Did crown monotremes began to diversify in the early Cretaceous?
Molecular evidence indicates crown monotremes diversified before has previously been indicated by fossils, but new assessments of fossils may support these findings.
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Teinolophos appears to show cranial innervation similar to the modern platypus Ornithorhicus
The modern platypus shows a greatly enlarged (hypertrophied) mandibular canal in the dentary. This is used to carry and enlarged V2 and V3 (maxillary and mandibular) branches of the trigeminal cranial nerve (CN V), which carries signals from the numerous electroreceptors in the highly sensitive platypus bill to the brain. This is lacked by echidnas, which have a relatively narrow canal (Rowe et al., 2008), as do therian mammals, despite echidnas possessing electroreceptors in their proboscis.
Following CT scanning of a fossil from the Early Creatceous flat rocks locality in Australia called Teinolophos, Rowe et al. (2008) [[PNAS 105,1238–1242]] found a similar structure in the dentary. They thus concluded, following a cladistic analysis using an updated dataset, that Teinolophos is a platypus and nests thus within crown monotremata. This puts crown monotremata as having diversified in the early Cretaceous, 80 million year prior to the earliest previously known crown monotreme fossil Obdurodon. Steropodon - another early Cretaceous fossil (older, at 110 million years) - also appears to show this morphology, with the edge of a mandibular canal being visible.
The teeth of Teinolophos are similar to platypus teeth
Though both modern echidnas and platypuses lack teeth (though the latter shows them embryonically), the late Oligocene-Miocene fossil Obdurodon from Riversleigh - which is clearly a platypus as it possesses a bill etc. - does display full dentition. This has allowed recognition of Palaeocene monotremes from South America in the form of Monotrematum (Pascual et al., 2002 [[Acta Palaeontologica Polonica 47(3): 487–492.]] ). Steropodon also shows similarity to Obdurodon in dentition, and also to Monotrematum (Pascal et al., 2002), with all of these hypothesized as platypuses.
No similar echidna fossil has been found however, and thus, though shared characters can be seen in the teeth of Obdurodon and Teinolophos, whether these are plesiomorphies for monotremes or not is difficult to tell. Though both Steropodon and Teinolophos are certainly monotremes based on dental morphology, localization as a platypus or otherwise is not possible based on tooth morphology.
The date found by Rowe et al. (2008) is consistent with a Triassic monotreme-therian split as estimated by Woodburne et al. (2003).
A monotreme-therian split does not indicate that crown monotremes began to diversify especially soon afterwards - the clades on the stem group could have made up the monotreme lineage for an indefinite time thereafter.
Despite optical evidence (Rich et al., 2001) seemingly having indicated presence of ossified postdentary bones on the jaw of Teinolophos, this has subsequently been found not to be the case using CT scanning. This makes the liklihood of Teinolophos as a crown monotreme yet more likely, with it thus sharing detatched auditory ossicles along with other crown monotremes.
Rowe et al.'s phylogeny contradicts that of Luo et al., breaking up Australosphenida
Luo et al. (2001b) [[Nature 409, 53-57]] controversially grouped the monotremes - following discovery of monotreme teeth in the form of Obdurodon and Monotrematum - along with the Madagascan fossil Ambondro, and also Asfaltomylos, Ausktribosphenos, the 'pseudotribosphenic' Shuotherium and other fossils as the Australosphenida; a group of Gondwanan tribosphenic mammals not part of the Laurasian radiation of tribosphenic mammals known as the boreosphenida. This grouping is broken up by the analysis of Rowe et al. (2008), which places Ambondro, Shuotherium etc. together, but with the monotremata excluded from 'Theriiformes' thus on the base of the therian mammal stem. If Luo et al. are correct, this is a reason to doubt the hypothesis.
Luo et al.'s phylogeny is highly controversial, and some have advocated removal of Shuotherium from the group, let alone the monotremes. Luo et al.'s work was not done in light of the new evidence presented by Rowe et al.(2008) either.
Ambondro, Ausktribosphenos, and Asfaltomylos all cluster consistently with therians in Rowe et al.'s (2008) tree, and monotremes basally.
The Australosphenida hypothesis requires dual origin of tribosphenic molars - the very thing which makes the hypothesis so well known and controversial -, and if undermined by lack of clustering of monotremes with Australosphenida it looks yet less certain.
Of even the most relaxed molecular clock estimates of Rowe et al. (2008), only one put a date for an echidna-platypus divergence overlapping with the fossil date. Most estimates range from between 17 and 80 mya, and even the oldest strict dates are predated by the fossil by 50%. This is very unsual, with molecular clocks usually overstimating age of divergence, partly as the 'very first' organism/population on each branch of a divergence is unlikely to become fossilized and therefore at least a short 'ghost lineage' would be expected. Often this ghost lineage is indeed much larger than expected, casting into doubt the clocks themselves for this reason.
Rowe et al. (2008) do not claim that the molecular dating methods are simply wrong, but rather that the results they produce are the result of slow evolution on the part of monotremes leading to understimations when evolution rates are calibrated using other taxa. This correlates to their slower metabolism, ventilation rates compared to therian mammals of similar size, and low body temperature (Bech et al., 1992). Such rate slowdowns present a challange to use of molecular clocks, as they are "difficult to accommodate" (Rowe et al.) even when relaxed molecular clock estimates are used. The 1000-fold lower diversity of modern monotremes compared to therians (5 versus ~5000 species) may also be evidence of a slowed down evolutionary rate.
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