Age determination in dinosaurs

Age determination in dinosaurs is mainly used to determine the approximate ontogenetic age of a dinosaur when the animal died.

History

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, longevity estimates for the sauropod Hypselosaurus priscus range from a few decades to several hundred years. However, it has been shown that most dinosaur bones have growth lines that are visible in thin sectioned material viewed under a polarized light source.

Growth lines

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.
Two types of growth lines exist: annuli, and lines of arrested growth. 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 are avascular. They are, however, much thinner, and have relatively fewer bone fibers by volume. When modern animals deposit LAGs, one LAG is typically deposited annually, caused by the slowing or absence of growth during the harshest seasons of the year. As a result, each LAG is typically interpreted as representation of a year of growth. 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 in extremely close proximity to each other. Such are referred to as "double LAGs", and interpreted as 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 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 instead 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. 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, which is avascular bone composed of slowly deposited parallel-fibered bone.

Sexual maturity

Sexual maturity is inferred when intrinsic fibres of the bone become much more well organized, which may be accompanied by the slowing growth of the animal. It may also be inferred when putative sociosexual structures become well developed, such as the cranial domes of pachycephalosaurs. As a result, there are two abrupt decreases in growth rate across a dinosaur's lifespan: the first when it reaches sexual maturity, and the second when it reaches skeletal and somatic maturity.

Comparison to birds

Sexual maturity typically precedes skeletal maturity in non-avian dinosaurs and indeed most amniotes to varying degrees. In contrast, most modern birds experience an intense growth spurt early in life, and reach sexual maturity well after growing to maximum size, with kiwis as an exception. 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 results 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. 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.
In 2026, histology of Tyrannosaurus rex specimens revealed in over half of the studied specimens the presence of growth marks that only became visible when viewed under cross-polarized light. While there is some evidence to support the assumption that these growth marks are also annual, confirmation requires further investigation of their presence in other specimens and taxa.

Species	Estimated age of oldest measured specimen	Specimen #	Notes
Manidens condorensis	< 1 year	MPEF-PV 3211	Under a year old, considered a late juvenile or subadult.
Zavacephale rinpoche	2+ years	MPC-D 100/1209	Skeletally immature, with no EFS. Age estimate is based on the presence of 2 "cyclical growth marks". The cranial dome is well-developed, such that if the hypothesis that the cranial dome in pachycephalosaurs is a sociosexual structure is correct, implies that the animal had reached sexual maturity.
Yamaceratops dorngobiensis	3 years	MPC-D 100/553	Juvenile with 3 preserved LAGs, showing very little bone remodelling.
Liaoceratops yanzigouensis	3+ years	IVPP V17910	A subadult.
Psittacosaurus ''sibiricus	4+ years	KOKM 4652/1	A subadult that was least 4 years old. Larger specimens were histologically sampled, but no growth marks were observed in them.
Lesothosaurus diagnosticus	6 years		Several skeletally immature elements from a bonebed.
Psittacosaurus mongoliensis	9 years	PIN 698/1977	One of the largest known Psittacosaurus specimens. However, it likely could've grown noticeably larger as its growth rate had not yet plateaued.
Psittacosaurus lujiatunensis	11 years	ZMNH M8138	Although sexually mature, it likely could've grown noticeably larger as its growth rate had not yet plateaued.
Yinlong downsi	10+ years	IVPP V18637	An adult. 10 lines of arrested growth are visible, but partial erasure of the innermost LAGs prevent precise age estimation.
Laquintasaura venezuelae	10-12 years	MBLUZ P.5012	Scapula fragment of a skeletally mature adult, with a minimum estimated age of 10-12 years, showing an EFS.
Cerasinops hodgskissi	11 years	MOR 300	A "relatively mature" individual that was still slowly growing when it died at an estimated maximum age of 11 years.
Einiosaurus procurvicornis	6 years		The oldest known Einiosaurus specimens were all just 6 years old at time of death. None of them were fully grown adults, although they were sexually mature.
Pachyrhinosaurus sp.	19 years	UAMES 3551	A large femur from the Prince Creek Formation. The animal was still actively growing at time of death.
Hypacrosaurus stebingeri	13 years	MOR 549	An adult with an EFS. Sexual maturity occurred at 2-3 years of age.
Probrachylophosaurus bergei	14 years	MOR 2919	The specimen's growth rate had slowed, but lacked an EFS. It was not yet skeletally mature, although it was close to being so. It probably became sexually mature at 5 years old.
Dryosaurus altus	≤ 15 years	CM 1949	A femur lacking an EFS. The specimen only preserved LAGs at the outside perimeter of the bone. Estimation of the full number of LAGS that were originally present was done by comparing it to ontogenetically much younger specimens that preserved LAGs close to the bone cortex.
Maiasaura peeblesorum	15 years		An adult.
Edmontosaurus annectens	18+ years	ROM 73853	An adult whose growth rate was beginning to plateau.
Dysalotosaurus lettowvorbecki	20 years		An adult. Out of several bone-beds, this specimen's femur was the largest recovered.
Miragaia longicollum	25 years	ML 433	A skeletally mature adult, based on the presence of an EFS. Skeletal maturity occurred at 21-22 years of age. Sexual maturity occurred at 14-15 years of age.
Draconyx loureiroi	31 years	ML 439	A fully grown adult, based on the presence of an EFS. Sexual maturity occurred at 12-17 years of age.

Species	Estimated age of oldest measured specimen	Specimen #	Notes
Shuvuuia deserti	2 years	MGI 100/975	A juvenile.
Raptorex kriegsteini	2-3 years	LH PV18	A juvenile tyrannosaurid of controversial affinities. It is disputed whether it belongs to its own genus or is an immature Tarbosaurus. It was originally considered 5-6 years old, but this was challenged by a later histological study.
Mei long	2-4+ years	DNHM D2154	An adult whose growth had mostly stopped, and there is a structure that resembles an EFS.
Aniksosaurus darwini	3+ years		Two specimens were found to have been at least 3 years old at death, and are considered juveniles or subadults due to the absence of an OCL.
Albinykus baatar	3 years	MAE PSS 04-18	An adult.
Kiyacursor longipes	3+ years	KOKM 5542	A subadult individual that lack an EFS.
Berthasaura leopoldinae	3-4 years	MN-7821-V	Subadult that did not yet reach sexual maturity.
Haplocheirus sollers	4 years	IVPP V15988	A juvenile.
Daliansaurus liaoningensis	4 years	DNHM D2885	A subadult that was close to adulthood.
Liaoningvenator curriei	4+ years	DNHM D3012	Lacks an EFS and was still growing, but was close to skeletal maturity.
Talos sampsoni	4-6 years	UMNH VP 19479	An immature specimen lacking an EFS. Growth was beginning to slow down.
Troodontidae nov. sp.	6 years	IGM 100/1129	A new species of troodontid that is yet to be described as of 2025. Although skeletally immature and lacking an EFS, it was found brooding a clutch of eggs.
Eoneophron infernalis	6 years	CM 96523	Close to if not already skeletally mature.
Koleken inakayali	6+ years	MPEF-PV 10826	An immature individual that had not yet reached sexual maturity.
Moros intrepidus	6-7 years	NCSM 33392	A subadult.
Megapnosaurus rhodesiensis	7 years
Bannykus wulatensis	8 years	IVPP V25026	An adult.
Oviraptor philoceratops	8 years	AMNH FARB 6517	Was brooding a clutch of eggs when it died, however, it lacks an EFS and was still actively growing.
cf. Anzu wyliei	8+ years	ROM VP 65884	A partial skeleton of what is likely an Anzu wyliei, as it was from the Hell Creek Formation. The individual was at least 8 years old and actively growing when it died, and would've been approximately the same size as the A. wyliei holotype.
Rativates evadens	8+ years	ROM 1790	A subadult or adult, near or at the onset of somatic maturity.
Elemgasem nubilus	8+ years	MCF-PVPH-380	Sexually mature but not yet somatically mature, as it was still growing and lacked an EFS.
Xiyunykus pengi	9 years	IVPP V22783	A subadult.
Shishugounykus inexpectus	9 years	IVPP V23567	An adult.
Niebla antiqua	9+ years	MPCN-PV-796	An adult that had reached somatic maturity based on the presence of an EFS.
Chirostenotes pergracilis	9+ years	UALVP 59400	A partial skeleton that was at least 9 years old at time of death. Growth had considerably slowed by this time and the individual was approaching its maximum body size.
Wiehenvenator albati	9+ years		Considered a subadult, as it lacked an EFS despite its growth rate slowing down. Although 9 years is the estimated minimum age at death, the specimen may have been well over 10 years old as intense secondary remodelling of the bone could have obstructed some LAGs.
Limusaurus inextricabilis	10 years	IVPP 20099	An adult with an EFS, that attained somatic maturity when 6 years old.
Xixianykus zhangi	11 years	XMDFEC V0011	An adult.
Ceratosaurus magnicornis	11+ years	MWC 1	One of the largest known Ceratosaurus specimens, but not the largest. It is the holotype of C. magnicornis, although the validity of the species is disputed. Unfused sutures in the skull indicate it had not yet reached adult size.
Aucasaurus garridoi	11+ years	MCF-PVPH-236	An adult.
Masiakasaurus knopfleri	12 years	FMNH PR 2123	Multiple models were used to create age estimates of the specimen. The most reliable model as judged by the study team yielded an age of 12 years at time of death. Growth rate of the animal had plateaued, and it had reached maximum size.
Guanlong wucaii	12 years	IVPP V14531	An adult that had reached full size by the time it was 7 years old.
Anteavis crurilongus	12+ years	PVSJ 1085	The presence of an EFS was interpreted as an indication of somatic maturity. The degree of skeletal fusion indicates that it was at least a young adult close to skeletal maturity. Around 16 LAGs can be observed in the dorsal ribs, however the lamellar condition of the bone tissue prevents a precise count; LAGs from early in life may have also been obstructed.
Citipati osmolskae	13 years	IGM 100/979, IGM 100/1004	Both specimens were found brooding a clutch of eggs.
Vespersaurus paranaensis	13 years	CP.V 2385	An adult with an EFS.
Asiatyrannus xui	13+ years	ZMNH M30360	Initially interpreted as a subadult that was approaching somatic maturity, it has more recently been argued to be more immature than was initially concluded.
Deinonychus antirrhopus	13-14 years	AMNH 3015	The specimen is closely associated with an egg, which was positioned under its belly.
Nanotyrannus lancensis	14+ years	NCSM 40000	Nicknamed "Bloody Mary", the specimen shows the beginnings of an EFS and was close to somatic maturity, but skeletal growth had not fully stopped.
Lourinhanosaurus antunesi	14-17 years	ML 370	A skeletally immature individual. It is unknown if it reached sexual maturity.
Alpkarakush kyrgyzicus	17+ years	IGB 2/33	The specimen has a minimum estimated age of 17 years, but is most likely older. It was a subadult that was close to fully grown, yet still slowly growing based on the absence of an EFS.
Troodon formosus	18 years	MOR 748	An adult brooding a clutch of eggs. It had reached full adult size and had an EFS. The validity of the genus Troodon is currently controversial.
Oviraptorinae indet.	18 years	IVPP V9608	A skeletally mature adult with an EFS, found brooding a clutch of eggs. This partially complete specimen from Bayan Mandahu was initially tentatively referred to Oviraptor philoceratops, but this is now disputed.
Saurornitholestes	19 years		Based on lines of arrested growth.
Joaquinraptor casali	19+ years	UNPSJB-PV 1112	Sexually mature based on the reduction in spacing of LAGs. Its somatic maturity is uncertain, but is noted to lack an EFS.
Acrocanthosaurus atokensis	21+ years	OMNH 10146	Skeletally mature based on the presence of an EFS.
Gorgosaurus libratus	22 years
Iberospinus natarioi	23-25 years	ML 1190	The specimen died near the onset of skeletal maturity. Sexual maturity occurred at 13-15 years of age.
Saltriovenator zanellai	24+ years	MSNM V3664	A subadult close to the onset of somatic maturity, based on the presence of an incipient EFS.
Daspletosaurus torosus	26 years
Albertosaurus sarcophagus	28 years	TMP 2004.56.48	Excavated from the Dry Island Albertosaurus bonebed, this specimen has an estimated body length of 10.1 metres.
Allosaurus fragilis	22-28 years	AMNH 680, MHNG GEPI V2567b	AMNH 680 is the largest definitive Allosaurus specimen, with an estimated length of 9.7 metres. It has been referred to A. fragilis. Histology of the femur and tibia yielded age estimations of 22 and 28 years, respectively. There is no presence of an EFS. MHNG GEPI V2567 is a composite skeleton likely representing multiple individuals. The individual represented by the tibia was around 26 years old at time of death, having reached skeletal maturity at least 5 years prior, when it developed an EFS.
Tyrannosaurus rex	30+ years	FMNH PR 2081, RGM 792.000	Specimen FMNH PR 2081 (Sue) is estimated to have been 27-33 years old at death. RGM 792.000 (Trix) is also estimated to have reached over 30 years of age, but no formal literature has been published on this as of 2025. The species may have been able to attain longer lifespans; a 2026 histological study of 17 specimens estimated that growth would only plateau at 35-40 years. Bone core sampling methodology differed from that of previous studies such as those done on FMNH PR 2081, such that it may have impacted the number of growth marks that could be observed.
Meraxes gigas''	39-53 years	MMCh-PV 65	Reached skeletal maturity approximately 4 years before death.