmer winters for the hippopotamus and other species whose analogs are today found withdrawn toward the tropical regions.23
An equable climate is also reinforced by the high diversity of animals in the Ice Age sediments:
In modern floras and faunas there is a positive correlation between increased species diversity and decreased climatic variability as measured by winter-summer differences in mean temperature … thus the degree of diversity in late Pleistocene disharmonious biotas suggests that they existed during times when the climate was equable and seasonal extremes in temperature and effective moisture were reduced … .24
An equable climate would also explain the mix of warm- and cold-tolerant plants during the Ice Age, for instance in France:
The implications of the botanical co-occurrences seemed clear to Saporta: only a humid, equable climate would have allowed such an association.25
Although an equable climate enjoys much support from the fossils, it still cannot be explained by uniformitarian Ice Age simulations. It certainly is not expected from standard ideas of what an ice age should be like. There are of course a few wild guesses. For instance, Stuart26 states:
The apparently paradoxical situation where Wisconsinan [late Ice Age] winters were less severe than today is thought to have resulted from the vast ice sheet preventing arctic air from sweeping across the Plains.
From a meteorological point of view, the ice sheets could not have blocked Arctic air. They would have created Arctic air that would have no trouble spilling southward away from the ice sheets. Nevertheless, Stuart does admit that disharmonious associations are paradoxical because winters would have to have been warmer, even at the peak of the Ice Age. This leaves him and other scientists with the question: how can there be warmer winters during an ice age?