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===Other definitions=== ==== Long-enduring ==== [[File:Rhynchocyon petersi from side.jpg|thumb|[[Elephant shrew]]s resemble the extinct ''[[Leptictidium]]'' of [[Eocene]] Europe.]] A living taxon that lived through a large portion of [[geologic time]]. The [[Australian lungfish]] (''Neoceratodus fosteri''), also known as the Queensland lungfish, is an example of an organism that meets this criterion. Fossils identical to modern specimens have been dated at over 100{{nbsp}}million years old. Modern Queensland lungfish have existed as a species for almost 30{{nbsp}}million years.<ref>{{Cite web |title=Ann Kemps Lungfish - Queensland - Australia |url=http://annekempslungfish.com.au/lungfish_habitat.html |access-date=2025-01-18 |website=annekempslungfish.com.au}}</ref> The contemporary [[nurse shark]] has existed for more than 112{{nbsp}}million years, making this species one of the oldest, if not actually the oldest extant vertebrate species. ==== Resembles ancient species ==== A living taxon [[Morphology (biology)|morphologically]] and/or [[Physiology|physiologically]] resembling a fossil taxon through a large portion of geologic time (morphological stasis).<ref>{{cite web|url=http://www.uchospitals.edu/news/2006/20061026-lamprey.html |title=The University of Chicago Medical Center: ''Scientists find lamprey a 'living fossil' '' |publisher=Uchospitals.edu |date=2006-10-26 |access-date=2012-05-16}}</ref> ==== Retains many ancient traits ==== [[File:Liphistius malayanus 44930086.jpg|thumb|More primitive [[trapdoor spider]]s, such as this ''[[Liphistius]] malayanus'', have segmented plates on the dorsal surface of the abdomen and cephalothorax, a character shared with scorpions, making it probable that after the spiders diverged from the scorpions, the earliest unique ancestor of trapdoor species was the first to split off from the lineage that contains all other [[Extant taxon|extant]] spiders.]] A living taxon with many characteristics believed to be primitive. This is a more neutral definition. However, it does not make it clear whether the taxon is truly old, or it simply has many plesiomorphies. Note that, as mentioned above, the converse may hold for true living fossil taxa; that is, they may possess a great many derived features ([[autapomorphy|autapomorphies]]), and not be particularly "primitive" in appearance. ==== Relict population ==== Any one of the above three definitions, but also with a [[relict distribution]] in [[Refugium (population biology)|refuges]]. Some paleontologists believe that living fossils with large distributions (such as ''[[Triops cancriformis]]'') are not real living fossils. In the case of ''Triops cancriformis'' (living from the [[Triassic]] until now), the Triassic specimens lost most of their appendages (mostly only [[carapace]]s remain), and they have not been thoroughly examined since 1938. ==== Low diversity ==== Any of the first three definitions, but the [[clade]] also has a low taxonomic diversity (low diversity lineages). [[Oxpecker]]s are [[morphology (biology)|morphologically]] somewhat similar to [[starlings]] due to shared plesiomorphies, but are uniquely adapted to feed on parasites and blood of large land mammals, which has always obscured their relationships. This lineage forms part of a radiation that includes [[Sturnidae]] and [[Mimidae]], but appears to be the most ancient of these groups. [[Biogeography]] strongly suggests that oxpeckers originated in eastern [[Asia]] and only later arrived in Africa, where they now have a relict distribution.<ref name="Zuccon"/> The two living species thus seem to represent an entirely extinct and (as [[Passerida]] go) rather ancient lineage, as certainly as this can be said in the absence of actual fossils. The latter is probably due to the fact that the oxpecker lineage never occurred in areas where conditions were good for fossilization of small bird bones, but of course, fossils of ancestral oxpeckers may one day turn up enabling this theory to be tested.
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