( B) Chromosomal syntenic relationship of two billfishes and stickleback each line represents a syntenic block of 10 or more zones from the results of LAST with a similarity of 75% or more and length > 1,000 bp. Endothermy is indicated with red vertical bars (see legend) at the branches. Cz: Cenozoic, Pg: Paleogene, K: Cretaceous, J: Jurassic, Tr: Triassic, P: Permian, MIS: Carboniferous, D: Devonian, S: Silurian, O: Ordovician, C: Cambrian. Geological periods and ages (million years ago, MYA) are shown at the bottom.

2017), and the divergence time of each node were retrieved from the website TimeTree ( August24, 2020), which were estimated from a population of studies ( Kumar et al. ( A) Phylogenetic tree of vertebrate orders and major groups of endothermic animals (the backbone topology is from a previous study of bony fishes ( Betancur-R et al. Origins of endothermy in vertebrates and genome assemblies for two billfishes. Several features have been found to be necessary for endothermy in teleosts and chondrichthyans, including large body size, a heat source (specific heat-generating organ or high basal metabolic rate), and heat exchangers (retia mirabilia) for conserving the heat ( Block 1991). It is harder for organisms to keep warm in water than on land, because of the high heat capacity of water and the rapid rate of heat loss in aqueous environments ( Block 1991 Tullis, Block and Sidell 1991). In teleosts, it is represented by Istiophoriformes (billfishes), Scombriformes (tunas and butterfly kingfish), and Lampriformes (ophas) ( Block and Finnerty 1994 Dickson and Graham 2004 Runcie et al. Endothermy is not only present in mammals and birds, it has also independently evolved at least five times in teleosts and chondrichthyans ( Block and Finnerty 1994). Endothermy, maintaining an elevated body temperature and controlling rate of heat production ( Hulbert and Else 2000 IUPS Thermal Commission 2001 Koteja 2004 Clarke and Pörtner 2010 Lovegrove 2017), has been a revolutionary innovation for organisms, allowing them to overcome environmental constraints and adapt to new ecological niches ( Clarke and Pörtner 2010 Hedrick and Hillman 2016).

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