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Age determination in dinosaurs

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

History

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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.[1] 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.[2]

Growth lines

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Two types of growth lines exist: annuli, and lines of arrested growth (LAGs).[3] 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, similar to annuli, are found between zones are avascular. They are, however, much thinner, and have relatively fewer bone fibers by volume.[3] 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.[4] 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.[5]

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.[6] 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".[citation needed]

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.[3]

Maturity

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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.[7][8][9] 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.[10]

Sexual maturity on the other hand is inferred when intrinsic fibres of the bone become much more well organized, which may be accompanied by the slowing growth of the animal.[11] 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.[9][11]

Sexual maturity typically precedes skeletal maturity in non-avian dinosaurs and indeed most amniotes to varying degrees. In contrast, modern birds (ornithurans) experience an intense growth spurt early in life, and reach sexual maturity well after growing to maximum size.[10] 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.[12]

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.[13]

Longevity

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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.[14] 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.[15]

Age estimation of mature titanosaurs is difficult. Intense secondary remodelling of the bone occurs after titanosaurs reach sexual maturity, obstructing LAGs.[16]

Ornithischians
Species Estimated age of oldest measured specimen Specimen # Notes
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.[17]
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.[18]
Psittacosaurus lujiatunensis 11 years ZMNH M8138 Although sexually mature, it likely could've grown noticeably larger as its growth rate had not yet plateaued.[19]
Yinlong downsi 10+ years IVPP V18637 An adult. 10 lines of arrested growth (LAG) are visible, but partial erasure of the innermost LAGs prevent precise age estimation.[20]
Cerasinops hodgskissi 11 years MOR 300 (holotype) A "relatively mature" individual that was still slowly growing when it died at an estimated maximum age of 11 years.[21]
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.[22]
Pachyrhinosaurus sp. 19 years UAMES 3551 A large femur from the Prince Creek Formation. The animal was still actively growing at time of death.[23]
Hypacrosaurus stebingeri 13 years MOR 549 An adult with an EFS. Sexual maturity occurred at 2-3 years of age.[24]
Probrachylophosaurus bergei 14 years MOR 2919 (holotype) 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.[9]
Maiasaura peeblesorum 15 years An adult.[25]
Edmontosaurus annectens 18+ years ROM 73853 An adult whose growth rate was beginning to plateau.[26]
Dysalotosaurus lettowvorbecki 20 years An adult. Out of several bone-beds, this specimen's femur was the largest recovered.[27]
Miragaia longicollum 25 years ML 433 (holotype) 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.[28]
Draconyx loureiroi 31 years ML 439 (holotype) A fully grown adult, based on the presence of an EFS. Sexual maturity occurred at 12-17 years of age.[28]
Sauropodomorphs
Species Estimated age of oldest measured specimen Specimen # Notes
Amargasaurus cazaui 10+ years MACN PV N15 (holotype) The specimen was sexually mature. Despite the absence of an EFS, it is unknown if it had reached somatic maturity, as the bones experienced erosion that could've destroyed an EFS.[8]
Lirainosaurus astibiae 11+ years MGUV 17166 A humerus from what was considered a young adult. It had reached 69% of the body size of the largest known Lirainosaurus specimens (although such specimens were likely still actively growing due to lack of an EFS). Obstruction of LAGs from intense secondary remodelling of bone occurs as this species approaches maturity, making age estimation difficult. A degree of remodelling is even seen in this relatively young specimen.[29]
Massospondylus carinatus 20+ years Several specimens have been estimated to be 20 years old at minimum. None of them reached skeletal maturity, as they all lack an EFS in the long bones, but growth rate had slowed to the point where they were very close to having reached maximum size.[30]
Amargatitanis macni 19+ years MACN PV N53 (holotype) A somatically and skeletally mature adult, based on the presence of an EFS containing 6 LAGs.[8]
Ardetsaurus viator 22 years MAB011899 (holotype) Nicknamed "Brösmeli", this individual was an adult. It reached sexual maturity at 13 years old, and skeletal maturity at 17 years old.[31]
Plateosaurus engelhardti 26-27 years IFG uncatalogued, NAA A 9[32] The IFG specimen is estimated to have been 10 metres long, the maximum documented size in Plateosaurus. It had already reached skeletal maturity several years before death. The animals of the bonebed it was from are presumed to have died by accident, so it is assumed that it had not reached the maximum lifespan of the species. The NAA specimen reached a similar age, but belonged to an individual only 6.5 metres long.[32]
Apatosaurus sp. 30+ years BYU 601-17328,[33]SMA 0014 SMA 0014 was at least 30 years old at time of death, having reached an estimated 91% of its maximum size.[34]
Mamenchisaurid indet. 26-31 (45?) years SGP 2006/9 The Giant's Tomb mamenchisaurid consisted of a right humerus and left ulna. This massive individual represented the largest known mamenchisaurid when described in 2011.[35] Histological analyses place it at 26-31 years old at death, while conflicting data from body mass growth models suggest it was around 45 years old.[33]
Mussaurus patagonicus 29-34 years MLP 68-II-27-1 A subadult that reached sexual maturity 3-6 years before death. The age estimate is tentative, as it is difficult to account for the number of cyclical growth marks (CGM) destroyed during growth. Much older, even "senile" specimens are known, but none are well preserved enough for an age estimation.[36]
Janenschia robusta 38 years NHUB Nr. 22 An adult. The initial study using growth rings posited it may have actually been 39 years old at death, as bone remodelling may have obliterated one growth line. Sexual maturity was reached at 11 years of age.[37] A later study calculating the rate of bone apposition yielded much lower estimates of the age at death.[38]
Camarasaurus sp. 40 years SMA 0002 This nearly complete specimen is also known as "E.T.", and reached adult size at 34 years old,[39] despite being unusually small for a Camarasaurus.[28] The specimen is nearly complete, and is geologically the oldest Camarasaurus specimen in the Morrison Formation.[28]
Lapparentosaurus 43 years[40][verification needed]
Narindasaurus 43 years[40][verification needed]
Diplodocus carnegii 34 years CM 94 (paratype) Skeletally mature at a maximum estimated age of 34 years. It may have been at the same stage in life as the D. hallorum holotype despite being lesser in age, as they shared similar bone tissue composition.[41]
Diplodocus hallorum 60 years NMMNH P-25079 (holotype) The holotype of D. hallorum (formerly Seismosaurus) had only 'recently' reached skeletally maturity before dying at a maximum estimated age of 60 years.[41]
Supersaurus vivianae Unknown WDC DMJ-021 Also known as "Jimbo", this specimen is considered to be the oldest known dinosaur. However, its exact age is unknown, as extensive remodelling of the bone due to age had obscured most of the lines of arrested growth.[41]
Non-avian theropods
Species Estimated age of oldest measured specimen Specimen # Notes
Shuvuuia deserti 2 years MGI 100/975 A juvenile.[42]
Raptorex kriegsteini 2-3 years LH PV18 (holotype) 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.[43]
Mei long 2-4+ years DNHM D2154 An adult whose growth had mostly stopped, and there is a structure that resembles an EFS.[44]
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.[45]
Albinykus baatar 3 years MAE PSS 04-18 (holotype) An adult.[42]
Kiyacursor longipes 3+ years KOKM 5542 (holotype) A subadult individual that lack an EFS.[46]
Haplocheirus sollers 4 years IVPP V15988 (holotype) A juvenile.[42]
Daliansaurus liaoningensis 4 years DNHM D2885 (holotype) A subadult that was close to adulthood.[47]
Liaoningvenator curriei 4+ years DNHM D3012 (holotype) Lacks an EFS and was still growing, but was close to skeletal maturity.[48]
Talos sampsoni 4-6 years UMNH VP 19479 (holotype) An immature specimen lacking an EFS. Growth was beginning to slow down.[49]
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.[50]
Eoneophron infernalis 6 years CM 96523 (holotype) Close to if not already skeletally mature.[51]
Koleken inakayali 6+ years MPEF-PV 10826 An immature individual that had not yet reached sexual maturity.[52]
Moros intrepidus 6-7 years NCSM 33392 (holotype) A subadult.[53]
Megapnosaurus rhodesiensis 7 years[54]
Bannykus wulatensis 8 years IVPP V25026 (holotype) An adult.[42]
Oviraptor philoceratops 8 years AMNH FARB 6517 (holotype) Was brooding a clutch of eggs when it died, however, it lacks an EFS and was still actively growing.[50]
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.[55]
Rativates evadens 8+ years ROM 1790 (holotype) A subadult or adult, near or at the onset of somatic maturity.[56]
Elemgasem nubilus 8+ years MCF-PVPH-380 (holotype) Sexually mature but not yet somatically mature, as it was still growing and lacked an EFS.[11]
Xiyunykus pengi 9 years IVPP V22783 (holotype) A subadult.[42]
Shishugounykus inexpectus 9 years IVPP V23567 (holotype) An adult.[42]
Niebla antiqua 9+ years MPCN-PV-796 (holotype) An adult that had reached somatic maturity based on the presence of an EFS.[57]
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.[58]
Wiehenvenator albati 9+ years (holotype) 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.[59]
Limusaurus inextricabilis 10 years IVPP 20099 An adult with an EFS, that attained somatic maturity when 6 years old.[60]
Xixianykus zhangi 11 years XMDFEC V0011 (holotype) An adult.[42]
Ceratosaurus magnicornis 11+ years[61] MWC 1 One of the largest known Ceratosaurus specimens,[61], 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.[62]
Aucasaurus garridoi 11+ years MCF-PVPH-236 (holotype) An adult.[63]
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.[64]
Guanlong wucaii 12 years IVPP V14531 (holotype) An adult that had reached full size by the time it was 7 years old.[65]
Citipati osmolskae 13 years IGM 100/979,

IGM 100/1004

Both specimens were found brooding a clutch of eggs.[50]
Vespersaurus paranaensis 13 years CP.V 2385 An adult with an EFS.[66]
Asiatyrannus xui 13+ years ZMNH M30360 (holotype) Initially interpreted as a subadult that was approaching somatic maturity,[67] it has more recently been argued to be more immature than was initially concluded.[68]
Deinonychus antirrhopus 13-14 years AMNH 3015 The specimen is closely associated with an egg, which was positioned under its belly.[50]
Lourinhanosaurus antunesi 14-17 years ML 370 (holotype) A skeletally immature individual. It is unknown if it reached sexual maturity.[28]
Alpkarakush kyrgyzicus 17+ years IGB 2/33 (holotype) 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.[69]
Troodon formosus 18 years MOR 748 An adult brooding a clutch of eggs. It had reached full adult size and had an EFS.[50] 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.[50] This partially complete specimen from Bayan Mandahu was initially tentatively referred to Oviraptor philoceratops, but this is now disputed.[70]
Saurornitholestes 19 years Based on lines of arrested growth.[71]
Acrocanthosaurus atokensis 21+ years OMNH 10146 (holotype) Skeletally mature based on the presence of an EFS.[72]
Gorgosaurus libratus 22 years[73]
Iberospinus natarioi 23-25 years ML 1190 (holotype) The specimen died near the onset of skeletal maturity. Sexual maturity occurred at 13-15 years of age.[74][28]
Saltriovenator zanellai 24+ years MSNM V3664 (holotype) A subadult close to the onset of somatic maturity, based on the presence of an incipient EFS.[75]
Daspletosaurus torosus 26 years[73]
Albertosaurus sarcophagus 28 years[76] TMP 2004.56.48 Excavated from the Dry Island Albertosaurus bonebed, this specimen has an estimated body length of 10.1 metres.[73]
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.[77] It has been referred to A. fragilis.[78] Histology of the femur and tibia yielded age estimations of 22 and 28 years, respectively. There is no presence of an EFS.[79] 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.[80]
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.[81] 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.[82]
Meraxes gigas 39-53 years MMCh-PV 65 (holotype) Reached skeletal maturity approximately 4 years before death.[6]

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