Abstract
BackgroundDuring their evolution in the Late Cretaceous, mosasauroids attained a worldwide distribution, accompanied by a marked increase in body size and open ocean adaptations. This transition from land-dwellers to highly marine-adapted forms is readily apparent not only at the gross anatomic level but also in their inner bone architecture, which underwent profound modifications.Methodology/Principal FindingsThe present contribution describes, both qualitatively and quantitatively, the internal organization (microanatomy) and tissue types and characteristics (histology) of propodial and epipodial bones in one lineage of mosasauroids; i.e., the subfamily Mosasaurinae. By using microanatomical and histological data from limb bones in combination with recently acquired knowledge on the inner structure of ribs and vertebrae, and through comparisons with extant squamates and semi-aquatic to fully marine amniotes, we infer possible implications on mosasaurine evolution, aquatic adaptation, growth rates, and basal metabolic rates. Notably, we observe the occurrence of an unusual type of parallel-fibered bone, with large and randomly shaped osteocyte lacunae (otherwise typical of fibrous bone) and particular microanatomical features in Dallasaurus, which displays, rather than a spongious inner organization, bone mass increase in its humeri and a tubular organization in its femora and ribs.Conclusions/SignificanceThe dominance of an unusual type of parallel-fibered bone suggests growth rates and, by extension, basal metabolic rates intermediate between that of the extant leatherback turtle, Dermochelys, and those suggested for plesiosaur and ichthyosaur reptiles. Moreover, the microanatomical features of the relatively primitive genus Dallasaurus differ from those of more derived mosasaurines, indicating an intermediate stage of adaptation for a marine existence. The more complete image of the various microanatomical trends observed in mosasaurine skeletal elements supports the evolutionary convergence between this lineage of secondarily aquatically adapted squamates and cetaceans in the ecological transition from a coastal to a pelagic lifestyle.
Highlights
Mosasauroidea includes medium-sized to giant lizards that evolved in the oceans of the Late Cretaceous between 98 and 66 Ma ago (e.g. [1,2])
Three general morphotypes have been recognized [6,7] that seemingly illustrate progressive steps in the adaptation of mosasauroids to open-ocean habitats, as well as increasing abilities for more energy-efficient swimming. These morphotypes include: (1) rather small forms that display terrestrial-like limbs and a typical squamate pelvic girdle; (2) taxa possessing plesiopedal limbs but where the pelvic girdle has been transformed so that there is no sacral contact with the ilium; and (3) forms that have both paddle-like limbs and a hydropelvic anatomy
In the present contribution, we describe, both qualitatively and quantitatively, microanatomical and histological specializations observed in middiaphyseal sections of pro- and epipodial bones in one lineage of mosasauroids; i.e., the subfamily Mosasaurinae, and compare these features with those observed in extant squamates and extant and extinct semi-aquatic to fully marine amniotes
Summary
Mosasauroidea includes medium-sized to giant lizards that evolved in the oceans of the Late Cretaceous between 98 and 66 Ma ago (e.g. [1,2]). Three general morphotypes have been recognized [6,7] that seemingly illustrate progressive steps in the adaptation of mosasauroids to open-ocean habitats, as well as increasing abilities for more energy-efficient swimming. These morphotypes include: (1) rather small forms (typically ,2 meters long) that display terrestrial-like (plesiopedal) limbs and a typical squamate pelvic girdle (i.e., a plesiopelvic anatomy); (2) taxa (from 3 to 6 meters long) possessing plesiopedal limbs but where the pelvic girdle has been transformed so that there is no sacral contact with the ilium (i.e., a hydropelvic anatomy); and (3) forms (from 4 to 15 meters long) that have both paddle-like (hydropedal) limbs and a hydropelvic anatomy. We conclude with a discussion on the implications for mosasaurine evolution, including increased growth rates and elevated basal metabolic rates
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