Scientific consensus establishes that while non-avian dinosaurs are extinct, avian dinosaurs—modern birds—survive and thrive today, meaning dinosaurs technically still exist.
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.
Unlike mammals, reptiles typically lack large muscles and ligaments that connect the zygoma to the mandible. Dinosaur craniomandibular soft tissue reconstructions, often based on the rationale of extant phylogenetic bracketing, follow this general rule. However, descending flanges from the zygomata of hadrosaurs, heterodontosaurids, and psittacosaurids have been used to argue for a masseter‐like muscle in these dinosaur taxa. We examined dinosauriform skulls for osteological indicators of connective tissue entheses on the zygoma and mandible, and subsequently sectioned 10 specimens for histological evidence. Osteological indicators were found on the zygoma in most sampled dinosauriforms, which range from rugosities to large descending processes, and morphologically resemble known muscular and ligamentous entheses. Similarly, rugose features oriented towards the zygoma were found on the mandible in sampled dinosauriforms, many having previously been interpreted as entheses for the adductor mandibulae muscle group. Serial histological sectioning of ceratopsid, hadrosaurid, and tyrannosaurid jugal and surangular rugosities reveals an external cortex rich in collagen fibres, strongly resembling entheseal fibres. Jugal entheseal fibres are usually oriented ventrally towards the surangular, and in hadrosaurids and tyrannosaurids these are parallel to macroscopic striations on the surfaces of the jugal flange. Histological sections of extant chicken buccal regions show similar entheseal fibres in the attachments of the jugomandibular ligament on the jugal and of the adductor musculature on the mandible. We hypothesise a strong connective tissue structure bridging the zygoma and mandible in dinosaurs, termed the ‘exoparia’. This structure's size and proximity to the craniomandibular joint would be advantageous in stabilising the mandible relative to the cranium during jaw movement, particularly in dinosaurs thought to process their masticate. A ligamentous or muscular identity for the exoparia cannot be determined with the available data, but the size and shape of the zygomatic entheses in many dinosaurs are more consistent with a muscular attachment. Possible antecedents in non‐dinosauriform archosaurs and derivations in modern birds may exist, but the homology of the exoparia is currently unknown. These results highlight the complex soft tissue evolution of dinosaurs and caution against simplified phylogenetic model‐based approaches to tissue reconstruction that ignore contrasting osteological signals.
approximately 66 mya. Dinosaurs can therefore be divided into avian dinosaurs—birds—and the extinct non-avian dinosaurs, which are all dinosaurs other than birds
Dinosaurs are a diverse group of warmblooded reptiles of the clade Dinosauria. They existed through most of the Mesozoic era, first appearing early in the Triassic period. They became the dominant terrestrial vertebrates after the Triassic–Jurassic extinction event 201.3 mya and their dominance continued throughout the Jurassic and Cretaceous periods. The fossil record shows that birds are feather
All non-avian dinosaurs and most lineages of birds became extinct in a mass extinction event, called the Cretaceous–Paleogene (K-Pg) extinction event, at the end of the Cretaceous period. Above the Cretaceous–Paleogene boundary, which has been dated to 66.038 ± 0.025 million years ago, fossils of non-avian dinosaurs disappear abruptly; the absence of dinosaur fossils was historically used to assign rocks to the ensuing Cenozoic. The nature of the event that caused this mass extinction has been extensively studied since the 1970s, leading to the development of two mechanisms that are thought to have played major roles: an extraterrestrial impact event in the Yucatán Peninsula, along with flood basalt…
mankind evolved. Evolutionary descendants of dinosaurs still live today, including the birds. For the literal … did not become extinct after all, but instead still exist today on our birdfeeders (see Question 9). Table … popular idea. At the time of the Genesis flood, dinosaurs still lived on earth. Smaller, youthful representatives
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