Small body size and extreme cortical bone remodeling indicate phyletic dwarfism inMagyarosaurus dacus(Sauropoda: Titanosauria)

Abstract

Sauropods were the largest terrestrial tetrapods (>10 5 kg) in Earth's history and grew at rates that rival those of extant mammals. Magyarosaurus dacus, a titanosaurian sauropod from the Upper Cretaceous (Maastrichtian) of Romania, is known exclusively from small individuals (<10 3 kg) and conflicts with the idea that all sauropods were massive. The diminutive M. dacus was a classical example of island dwarfism (phyletic nanism) in dinosaurs, but a recent study suggested that the small Romanian titanosaurs actually represent juveniles of a larger-bodied taxon. Here we present strong histological evidence that M. dacus was indeed a dwarf (phyletic nanoid). Bone histological analysis of an ontogenetic series of Magyarosaurus limb bones indicates that even the smallest Magyarosaurus specimens exhibit a bone microstructure identical to fully mature or old individuals of other sauropod taxa. Comparison of histologies with large-bodied sauropods suggests that Magyarosaurus had an extremely reduced growth rate, but had retained high basal metabolic rates typical for sauropods. The uniquely decreased growth rate and diminutive body size in Magyarosaurus were adaptations to life on a Cretaceous island and show that sauropod dinosaurs were not exempt from general ecological principles limiting body size.bone histology | Sauropoda | secondary osteon | nanism | island fauna S auropod dinosaurs were the largest animals that ever roamed the surface of the Earth (1, 2). Gigantic size was acquired early in the evolutionary history of the group, in the Late Triassic (3). Recent studies of bone histology have shown that sauropods attained their gargantuan sizes by an evolutionary increase in their growth rate to levels comparable to those of extant endothermic mammals (4, 5). However, not all sauropods were multi-ton animals. Some titanosaurs are known to have had relatively small body sizes by sauropod standards; e.g., the South American Neuquensaurus australis reached a body length of about 7-9 m (6, 7), and its body mass is estimated at 3,500 kg. The recently described basal macronarian Europasaurus holgeri from the Late Jurassic of Germany (8) was even smaller, with a total estimated adult body length of approximately 6.2 m and a body mass of 800 kg.Another small-bodied titanosaurian sauropod, Magyarosaurus dacus, is known from the Upper Cretaceous (Maastrichtian) continental formations of the Hat xeg Basin of Romania (9, 10). These strata contain an array of relatively small-bodied dinosaur taxa, including the basal hadrosaurid Telmatosaurus (11), and two species of the noniguanodontian euornithopod Zalmoxes (12). In a famous early evolutionary hypothesis involving dinosaurs, the small body size of these taxa prompted the brilliant Hungarian paleontologist Franz Baron Nopcsa to hypothesize that, like Mediterranean dwarf proboscideans (13), the Hat xeg dinosaurs evolved their diminutive body size on a paleo-island (14, 15). Later, however, rare larger titanosaur bones were recovered from the Hat xeg Basin as well and describ...

A hypothesis of differential secondary bone formation in dinosaurs

“…Another example may be provided by dwarf sauropods (e.g., Benton et al, 2010;D'Emic and Wilson, 2012;Stein et al, 2010) whose long bones show extensive secondary remodeling. Biomechanical stresses could have been involved, but because all bones tend to be substantially affected by Haversian remodeling, and the long bones of much larger related sauropods (with much higher proportional stresses) are less so, we find no support for a biomechanical hypothesis.…”

The youngest sauropod evidence in Europe

“…The external primary bone is lost, probably due to the erosion of the bone surface. However, extreme remodelling in Magyarosaurus dacus includes loss of the cortical bone (Stein et al 2010). The Haversian bone is composed of relatively large and densely packed secondary osteons (dense Haversian tissue) ( Figure 5).…”

An aberrant island-dwelling theropod dinosaur from the Late Cretaceous of Romania

“…Attributing a causal relationship between an island habitat and the unusual morphology of inhabitant taxa is difficult (1,2). However, given the fact that Late Cretaceous Romania was clearly an island, numerous contemporaries of Balaur exhibit dwarfing and other morphological modifications seen in recent island-dwelling taxa (4)(5)(6)(7)(8), and the extreme anatomical differences between Balaur and its closest relatives, we hypothesize that the aberrant nature of the dromaeosaurid is likely due to the island effect. As such, this is a unique record of an island effect in a nonavian theropod.…”

“…The vertebrates inhabiting these islands have long been recognized as peculiar (4,5). Herbivorous dinosaurs were dwarfed compared to close relatives (5)(6)(7)(8) and formed endemic clades that are remarkably primitive relative to contemporaries from other continents, a pattern also seen in lizards, turtles, crocodylians, and mammals (2,5,9). As a result, Late Cretaceous assemblages in Romania and elsewhere in Europe are considered prime examples of abnormal island faunas in the fossil record (2).…”