An early Pangaean vicariance model for synapsid evolution

Abstract

Genetic isolation due to geographic separation (vicariance) is the best understood cause of vertebrate speciation. nevertheless, it has never been demonstrated in the fossil record across a wide range of taxa. Here, by reviewing in-depth the available data of the Late Palaeozoic (~ 350-250 million years ago), i reconstructed an early pangaean junction-disjunction palaeogeographic model and showed that it coincides strongly with time-calibrated cladograms of the Late palaeozoic synapsids (the primitive ancestors of modern mammals). the temporal development of the vicariant topology seems to fit closely with the emergence rhythm of the recovered synapsid taxa, suggesting vicariance due to pangaean separation as the cause of early amniote evolution. the inferred vicariant topology also accounts for the observed pattern in the north American marine biostratigraphic units. Accordingly, the model demonstrates the link between the evolution of life on earth and palaeogeographic evolution and strongly supports allopatric speciation through vicariance as the prominent mode of amniote evolution. furthermore, correlations between state-of-the-art biochronostratigraphic charts and this palaeogeographic model suggest that the arido-eustasy model can explain the mid-permian biotic extinction event and depositional cycles, such as the pre-Zechstein of the central european Basin. Vicariance is the geographical separation of previously sympatric populations due to the development of geographical and/or ecological barriers to gene flow 1. Through vicariance, conspecific populations become genetically isolated and subsequently accumulate different mutations that render them reproductively incompatible resulting in the creation of new species 2. One common way for temporally heterogeneous geographic barriers to form is via eustatic sea-level changes that produce seaways on low profile intercontinental land bridges 3. Accordingly, transgressive stages, which occur when sea-levels are high, are expected to coincide with vicariance of the terrestrial biota, whereas regressive stages, which occur when sea-levels are low, are expected to coincide with geodispersals, and the opposite pattern is expected for marine biota 4. To verify a vicariance pattern, a junction-disjunction palaeogeographic model must correlate with phylogenetic topologies of multiple taxa 5. Although vicariance is the best understood mode of speciation 1,2 , due to the absence of detailed scenarios of palaeogeographic changes and sufficiently resolved and representative phylogenetic trees, allopatric speciation has not been confirmed for a wide range of taxa in the vertebrate fossil record. Based on the most apparent episodes of sea-level change that affected the southern connections between the Uralian Seaway (URS) and the Palaeotethys, an early Pangaean junction-disjunction palaeogeographic model was reconstructed and compared to time-calibrated consensus cladograms ("clado-stratigraphic patterns") of Late Palaeozoic vertebrates to determine whether a vicariance pat...

Evidence for a recent (0.65 Ma) formation of the Isthmus of Panama

“…Such a recent resemblance in the molluscan assemblages on either side of the isthmus cannot easily be explained if the final emergence of the isthmus occurred at 3 Ma. Likewise, the occurrence of the atlantiphile tonnoidean gastropod species Linatella caudata in the earlyto-mid Pleistocene Armuelles Formation [103] in Pacific Panama indicate the existence of shallow-water interoceanic exchange [96] between the Pacific and the Caribbean during the whole Early Pleistocene [94,101]. Continued Early or Middle Pleistocene connections are also demonstrated by (now Atlantic) in the Armuelles Formation of Pacific Panama [104].…”

“…Most of the relevant species are marine species for which, if a long-distance dispersal and/or a "sweepstake" dispersal is excluded, there is no way of dispersal to either side of the isthmus other than an exposed transisthmian seaway. The above two biogeographical concepts are used frequently to support phylogeographical conclusions, adding "noise" to the biogeographical signal; however, they might be the result of poorly understood palaeogeographic evolution and/or inaccurate dates for clade divergences due to incorrectly calibrated molecular clocks [94]. This is probably also the case in the biogeography of the Isthmus of Panama, where molecular divergence dates earlier than the "benchmark" date of 3 Ma have been easily attributed to "sweepstake" dispersals.…”

“…[82]). Terrestrial migrations across an isthmus provide evidence that the isthmus was exposed as a land bridge at the time of the mi- *not considered as geminate species by the original authors **stable molecular clock percentage characterizing a specific taxonomic rank gration, whereas the presence of related marine biota on both sides of an isthmus suggests the former presence of a seaway through the isthmus [94]. Evidence from terrestrial biota that might have migrated through a subaerial exposure of the Isthmus of Panama 1-0.89 Ma ago can be found in the concept of the Great American Biotic nterchange [95] and, more specifically, within the third terrestrial migration event (GABI 3) Specifically, the third dispersal event of the Great American Biotic Interchange (GABI 3) between North and South America took place ~0.9 Ma [95], resulting in the presence of opossum (Didelphis) in North America ~0.9 Ma (0.8-1.0 Ma) [95] and that of the jaguarundi Herpailurus, the cervid Paraceros, the peccary Pecari, and the tayassuid Tayassu in imprecisely dated Bonaerian beds (Middle Pleistocene, ~0.7 Ma) in South America [95][96].…”

Croizat’s form-making, RNA networks, and biogeography

“…In this context, we note a recent analysis of the distribution of fossil taxa and their palaeoclimate that supports the conclusion that vertebrates did not cross the relatively narrow Palaeozoic marine Ural Seaway (between Siberia and Baltica) via random or ‘sweepstakes’ dispersal. Brikiatis ( 2020 ) investigated the impact of arido-eustatic cycles on the Palaeozoic evolution of vertebrates and copepods and constructed a vicariance model that was highly predictive of fossil distributions and in agreement with sea level stands and paleoclimatic records. He concluded that vicariance, i.e., in situ evolution, was the predominant mode of vertebrate evolution (also supported by recent, more accurate radioisotopic dating of fossiliferous rock containing early synapsid taxa).…”

Evidence for a Recent Formation of the Isthmus of Panama 0.6 Mya

“…62 ). Terrestrial migrations across an isthmus provide evidence that the isthmus was exposed as a land bridge at the time of the migration, whereas the presence of related marine biota on both sides of an isthmus suggests the former presence of a seaway through the isthmus 131 . Evidence from terrestrial biota for the formation of the Isthmus of Panama 1-0.89 Mya has been reviewed under the concept of the Great American Biotic Interchange 17 and, more speci cally, the third terrestrial migration event (GABI 3).…”