Age determination in dinosaurs

Early attempts to estimate the longevity of dinosaurs used allometric scaling principles. Ages were determined by dividing individual mass estimates by rates of growth for similar, extant taxa. For very large individuals, growth rates were extrapolated to dinosaur proportions using regression analysis. The results of these investigations have been extremely variable as they depend on mass estimates and growth rates that are highly at odds with one another. For example, using this method resulted in longevity estimates for the sauropod Hypselosaurus priscus that ranged from a few decades to several hundred years. However, it was later shown that most dinosaur bones have growth lines that are visible in thin-sectioned material viewed under a polarized light source. ==Growth lines==

''. Three LAGs are marked with white lines (A) and white arrowheads (B). Studies on extant vertebrates indicate that the vascularized zones form during moderate to rapid skeletogenesis, and that abrupt metabolic disruptions of bone formation can trigger growth line deposition. Histological examinations have revealed that annuli are composed of thin layers of avascular bone with parallel-aligned bone fibers. The growth line annuli are found compressed between broad vascularized regions of bone with randomly oriented fibrillar patterns, known as zones. Lines of arrested growth (LAGs) Lines of arrested growth, similar to annuli, are found between zones that are avascular. They are, however, much thinner and have relatively fewer bone fibers by volume. This is further supported by the fact that in modern animals, annuli and LAGs may be deposited in synchrony with endogenous biorhythms. For example, captive crocodilians exposed to constant temperature, diet, and photoperiod still exhibit the periodic and cyclical skeletal growth banding of their wild counterparts. However, the total number of observed LAGs is not directly reflective of an individual animal's age. Two LAGs may sometimes occur when extremely close to each other. Such are referred to as "double LAGs" and interpreted as a representation of one year of growth instead of two. In addition, resorption of internal and external bone proceeds even as new cortical bone continues to be deposited, so that growth lines deposited early in development may need to be inferred from the width of such remodelled bone, and the width between any LAGs that are preserved. This is referred to as "retrocalculation". == Maturity ==

Within academic studies, the meaning of terms such as "adult", "subadult", "juvenile", etc., can be ambiguous, and their meaning may differ between different studies. For instance, some authors consider animals to have reached adulthood upon reaching sexual maturity, while others only consider skeletally mature animals as adults. In addition, what one author considers "subadult" may be considered "juvenile" by another. As a result, the practice of using such terms has been discouraged, as well as references to "somatic" maturity, in favour of referring to specimens' "skeletal" maturity. Skeletal maturity A common indicator of skeletal maturity and somatic maturity in dinosaurs is the presence of an external fundamental system (EFS). An EFS is a grouping of closely spaced LAGs in the outermost cortex, caused by the animal's relatively slow growth as it approaches final size. It is often accompanied by a loss of vascularity; another indicator of skeletal and somatic maturity is the presence of an outer circumferential layer (OCL), which is avascular bone composed of slowly deposited parallel-fibered bone. The approximate degree of skeletal maturity that an animal has reached can also be inferred from the degree of fusion or co-ossification of bones. It may also be inferred when putative sociosexual structures become well developed, such as the cranial domes of pachycephalosaurs. In 2019, the Early Cretaceous enanthiornithine bird Avimaia was found gravid with an unlaid egg despite not having reached skeletal maturity, indicating that non-ornithuran birds still attained sexual maturity before skeletal maturity. With the exception of at least some species of flightless palaeognaths, all modern birds reach skeletal maturity within their first year of life, contrasting with non-avian dinosaurs. ==Longevity==

The data resulting from pioneering efforts to age dinosaur fossils using growth ring counts is used in conjunction with mass estimates in order to infer the metabolic status and growth rates of dinosaurs. It has been reported that chasmosaurines do not preserve LAGs, unlike other ceratopsians, making age determination of the group difficult. This is because chasmosaurines exhibit a continuous and uninterrupted growth pattern, which does not produce LAGs in the bone. In contrast, early-diverging neoceratopsians and centrosaurines exhibit cyclic, interrupted growth patterns, which result in the presence of LAGs. Unpublished histological studies of Utahceratops and Kosmoceratops appear to dispute the absence of LAGs in chasmosaurines. Some, such as Triceratops, do preserve LAGs, but only LAGs from later stages of life, obstructing age determination. Age estimation of mature titanosaurs is difficult. Intense secondary remodelling of the bone occurs after titanosaurs reach sexual maturity, obstructing LAGs. Similar high levels of remodelling is also seen in the euhelopodid Phuwiangosaurus. The Javelina Formation titanosaur specimen TMM 41541-1 from the Maastrichtian may be referable to Alamosaurus, although this pends the full description of the specimen. Up to 12 LAGs could be observed in the dorsal ribs, but the true age of the animal is still obfuscated by the secondary bone remodelling that is characteristic of titanosaurs. The presence of an EFS in only the posterior ribs suggests that growth did not stop at the same time in all parts of the skeleton. == See also ==