Abstract
Current patterns of biological distribution result from the deep past. Of particular interest, some closely related species appear at high latitudes of both hemispheres, but not in between, a pattern known as antitropical distribution. However, the timing, pathway, and drivers of antitropical distributions remain mostly unknown. Here we describe a new fossil, a left tympanic bulla (part of the ear bones), from the Middle/Late Pleistocene (0.78–0.01 mya, but not excluding the possibility of Holocene in age, as the specimen was dredged from the sea bottom and the geological horizon remains uncertain) of Taiwan. The tympanic bulla is diagnostic in baleen whales, and this specimen shows morphological features that are identical to extant Eubalaena, including: relatively large size (the anteroposterior length is 117 mm); rectangular outline in medial view; short anterior lobe, judging from the remaining of the lateral furrow; squared anterior margin; prominent transverse crease on the involucrum; transversely compressed in anterior view; well-developed and rounded outer lip; and parallel involucral and main ridges. Although incomplete, the morphological characters and overall similarity to extant Eubalaena allow a reliable taxonomic assignment to Eubalaena sp. The occurrence of a Pleistocene Eubalaena on the southern margin of the western North Pacific is the first balaenid fossil evidence indicative of the biotic interchange between two hemispheres leading to the origin of antitropical distribution in the Pleistocene; alternatively, this specimen might merely represent an extra-limital record of the North Pacific Eubalaena. Furthermore, this find suggests that the Eubalaena interchange, being one of the largest species displaying antitropical distribution pairs in the history of life, likely took place along the western Pacific. Notably, this does not preclude the Eubalaena interchange from other routes, such as the eastern Pacific or the Atlantic Ocean, and future finds should test the scenario for the biotic interchange between Northern and Southern Hemispheres of Eubalaena.
Highlights
IntroductionThe antitropical distribution in which closely related species appear at high latitudes of the Northern and Southern Hemispheres, but not in the tropical regions, is singular and remarkable
Biogeographic patterns are dynamic and constantly evolving
The faunal composition based on the published fossils includes marine and terrestrial mammals [20,21,22], reflecting the shallow bed of the Taiwan Strait that converted between sea and land during the interglaciations and glaciations in the Pleistocene
Summary
The antitropical distribution in which closely related species appear at high latitudes of the Northern and Southern Hemispheres, but not in the tropical regions, is singular and remarkable. Recognition of such discontinuous distributions in latitude has been noted since Darwin [1], but the details, including the pathway, timing, and drivers of antitropical distributions remain largely unspecified and speculative [2, 3]. Given its geological and geographic occurrence, this Pleistocene Eubalaena lends support to the glaciation/interglaciation hypothesis It implies that the Eubalaena interchange may have taken place between Northern and Southern Hemispheres along the western Pacific (not excluding the possibility of other dispersal routes, such as the eastern Pacific or the (2019) 5:37
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