Abstract
AbstractThe typical marine animal has increased in biovolume by more than two orders of magnitude since the beginning of the Cambrian, but the causes of this trend remain unknown. We test the hypothesis that the efficiency of intra-organism oxygen delivery is a major constraint on body-size evolution in marine animals. To test this hypothesis, we compiled a dataset comprising 13,723 marine animal genera spanning the Phanerozoic. We coded each genus according to its respiratory medium, circulatory anatomy, and feeding mode. In extant genera, we find that respiratory medium and circulatory anatomy explain more of the difference in size than feeding modes. Likewise, we find that most of the Phanerozoic increase in mean biovolume is accounted for by size increase in taxa that accomplish oxygen delivery through closed circulatory systems. During the Cambrian, water-breathing animals with closed circulatory systems were smaller, on average, than contemporaries with open circulatory systems. However, genera with closed circulatory systems superseded in size genera with open circulatory systems by the Middle Ordovician, as part of their Phanerozoic-long trend of increasing size. In a regression analysis, respiratory and circulatory anatomy explain far more size variation in the living fauna than do feeding modes, even after accounting for taxonomic affinity at the class level. These findings suggest that ecological and environmental drivers of the Phanerozoic increase in the mean size of marine animals operated within strong, anatomically determined constraints.
Highlights
The biovolume of the average-sized marine animal genus has increased by more than two orders of magnitude since the beginning of Cambrian time (Fig. 1) (Heim et al 2015)
Physiological Anatomy.—We considered the medium from which an animal extracts oxygen and whether the animal has an open or closed circulatory system
The mean size of water-breathing genera with closed circulation is more than two orders of magnitude larger than the mean size of genera that are water breathers with open circulatory systems
Summary
The biovolume of the average-sized marine animal genus has increased by more than two orders of magnitude since the beginning of Cambrian time (Fig. 1) (Heim et al 2015). The proposed causes of this size increase include both an environment more permissive of larger size due to increasing availability of oxygen in seawater (Berner et al 2007; Dahl et al 2010) and ecosystems more capable of supporting large animals through increasing levels of primary productivity (Tappan and Loeblich 1973; Peters 1983; Vidal and Knoll 1983; Bambach 1993; Thiebaux and Dickie 1993; Bambach 1999; Vermeij 2011, 2016). Not all phyla and classes experienced similar trends in size (Fig. 2). Chordates have become substantially larger, on average, over time, whereas mollusks have been nearly constant in mean size since the Ordovician. IsPeasd/dbreys/s:43.04./2)0,5.w15h7.i2c4h1,poenr0m2 iNtosvu2n02r1esattr1ic5t:2e1d:51r,es-uubsjee,ctdtoistthreibCuamtiobrnid, gaenCdorereteprrmosdoufction in use, availaanblye amt hetdtpius:/m/w,wpwr.ocvamidberiddgteh.oergo/rciogrien/taelrmwso. hrkttpiss:/p/drooi.poergr/l1y0.c1i0t1e7d/.pab.2020.16
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