The formation of the Isthmus of Panama stands as one of the greatest natural events of the Cenozoic, driving profound biotic transformations on land and in the oceans. Some recent studies suggest that the Isthmus formed many millions of years earlier than the widely recognized age of approximately 3 million years ago (Ma), a result that if true would revolutionize our understanding of environmental, ecological, and evolutionary change across the Americas. To bring clarity to the question of when the Isthmus of Panama formed, we provide an exhaustive review and reanalysis of geological, paleontological, and molecular records. These 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.

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Formation of the Isthmus of Panama

In recent years, the view that Pleistocene climatic events played a major role in the evolution of the biotas of southern, primarily tropical continents has begun to displace the previously held conviction that these areas remained relatively stable during the Quaternary. Studies of speciation patterns of high Andean plant and avian taxa (7-14) have led to the conclusion that Pleistocene climatic events were the factors that ultimately shaped the patterns now observed in the paramo-puna and the related Patagonian flora and fauna. The final uplift of the Andes at the end of the Tertiary automatically limits the age of the high Andean habitats and their biotas to the Quaternary. Within this period, the number of ecological fluctuations caused by the glaciations could easily have provided the mechanism behind the patterns now present in these habitats (Appendix, 1; Figs. 1 and 2; Table 1). In glacial periods, when vegetation belts, were lowered, organisms in the paramo-puna habitat were allowed to expand their ranges. In interglacial periods, these taxa were isolated on disjunct peaks, where differentiation could occur. At times of ice expansion, glacial tongues and lakes provided local barriers to gene exchange, whereas in warm, interglacial times, dry river valleys were a major deterrent to the interbreeding of populations on different mountains (Fig. 2; Table 2). A preliminary analysis of about 10 to 12 percent of the total South American avifauna (14), subsequent to the study of the high Andean biota, suggested that the birds of all the major habitats of the continent possess, with about equal frequency, similar stages of speciation. This correspondence in levels of evolution indicated that the avifauna of vegetation zones which were thought to have been more stable (for example, tropical rainforests) are as actively speciating as are those of the more recent paramo-puna habitats. More intensive work on lowland tropical taxa (16, 19-21) and recent work on montane forest elements (40) now justify the conclusion that the floras and faunas of these areas were also greatly affected by Pleistocene climatic shifts. In the broad region of South America that lies within the tropics, a series of humid-arid cycles (Appendix, 6, 8-10) drastically and repeatedly altered vegetation patterns during the Quaternary. Both montane and lowland rainforests were fragmented during dry periods and were able to reexpand during humid phases. Speciation of forest elements was initiated-and sometimes completed-in isolated patches of the fragmented forest. Secondary contact, with hybridization or reunition of populations that did not become reproductively isolated, occurred in periods of reexpansion. These biological data, combined with supportive geological evidence (Appendix, 1-11), show that climatic events during the last million or so years have affected the biota of South America as much as the Pleistocene glacial changes affected the biotas of Eurasia and North America. Since most of South America lies within tropical latitudes, it is suggested here that part of the diversity of species in the tropical areas of this continent is due to two historical factors: the lack of wholesale elimination of species (compared with northern and high latitudes), and ample opportunity for speciation in successive periods of ecological isolation. The apparent paradox of the wealth of species in the "stable tropics" is partially explained by the fact that the tropics have probably been quite unstable, from the point of view of their biotas, during the Pleistocene and perhaps part of the Tertiary.

https://science.sciencemag.org/content/sci/173/3999/771.full.pdf

Pleistocene Changes in the Fauna and Flora of South America

Neogene stratigraphy, paleoceanography and paleobiogeography in northwest South America and the evolution of the Panama seaway

Aim Predictive models of species’ distributions use occurrence records and environmental data to produce a model of the species’ requirements and a map of its potential distribution. To determine regions of suitable environmental conditions and assess biogeographical questions regarding their ranges, we modelled the potential geographical distributions of two spiny pocket mice (Rodentia: Heteromyidae) in northwestern South America.

Geographical distributions of spiny pocket mice in South America: insights from predictive models

This paper documents congruence in geographical patterns of speciation for four clades of birds having taxa endemic to the same areas within the Neotropics. Two genera, Pionopsitta parrots and Selenidera toucans, corroborate a well known biogeographic disjunction in which taxa endemic to southern Central America and the Choco region of northwestern South America are the sister-group to a radiation within the Amazon basin. These two genera, along with two lineages within the toucan genus Pteroglossus, also document a pattern of historical interrelationships for four well known areas of endemism within Amazonia: Guyanan + (Belem-Para + (Inambari + Napo)). These generalized historical patterns are interpreted to have arisen via fragmentation (vicariance) of a widespread ancestral biota. A review of the paleogeographic evidence suggests that these vicariance events could have originated as a result of several different mechanisms operating at various times during the Cenozoic. The inference that diversification of the Neotropical biota is primarily the result of the most recent of these possible vicariance events, namely isolation within Quaternary forest refugia, is unwarranted, given present data. These patterns of historical congruence are also interpreted as direct evidence against the hypothesis that diversification of the forest biota was a consequence of parapatric differentiation along recently established ecological gradients.

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Patterns and processes of diversification: speciation and historical congruence in some neotropical birds.

The existence of a continuous landbridge connecting Central and South America during the uppermost Cretaceous and/or the lowermost Tertiary  cannot be proven on the basis of presently available field evidence. Scattered outcrops of Upper Cretaceous and Paleocene to Lower Eocene shales indicate the continuation of sedimentation in northwestern Colombia during these time intervals. The volcanic "basement complex" consists mostly of basaltic igneous rocks which intrude Middle Eocene shales and limestones. The pre-Upper Eocene orogeny and the strong pre-Upper Miocene orogeny led to the temporary uplift of the "Cuchillo bridge" which connected the Western Andes of Colombia with the Serrania del Darien of eastern Panama. Both these orogenies were followed by periods of subsidence when parts of the previously uplifted areas foundered and were covered by the Oligo-Miocene and Upper Miocene-Pliocene seas, respectively. The main uplift of the northern Andes and of the mountains of southern Central America took place during the uppermost Pliocene and lowermost Pleistocene; vertical movements continued throughout the Pleistocene to the present day. The final connection of Central and South America was established during the Upper Pliocene. The exchange of the Middle and South American faunas during the generally warm Tertiary period was mainly influenced by the emergence and subsidence of low islands and partial land bridges in southern Central America and northwestern Colombia. On the other hand, faunal migrations in these regions during the Quaternary were mainly determined by the following events: (1) The closing of the last water gap separating Central and South America in the Uraba region during the late Pliocene.  (2) The strong uplift of the South and Central American mountains which caused the expansion of dense tropical forests along the foot hills on the windward side and the increased aridity of the lowlands on the leeward side. The mountains themselves formed new zoogeographic barriers which had not existed before, (3) The alternation of cold-wet and warm-dry climatic phases throughout the Quaternary. These climatic changes influenced the distribution of forest and non-forest vegetation and also determined the changing altitudinal position of the temperature life zones and their continuity or discontinuity along the slopes of the mountain ranges.  The Uraba region was a transit area for the faunas of the more open and drier vegetation during dry climatic periods of the past and is an important meeting place of various Central and South American forest faunas today.

Geologic-climatic history and zoogeographic significance of the Urabá region in northwestern Colombia

The bird fauna of the isolated arid Andean valley of the upper rio Patia, southwestern Colombia, resembles that of the Cauca Valley to the north.  The most prominent faunal element are widely distributed nonforest birds of the Tropical Zone which entered the Andean valleys of Colombia from the Caribbean lowlands to the north. The relations between the upper Patia fauna and the nonforest fauna of western Ecuador to the south are very restricted (e.g. Veniliornis callonotus). Montane species of the Upper Tropical Subtropical Zone that inhabit the upper Patia Valley include the antshrike Thamnophilus multistriatus, the nightingale-thrush C atharus aurantiirostris, the warbler Basileuterus culiciuorus and others.  Some of the Patia populations differ from conspecific Cauca Valley populations by paler plumage coloration reflecting the arid climate of the restricted mountain valley of the upper rio Patia, especially those of Forpus conspicillatus and C atharus aurantiirostris. However, these differences together with the c1inal nature of the geographical variation do not warrant taxonomic recognition as separate subspecies.

Jürgen Haffer

Papers

Geologic-climatic history and zoogeographic significance of the Urabá region in northwestern Colombia

On the avifauna of the upper Patía Valley, Southwestern Colombia