Literature

Last November, attention was drawn to a remarkable fossil find in Early Cretaceous rocks from Australia. The researchers discovered structures in the fossil that were just like the canal in a modern platypus that carries nerve fibers from the electrosensory glands in the bill to the brain. Further research has explored the significance of the fossil material coming from Cretaceous rocks, significantly earlier than had previously been thought possible.

The fossils are from the same family as the modern platypus (image credit: AAP & Joe Castro)

The researchers have named the fossil Teinolophos and refer to it as a "crown montreme". It has big implications for thinking about monotreme history:

"The finding that Teinolophos is a platypus indicates that the platypus and echidna clades diverged during or before the Early Cretaceous. This date is more ancient by a factor of 7 than the youngest, and 50% older than the oldest strict molecular clock estimates. The recent characterization of monotreme history as a "long-fuse" clade, whose diversification into platypus and echidna clades postdated the Cretaceous-Tertiary boundary, is difficult to reconcile with our more ancient divergence estimate, nor is there evidence of a diversity "explosion" at any time in monotreme history."
and:
"If the new position of Teinolophos is upheld, crown monotremes had originated and the platypus and echidna clades were established by the Early Cretaceous."

There are important implications for molecular clocks:

"our results suggest that different mammalian clades were subject to evolutionary rate heterogeneities that are incompatible with strict molecular clocks and difficult to accommodate even when relaxed molecular clock models are applied to mammalian history on a deep temporal scale."

This becomes another example of bradytely, akin to the Limulus example discussed recently. It means that many contributions to the literature about stem and crown monotremes have to be discarded in the light of this find. Furthermore, the hypothesis advanced in my Limulus blog - that organisms can be considered as reservoirs of type-specific biological information at the outset, and their subsequent history can result in various permutations of that biological information (e.g. niche specialisation) or to loss (degenerate forms) - appears to fit the platypus data as well.

The oldest platypus and its bearing on divergence timing of the platypus and echidna clades
Timothy Rowe, Thomas H. Rich, Patricia Vickers-Rich, Mark Springer, and Michael O. Woodburne
Proceedings of the National Academy of Sciences USA, January 29, 2008, 105(4), 1238-1242. (OPEN ACCESS ARTICLE)

ABSTRACT. Monotremes have left a poor fossil record, and paleontology has been virtually mute during two decades of discussion about molecular clock estimates of the timing of divergence between the platypus and echidna clades. We describe evidence from high-resolution x-ray computed tomography indicating that Teinolophos, an Early Cretaceous fossil from Australia's Flat Rocks locality (121-112.5 Ma), lies within the crown clade Monotremata, as a basal platypus. Strict molecular clock estimates of the divergence between platypus and echidnas range from 17 to 80 Ma, but Teinolophos suggests that the two monotreme clades were already distinct in the Early Cretaceous, and that their divergence may predate even the oldest strict molecular estimates by at least 50%. We generated relaxed molecular clock models using three different data sets, but only one yielded a date overlapping with the age of Teinolophos. Morphology suggests that Teinolophos is a platypus in both phylogenetic and ecological aspects, and tends to contradict the popular view of rapid Cenozoic monotreme diversification. Whereas the monotreme fossil record is still sparse and open to interpretation, the new data are consistent with much slower ecological, morphological, and taxonomic diversification rates for monotremes than in their sister taxon, the therian mammals. This alternative view of a deep geological history for monotremes suggests that rate heterogeneities may have affected mammalian evolution in such a way as to defeat strict molecular clock models, and to challenge even relaxed molecular clock models when applied to mammalian history at a deep temporal scale.

See also:

Miller, B. Fossil suggests platypus lived in dinosaur times, ABC News, Tue Jan 22, 2008.

Tyler, D. The "implausible" platypus continues to surprise, ARN Literature Blog, 23 November 2007.