Editing Dinosaur (section)

===Reproductive biology===
{{see also|Dinosaur egg}}
[[File:Gniazdo sieweczki RB.JPG|left|thumb|alt=Three bluish eggs with black speckling sit atop a layer of white mollusk shell pieces, surrounded by sandy ground and small bits of bluish stone|Nest of a [[plover]] (''[[Charadrius]]'')]]

All dinosaurs laid [[Amniote|amniotic egg]]s. Dinosaur eggs were usually laid in a nest. Most species create somewhat elaborate nests which can be cups, domes, plates, beds scrapes, mounds, or burrows.<ref name="Hansell">{{harvnb|Hansell|2000}}</ref> Some species of modern bird have no nests; the cliff-nesting [[Common murre|common guillemot]] lays its eggs on bare rock, and male [[emperor penguin]]s keep eggs between their body and feet. Primitive birds and many non-avialan dinosaurs often lay eggs in communal nests, with males primarily incubating the eggs. While modern birds have only one functional [[oviduct]] and lay one egg at a time, more primitive birds and dinosaurs had two oviducts, like crocodiles. Some non-avialan dinosaurs, such as ''[[Troodon]]'', exhibited iterative laying, where the adult might lay a pair of eggs every one or two days, and then ensured simultaneous hatching by delaying [[Broodiness#Broodiness in non-avian animals|brooding]] until all eggs were laid.<ref name="Varrichioetal.02">{{cite journal |last1=Varricchio |first1=David J. |last2=Horner |first2=John R. |last3=Jackson |first3=Frankie D. |year=2002 |title=Embryos and eggs for the Cretaceous theropod dinosaur ''Troodon formosus'' |journal=Journal of Vertebrate Paleontology |location=Milton Park, Oxfordshire |publisher=Taylor & Francis for the Society of Vertebrate Paleontology |volume=22 |issue=3 |pages=564–576 |doi=10.1671/0272-4634(2002)022[0564:EAEFTC]2.0.CO;2 |s2cid=85728452 |issn=0272-4634}}</ref>

When laying eggs, females grow a special type of bone between the hard outer bone and the [[Bone marrow|marrow]] of their limbs. This medullary bone, which is rich in [[calcium]], is used to make eggshells. A discovery of features in a ''Tyrannosaurus'' skeleton provided evidence of medullary bone in extinct dinosaurs and, for the first time, allowed paleontologists to establish the sex of a fossil dinosaur specimen. Further research has found medullary bone in the carnosaur ''Allosaurus'' and the ornithopod ''[[Tenontosaurus]]''. Because the line of dinosaurs that includes ''Allosaurus'' and ''Tyrannosaurus'' diverged from the line that led to ''Tenontosaurus'' very early in the evolution of dinosaurs, this suggests that the production of medullary tissue is a general characteristic of all dinosaurs.<ref name=LW08/>
[[File:Citipati AMNH.jpg|thumb|Fossil interpreted as a nesting [[oviraptoridae|oviraptorid]] ''[[Citipati]]'' at the [[American Museum of Natural History]]. ]]
Another widespread trait among modern birds (but see below in regards to fossil groups and extant [[megapodes]]) is parental care for young after hatching. [[Jack Horner (paleontologist)|Jack Horner's]] 1978 discovery of a ''[[Maiasaura]]'' ("good mother lizard") nesting ground in Montana demonstrated that parental care continued long after birth among ornithopods.<ref name=HM79/> A specimen of the [[oviraptoridae|oviraptorid]] ''[[Citipati|Citipati osmolskae]]'' was discovered in a [[Chicken#Broodiness|chicken-like brooding position]] in 1993,<ref name="search.eb"/> which may indicate that they had begun using an insulating layer of feathers to keep the eggs warm.<ref>{{harvnb|Currie|Koppelhus|Shugar|Wright|2004|pp=[http://thomas-hopp.com/pdf/DinoBrooding.pdf 234–250]|loc=chpt. 11: "Dinosaur Brooding Behavior and the Origin of Flight Feathers" by Thomas P. Hopp and Mark J. Orsen.}}</ref> An embryo of the basal sauropodomorph ''[[Massospondylus]]'' was found without teeth, indicating that some parental care was required to feed the young dinosaurs.<ref name=Reiszetal05/> Trackways have also confirmed parental behavior among ornithopods from the [[Isle of Skye]] in northwestern [[Scotland]].<ref name=BBCtracks/>

However, there is ample evidence of [[precociality]] or [[Precociality#Superprecociality|superprecociality]] among many dinosaur species, particularly theropods. For instance, non-[[Euornithes|ornithuromorph]] birds have been abundantly demonstrated to have had slow growth rates, [[megapode]]-like egg burying behavior and the ability to fly soon after birth.<ref>{{cite journal |first1=Zhonghe |last1=Zhou |first2=Fucheng |last2=Zhang |year=2004 |title=A Precocial Avian Embryo from the Lower Cretaceous of China |journal=Science |location=Washington, D.C. |publisher=American Association for the Advancement of Science |volume=306 |issue=5696 |page=653 |doi=10.1126/science.1100000 |issn=0036-8075 |pmid=15499011|s2cid=34504916 }}</ref><ref>{{cite journal |url=https://blogs.scientificamerican.com/tetrapod-zoology/drowned-cretaceous-bird-colony/ |url-status=live |title=A drowned nesting colony of Late Cretaceous birds |last=Naish |first=Darren |author-link=Darren Naish |date=May 15, 2012 |journal=Science |volume=306 |issue=5696 |page=653 |publisher=[[Scientific American#Website|Scientific American]] |doi=10.1126/science.1100000 |pmid=15499011 |s2cid=34504916 |archive-url=https://web.archive.org/web/20180925031243/https://blogs.scientificamerican.com/tetrapod-zoology/drowned-cretaceous-bird-colony/ |archive-date=September 25, 2018 |access-date=November 16, 2019}}</ref><ref>{{cite journal |last1=Fernández |first1=Mariela S. |last2=García |first2=Rodolfo A. |last3=Fiorelli |first3=Lucas |last4=Scolaro |first4=Alejandro |last5=Salvador |first5=Rodrigo B. |last6=Cotaro |first6=Carlos N. |last7=Kaiser |first7=Gary W. |last8=Dyke |first8=Gareth J. |author8-link=Gareth J. Dyke |display-authors=3 |year=2013 |title=A Large Accumulation of Avian Eggs from the Late Cretaceous of Patagonia (Argentina) Reveals a Novel Nesting Strategy in Mesozoic Birds |journal=PLOS ONE |location=San Francisco, CA |publisher=PLOS |volume=8 |issue=4 |page=e61030 |doi=10.1371/journal.pone.0061030 |pmid=23613776 |pmc=3629076 |bibcode=2013PLoSO...861030F |issn=1932-6203|doi-access=free }}</ref><ref>{{cite journal |last1=Deeming |first1=Denis Charles |last2=Mayr |first2=Gerald |author2-link=Gerald Mayr |date=May 2018 |title=Pelvis morphology suggests that early Mesozoic birds were too heavy to contact incubate their eggs |journal=[[Journal of Evolutionary Biology]] |location=Hoboken, NJ |publisher=Wiley-Blackwell on behalf of the [[European Society for Evolutionary Biology]] |volume=31 |issue=5 |pages=701–709 |doi=10.1111/jeb.13256 |pmid=29485191 |s2cid=3588317 |issn=1010-061X|url=http://eprints.lincoln.ac.uk/id/eprint/31436/13/31436%2031291%20Deeming_et_al-2018-Journal_of_Evolutionary_Biology.pdf |archive-url=https://web.archive.org/web/20200602094641/http://eprints.lincoln.ac.uk/id/eprint/31436/13/31436%2031291%20Deeming_et_al-2018-Journal_of_Evolutionary_Biology.pdf |archive-date=2020-06-02 |url-status=live }}</ref> Both ''Tyrannosaurus'' and ''Troodon'' had juveniles with clear superprecociality and likely occupying different ecological niches than the adults.<ref name="Varrichioetal.02"/> Superprecociality has been inferred for sauropods.<ref>{{cite journal |last1=Myers |first1=Timothy S. |last2=Fiorillo |first2=Anthony R. |year=2009 |title=Evidence for gregarious behavior and age segregation in sauropod dinosaurs |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |location=Amsterdam |publisher=Elsevier |volume=274 |issue=1–2 |pages=96–104 |doi=10.1016/j.palaeo.2009.01.002 |issn=0031-0182|bibcode=2009PPP...274...96M |url=http://doc.rero.ch/record/16633/files/PAL_E962.pdf |archive-url=https://web.archive.org/web/20200529011847/http://doc.rero.ch/record/16633/files/PAL_E962.pdf |archive-date=2020-05-29 |url-status=live }}</ref>

Genital structures are unlikely to fossilize as they lack scales that may allow preservation via pigmentation or residual calcium phosphate salts. In 2021, the best preserved specimen of a dinosaur's [[cloacal vent]] exterior was described for ''Psittacosaurus'', demonstrating lateral swellings similar to crocodylian musk glands used in social displays by both sexes and pigmented regions which could also reflect a signalling function. However, this specimen on its own does not offer enough information to determine whether this dinosaur had sexual signalling functions; it only supports the possibility. Cloacal visual signalling can occur in either males or females in living birds, making it unlikely to be useful to determine sex for extinct dinosaurs.<ref name="cloacal-vent">{{cite journal|doi=10.1016/j.cub.2020.12.039|title=A cloacal opening in a non-avian dinosaur|first1=Jakob|last1=Vinther|first2=Robert|last2=Nicholls|first3=Diane A.|last3=Kelly|journal=Current Biology|volume=31|pages=R1–R3|date=February 22, 2021|issue=4|publisher=Elsevier|pmid=33472049|s2cid=231644183|doi-access=free|bibcode=2021CBio...31.R182V }}</ref>