Penguin heat-retention structures evolved in a greenhouse Earth

Abstract

Penguins (Sphenisciformes) inhabit some of the most extreme environments on Earth. The 60+ Myr fossil record of penguins spans an interval that witnessed dramatic shifts in Cenozoic ocean temperatures and currents, indicating a long interplay between penguin evolution and environmental change. Perhaps the most celebrated example is the successful Late Cenozoic invasion of glacial environments by crown clade penguins. A major adaptation that allows penguins to forage in cold water is the humeral arterial plexus, a vascular counter-current heat exchanger (CCHE) that limits heat loss through the flipper. Fossil evidence reveals that the humeral plexus arose at least 49 Ma during a ‘Greenhouse Earth’ interval. The evolution of the CCHE is therefore unrelated to global cooling or development of polar ice sheets, but probably represents an adaptation to foraging in subsurface waters at temperate latitudes. As global climate cooled, the CCHE was key to invasion of thermally more demanding environments associated with Antarctic ice sheets.

Chatham Island Paleocene fossils provide insight into the palaeobiology, evolution, and diversity of early penguins (Aves, Sphenisciformes)

“…Outgroups. A volant distant outgroup taxon, Phaethon rubricauda (Phaethontiformes), was selected to better identify the evolutionary trajectory of character evolution as per the well-supported and commonly cited (e.g., Baker and Manwell, 1975;Raikow et al, 1988;Thomas et al, 2011; hypothesis that penguins are descended from aerially flighted ancestors . Studies of higher avian phylogenies have long recovered Phaethontiformes among aquatic birds (Cracraft, 1981(Cracraft, , 1988, part of a lineage sister to core waterbird clades (e.g., van Tuinen et al, 2001;Jarvis et al, 2014;Prum et al, 2015;Thomas, 2015;Houde et al, 2019).…”

“…Taxa such as these would have lived in a greenhouse interval with exceptionally warm poles (Zachos et al, 2008), predating the formation of the Circum-Ant-arctic Current, in a world before the southern polar ice cap; when ocean circulation and climate was drastically different as a result (Campbell and Hutching, 2007). While sub-tropical to tropical surface water temperatures existed (Hollis et al, 2009) in the warmer early Cenozoic (Zachos et al, 2008), the sub-surface water would have still been cooler than penguin body temperature (Ponganis et al, 2003;Thomas et al, 2011). Indeed, modern penguins have geographic distributions that are largely correlated with specific aquatic temperature ranges (see Stonehouse, 1967;Simpson, 1971: 374), and also possess numerous thermoregulatory adaptations related to survival in cooler waters (e.g., Frost et al, 1975;Kooyman et al, 1976;Thomas and Fordyce, 2007).…”

“…Indeed, modern penguins have geographic distributions that are largely correlated with specific aquatic temperature ranges (see Stonehouse, 1967;Simpson, 1971: 374), and also possess numerous thermoregulatory adaptations related to survival in cooler waters (e.g., Frost et al, 1975;Kooyman et al, 1976;Thomas and Fordyce, 2007). Thomas et al (2011) hypothesised that the evolution of the rete mirabile of the forelimb in early penguins would have promoted greater foraging duration at cooler sub-surface water temperatures, and increased their ability to forage for longer durations and greater distances. Evidence of a humeral plexus of this fashion has not been confidently observed in any Paleocene penguin, and the observation that the majority of these early taxa have been recovered in a relatively close proximity, near the east coast of New Zealand's South Island, may imply that a humeral plexus may not have yet evolved, effectively restricting them to a relatively near-shore foraging habitat .…”

Another look at tarsometatarsi of early penguins

“…Modern penguins are charac− terized by the presence of vascular counter−current heat exchangers. One of them, the humeral plexus, apparently arose at least 49 Ma (i.e., during the Eocene epoch) (Thomas et al 2011). In legs of most, if not all (Midtgård 1981), modern Sphenisciformes, the "extensive single−artery: multiple−vein" association exists as well (Frost et al1975).…”

Phylogenetically vetted and stratigraphically constrained fossil calibrations within Aves

“…The absence of substantial gaps in the early record of penguins, their relatively high preservation potential (due to dense bones and occurrence in shallow marine environments), and support for an area of origin in New Zealand (Slack et al, 2006;Thomas et al, 2011) all suggest that the appearance of penguins in the fossil record closely corresponds at least to the timing of their shift to a flightless marine ecology, if not to their split from their sister taxon Procellariiformes (Figure 4). Fossil Record of Related Clades.…”