Editing Megafauna (section)

===In terrestrial mammals===
[[File:Patagotitan vs Mammals Scale Diagram SVG Steveoc86.svg|thumb|upright=2|Large terrestrial mammals compared in size to one of the largest sauropod dinosaurs, ''[[Patagotitan]]'']]
Subsequent to the [[Cretaceous–Paleogene extinction event]] that eliminated the non-avian dinosaurs about {{period start|Paleogene}} [[Megaannum|Ma]] (million years) ago, terrestrial mammals underwent a nearly exponential increase in body size as they diversified to occupy the ecological niches left vacant.<ref name = "F.A.Smith">{{cite journal | last1=Smith|first1=F. A.|last2=Boyer|first2=A. G.|last3=Brown|first3=J. H.|last4=Costa|first4=D. P.|last5=Dayan|first5=T.|last6=Ernest|first6=S. K. M.|last7=Evans|first7=A. R.|last8=Fortelius|first8=M.|last9=Gittleman|first9=J. L.|last10=Hamilton|first10=M. J.|last11=Harding|first11=L. E.|last12=Lintulaakso|first12=K.|last13=Lyons|first13=S. K.|last14=McCain|first14=C.|last15=Okie|first15=J. G.|last16=Saarinen|first16=J. J.|last17=Sibly|first17=R. M.|last18=Stephens|first18=P. R.|last19=Theodor|first19=J.|last20=Uhen|first20=M. D.
  | title = The Evolution of Maximum Body Size of Terrestrial Mammals
  | journal = [[Science (journal)|Science]] | volume = 330 | issue = 6008 | pages = 1216–1219
  | date = 2010-11-26 | doi = 10.1126/science.1194830 |pmid=21109666|bibcode = 2010Sci...330.1216S|citeseerx=10.1.1.383.8581|s2cid=17272200}}</ref> Starting from just a few kg before the event, maximum size had reached ~{{Convert|50|kg|lb}} a few million years later, and ~{{Convert|750|kg|lb}} by the end of the [[Paleocene]]. This trend of increasing body mass appears to level off about 40 Ma ago (in the late [[Eocene]]), suggesting that physiological or ecological constraints had been reached, after an increase in body mass of over three orders of magnitude.<ref name = "F.A.Smith"/> However, when considered from the standpoint of rate of size increase per generation, the exponential increase is found to have continued until the appearance of ''[[Indricotherium]]'' 30 Ma ago. (Since generation time scales with ''body mass''<sup>0.259</sup>, increasing generation times with increasing size cause the log mass vs. time plot to curve downward from a linear fit.)<ref name = "Evans2012">{{cite journal
| last1=Evans|first1=A. R.|last2=Jones|first2=D.|last3=Boyer|first3=A. G.|last4=Brown|first4=J. H.|last5=Costa|first5=D. P.|last6=Ernest|first6=S. K. M.|last7=Fitzgerald|first7=E. M. G.|last8=Fortelius|first8=M.|last9=Gittleman|first9=J. L.|last10=Hamilton|first10=M. J.|last11=Harding|first11=L. E.|last12=Lintulaakso|first12=K.|last13=Lyons|first13=S. K.|last14=Okie|first14=J. G.|last15=Saarinen|first15=J. J.|last16=Sibly|first16=R. M.|last17=Smith|first17=F. A.|last18=Stephens|first18=P. R.|last19=Theodor|first19=J. M.|last20=Uhen|first20=M. D. 
 | title = The maximum rate of mammal evolution | journal = [[Proceedings of the National Academy of Sciences|PNAS]]
 | volume=109| issue=11| pages=4187–4190 | date = 2012-01-30
 | doi = 10.1073/pnas.1120774109 |pmid=22308461|pmc=3306709|bibcode=2012PNAS..109.4187E|doi-access=free}}</ref>

Megaherbivores eventually attained a body mass of over {{Convert|10,000|kg|lb}}. The largest of these, [[indricothere]]s and [[proboscid]]s, have been [[hindgut fermentation|hindgut fermenter]]s, which are believed to have an advantage over [[Foregut fermentation|foregut fermenter]]s in terms of being able to accelerate gastrointestinal transit in order to accommodate very large food intakes.<ref name = "Clauss">{{cite journal | last = Clauss | first = M. | author2 = Frey, R. | author3 = Kiefer, B. | author4 = Lechner-Doll, M. | author5 = Loehlein, W. | author6 = Polster, C. | author7 = Roessner, G. E. | author8 = Streich, W. J. | title = The maximum attainable body size of herbivorous mammals: morphophysiological constraints on foregut, and adaptations of hindgut fermenters | journal = [[Oecologia]] | volume = 136 | issue = 1 | pages = 14–27 | date = 2003-04-24 | doi = 10.1007/s00442-003-1254-z | pmid = 12712314 | bibcode = 2003Oecol.136...14C | s2cid = 206989975 | url = http://www.zora.uzh.ch/id/eprint/2393/2/Oecologia_body_size_2003V.pdf | access-date = 2019-07-13 | archive-date = 2019-06-08 | archive-url = https://web.archive.org/web/20190608152406/https://www.zora.uzh.ch/id/eprint/2393/2/Oecologia_body_size_2003V.pdf | url-status = dead }}</ref> A similar trend emerges when rates of increase of maximum body mass per generation for different mammalian [[clade]]s are compared (using rates averaged over [[macroevolution]]ary time scales). Among terrestrial mammals, the fastest rates of increase of ''body mass''<sup>0.259</sup> vs. time (in Ma) occurred in [[perissodactyl]]s (a slope of 2.1), followed by [[rodent]]s (1.2) and proboscids (1.1),<ref name = "Evans2012"/> all of which are hindgut fermenters. The rate of increase for [[artiodactyl]]s (0.74) was about a third of the perissodactyls. The rate for [[carnivora]]ns (0.65) was slightly lower yet, while [[primate]]s, perhaps constrained by their [[arboreal]] habits, had the lowest rate (0.39) among the mammalian groups studied.<ref name = "Evans2012"/>

Terrestrial mammalian carnivores from several [[eutheria]]n groups (the [[artiodactyl]] ''[[Andrewsarchus]]'' – formerly considered a [[Mesonychidae|mesonychid]], the [[oxyaenid]] ''[[Sarkastodon]]'', and the carnivorans ''[[Amphicyon]]'' and ''[[Arctodus]]'') all reached a maximum size of about {{Convert|1,000|kg|lb}}<ref name = "F.A.Smith"/> (the carnivoran ''[[Arctotherium]]'' and the [[hyaenodontid]] ''[[Simbakubwa]]'' may have been somewhat larger). The largest known [[metatheria]]n carnivore, ''[[Proborhyaena gigantea]]'', apparently reached {{Convert|600|kg|lb}}, also close to this limit.<ref name = "Sorkin"/> A similar theoretical maximum size for mammalian carnivores has been predicted based on the metabolic rate of mammals, the energetic cost of obtaining prey, and the maximum estimated rate coefficient of prey intake.<ref name = "Carbone">{{cite journal
  | last = Carbone | first = C. |author2=Teacher, A |author3=Rowcliffe, J. M.
  | title = The Costs of Carnivory | journal = [[PLOS Biology]]
  | volume = 5 | issue = 2, e22 | pages = 363–368
  | date = 2007-01-16 | doi = 10.1371/journal.pbio.0050022
  | pmid=17227145 | pmc=1769424 | doi-access = free }}</ref> It has also been suggested that maximum size for mammalian carnivores is constrained by the stress the [[humerus]] can withstand at top running speed.<ref name = "Sorkin">{{Cite journal
  | last = Sorkin | first = B.  | title = A biomechanical constraint on body mass in terrestrial mammalian predators
  | journal = [[Lethaia]] | volume = 41 | issue = 4 | pages = 333–347
  | date = 2008-04-10 | doi = 10.1111/j.1502-3931.2007.00091.x | bibcode = 2008Letha..41..333S }}</ref>

Analysis of the variation of maximum body size over the last 40 Ma suggests that decreasing temperature and increasing continental land area are associated with increasing maximum body size. The former correlation would be consistent with [[Bergmann's rule]],<ref name = "Ashton">{{cite journal
  | last = Ashton | first = K. G. |author2=Tracy, M. C. |author3=de Queiroz, A.
  | title = Is Bergmann's Rule Valid for Mammals?
  | journal = [[The American Naturalist]]
  | volume = 156 | issue = 4 | pages = 390–415
  | date = October 2000  | jstor = 10.1086/303400
  | doi = 10.1086/303400
  | pmid = 29592141 | s2cid = 205983729 }}</ref> and might be related to the [[Thermoregulation|thermoregulatory]] advantage of large body mass in cool climates,<ref name = "F.A.Smith"/> better ability of larger organisms to cope with seasonality in food supply,<ref name = "Ashton"/> or other factors;<ref name = "Ashton"/> the latter correlation could be explained in terms of range and resource limitations.<ref name = "F.A.Smith"/> However, the two parameters are interrelated (due to sea level drops accompanying increased glaciation), making the driver of the trends in maximum size more difficult to identify.<ref name = "F.A.Smith"/>