Editing Tyrannosaurus (section)

===Sexual dimorphism===
[[File:MUJA 04.jpg|thumb|upright|left|Skeleton casts mounted in a mating position, [[Jurassic Museum of Asturias]]]]
As the number of known specimens increased, scientists began to analyze the variation between individuals and discovered what appeared to be two distinct body types, or ''morphs'', similar to some other theropod species. As one of these morphs was more solidly built, it was termed the 'robust' morph while the other was termed '[[Wikt:gracile|gracile]]'. Several [[morphology (biology)|morphological]] differences associated with the two morphs were used to analyze [[sexual dimorphism]] in ''T. rex'', with the 'robust' morph usually suggested to be female. For example, the [[pelvis]] of several 'robust' specimens seemed to be wider, perhaps to allow the passage of [[Egg (biology)|eggs]].<ref name="carpenter1990">{{Cite book |title=Dinosaur Systematics: Approaches and Perspectives |last=Carpenter |first=K. |publisher=[[Cambridge University Press]] |year=1992 |isbn=978-0-521-43810-0 |editor-last=Carpenter |editor-first=K. |location=[[Cambridge]] |pages=141–145 |chapter=Variation in ''Tyrannosaurus rex'' |author-link=Kenneth Carpenter |editor-last2=Currie |editor-first2=P. J.}}</ref> It was also thought that the 'robust' morphology correlated with a reduced [[chevron (anatomy)|chevron]] on the first tail vertebra, also ostensibly to allow eggs to pass out of the [[reproductive system|reproductive tract]], as had been erroneously reported for [[crocodile]]s.<ref name="larson1994">{{Cite book |last=Larson |first=P. L. |year=1994 |editor-last=Rosenberg |editor-first=G. D. |editor2-last=Wolberg |editor2-first=D. L. |title=Dino Fest |publisher=The Paleontological Society Special Publications |chapter=''Tyrannosaurus'' sex |volume=7 |pages=139–155}}</ref>

In recent years, evidence for sexual dimorphism has been weakened. A 2005 study reported that previous claims of sexual dimorphism in crocodile chevron anatomy were in error, casting doubt on the existence of similar dimorphism between ''T. rex'' sexes.<ref name="ericksonetal2005">{{Cite journal |last1=Erickson |first1=G. M. |last2=Kristopher |first2=L. A. |last3=Larson |first3=P. |year=2005 |title=Androgynous rex&nbsp;– the utility of chevrons for determining the sex of crocodilians and non-avian dinosaurs |journal=Zoology (Jena, Germany) |volume=108 |issue=4 |pages=277–86 |doi=10.1016/j.zool.2005.08.001 |pmid=16351976 |bibcode=2005Zool..108..277E |url=http://doc.rero.ch/record/14378/files/PAL_E1827.pdf |archive-date=April 4, 2023 |access-date=February 2, 2023 |archive-url=https://web.archive.org/web/20230404163719/https://doc.rero.ch/record/14378/files/PAL_E1827.pdf |url-status=live }}</ref> A full-sized chevron was discovered on the first tail vertebra of Sue, an extremely robust individual, indicating that this feature could not be used to differentiate the two morphs anyway. As ''T. rex'' specimens have been found from [[Saskatchewan]] to [[New Mexico]], differences between individuals may be indicative of geographic variation rather than sexual dimorphism. The differences could also be age-related, with 'robust' individuals being older animals.<ref name="brochu2003" />

Only a single ''Tyrannosaurus'' specimen has been conclusively shown to belong to a specific sex. Examination of B-rex demonstrated the preservation of [[soft tissue]] within several bones. Some of this tissue has been identified as a medullary tissue, a specialized tissue grown only in modern birds as a source of [[calcium]] for the production of [[eggshell]] during [[ovulation]]. As only female birds lay eggs, medullary tissue is only found naturally in females, although males are capable of producing it when injected with female reproductive [[hormone]]s like [[estrogen]]. This strongly suggests that B-rex was female and that she died during ovulation.<ref name="schweitzeretal2005" /> Recent research has shown that medullary tissue is never found in crocodiles, which are thought to be the closest living relatives of dinosaurs. The shared presence of medullary tissue in birds and other theropod dinosaurs is further evidence of the close [[evolution]]ary relationship between the two.<ref name="schweitzeretal2007">{{Cite journal |last1=Schweitzer |first1=M. H. |last2=Elsey |first2=R. M. |last3=Dacke |first3=C. G. |last4=Horner |first4=J. R. |last5=Lamm |first5=E. T. |date=2007 |title=Do egg-laying crocodilian (''Alligator mississippiensis'') archosaurs form medullary bone? |journal=Bone |volume=40 |issue=4 |pages=1152–8 |doi=10.1016/j.bone.2006.10.029 |pmid=17223615 }}</ref>