Integrative paleophysiology of the metriorhynchoid Pelagosaurus typus (Pseudosuchia, Thalattosuchia)

Abstract

Paleophysiology is an emergent discipline. Organismic (integrative) approaches seem more appropriate than studies focusing on the variation of specific features because traits are tightly related in actual organisms. Here, we used such an organismic approach (including lifestyle, thermometabolism, and hunting behavior) to understand the paleobiology of the lower Jurassic (Toarcian) thalattosuchian metriorhynchoid Pelagosaurus typus. First, we show that the lifestyle (aquatic, amphibious, terrestrial) has an effect on the femoral compactness profiles in amniotes. The profile of Pelagosaurus indicates that it was amphibious, with a foraging activity in shallow marine environments (as suggested by the presence of salt glands) and thermoregulatory basking behavior in land (as suggested by the presence of osteoderms with highly developed ornamentation). As for the thermometabolism, we show that the mass-independent resting metabolic rate of Pelagosaurus is relatively high compared to the sample of extant ectothermic amniotes, but analysis of vascular canal diameter and inferences of red blood cell size refute the hypothesis suggesting incipient endothermy. Finally, the foraging behavior was inferred using two proxies. Pelagosaurus had a mass-independent maximum metabolic rate and an aerobic scope higher than those measured in the almost motionless Iguana iguana, similar to those measured in the sit-and-wait predator Crocodylus porosus but lower than those quantified in the active hunter Varanus gouldii. These results suggest that Pelagosaurus may have had a hunting behavior involving a slow sustained swimming or a patient waiting in shallow waters, and may have caught preys like gharials, using fast sideways sweeping motions of the head.

Keywords: Crocodylomorpha; paleohistology; paleophysiology.

FIGURE 1

Dorsal view of Pelagosaurus typus MNHN.F.RJN470 exposed at the Paleontology Gallery of the Muséum national d'Histoire naturelle (Paris). The two rows of dorsal osteorderms placed on the right and left sides of the specimen show extensive ornamentation. © Lilian Cazes, MNHN.

FIGURE 2

Pelagosaurus typus material analyzed in this study. (a) Left femur MNHN.F.RJN470 in dorsal, anterior, ventral and posterior views (from left to right) analyzed using CT‐scan © Philippe Loubry, MNHN. (b) Proximal right femur MNHN.F.RJN463, associated to ribs and osteoderms, in dorsal view, analyzed using paleohistology. © Lilian Cazes, MNHN.

FIGURE 3

Diaphyseal cross section of the right femur of Pelagosaurus typus MNHN.F.RJN463. (a) Complete cross section showing at least five lines of arrested growth (white arrows). (b) Outer layer at the dorsal side showing the process of formation of primary osteons (black arrows).

FIGURE 4

Drawings showing the bone tissue distribution on diaphyseal cross sections of (a) left femur MNHN.F.RJN470 and (b) right femur MNHN.F.RJN463 (this section is incomplete because the bone was broken at the diaphysis). Drawings are not to scale.

FIGURE 5

Mass‐independent resting metabolic rates quantified in a sample of extant amniotes and inferred in Pelagosaurus typus using a set of phylogenetic eigenvectors and the relative area of primary osteons. Dataset available in Table 1.

FIGURE 6

Phylogenetic logistic regression modeling the distribution of probabilities of being endothermic inferred for our sample of extant tetrapods using the harmonic mean of femoral vascular canal diameter as the explanatory variable (modified from figure 3 in Cubo et al., 2023).

FIGURE 7

Red blood cell sizes quantified in a sample of extant amniotes and inferred in Pelagosaurus typus using a set of phylogenetic eigenvectors and the harmonic mean of bone vascular canal diameter (sensu Huttenlocker & Farmer, 2017). Dataset analyzed by Cubo et al. (2023) available in Dryad at Cubo et al. (2022).

FIGURE 8

CT‐scan of left femur MNHN.F.RJN470 in ventral view (anterior part on the right). White arrows point to the openings of the two nutrient foramina.

FIGURE 9

Mass‐independent maximum metabolic rates quantified in a sample of extant amniotes and inferred in Pelagosaurus typus using a set of phylogenetic eigenvectors and blood flow rate (Q̇, cm³ s⁻¹; Seymour et al., 2019) computed using nutrient foramina size. Dataset available in Table 2.