Abstract
The sutures of the skulls of vertebrates are generally open early in life and slowly close as maturity is attained. The assumption that all vertebrates follow this pattern of progressive sutural closure has been used to assess maturity in the fossil remains of non-avian dinosaurs. Here, we test this assumption in two members of the Extant Phylogenetic Bracket of the Dinosauria, the emu, Dromaius novaehollandiae and the American alligator, Alligator mississippiensis, by investigating the sequence and timing of sutural fusion in their skulls. As expected, almost all the sutures in the emu skull progressively close (i.e., they get narrower) and then obliterate during ontogeny. However, in the American alligator, only two sutures out of 36 obliterate completely and they do so during embryonic development. Surprisingly, as maturity progresses, many sutures of alligators become wider in large individuals compared to younger, smaller individuals. Histological and histomorphometric analyses on two sutures and one synchondrosis in an ontogenetic series of American alligator confirmed our morphological observations. This pattern of sutural widening might reflect feeding biomechanics and dietary changes through ontogeny. Our findings show that progressive sutural closure is not always observed in extant archosaurs, and therefore suggest that cranial sutural fusion is an ambiguous proxy for assessing maturity in non-avian dinosaurs.
Highlights
The bones of the skulls of vertebrates are linked by different types of articulations, such as fibrous sutures in the cranial vault, the face and the palate, and cartilaginous synchondroses in the skull-base [1]
If the phylogenetic analysis produces an ontogram that is consistent with ontogeny, this would indicate that progressive sutural closure does occur during ontogeny and that it is a reliable indicator of maturity in this species
Growth series of the skulls of the emu (Dromaius novaehollandiae, n = 24; Table 1) and the American alligator (Alligator mississippiensis n = 50; Table 2), were analyzed in order to determine the hierarchy of sutural closure
Summary
Maceration or boiling often leads to disarticulation and may result in showing inaccurate sutural widths.We aimed to eliminate this taphonomical bias by using mostly specimens skeletonized by dermestid beetles and limited the use of macerated specimens (Table 2). We aimed to minimize this possibility by limiting the use of macerated specimens
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