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(Reverse date order)

1985
Two researchers are the first to define that caviomorphs are classed as order Rodentia, based on traditional taxonomy. This relies mainly on comparative morphology, and does not take into account molecular phylogenetics.
Reference: Luckett, W. and Hartenberger, J. (1985) 'Evolutionary relationships among rodents: Comments and conclusions.' Pp. 685-712 in Evolutionary Relationships Among Rodents: A Multidisciplineary Analysis. New York: Plenum Press.

1991
In a letter to the journal Nature, three researchers point out that literature about caviomorphs abounds with references referring to their 'convergent evolution', 'extremely rapid rates of substitution' and 'unique evolutionary mechanisms'. These researchers have data, based on analysis of guinea pig amino acid sequences, that imply caviomorphs became genetically diversified before primates separated from myomorph rodents. If this is true, then Rodentia and Caviomorpha are not the same clade, and so caviomorphs (or even hystricomorphs) should be elevated into their own, distinct order. It is suggested that this diversification occurred even before Lagomorpha differentiated- could this lead to the reclassification of degus into a new order, perhaps order Hystricomorpha?
Reference: Graur, D., Hide, W. and Li, W. (1991) 'Is the guinea-pig a rodent?' Nature, 351: 649-652.

May/June 1992
The Graur research team clarify their 1991 research. They state that caviomorphs appear to have branched off prior to the divergence of the myomorphs, lagomorphs, primates, chiropterans, artiodactyls and carnivores. They call for the possibility of elevating caviomorphs and possibly hystricomorphs from the order Rodentia into their own order, and for the re-evaluation of their evolution, along with further molecular studies. They also state that if caviomorphs remain in the order Rodentia, it cannot be ignored that the molecular evolution of caviomorph genes has been very unusual and further studies are needed. However, it is known that the method these researchers used is not always 100 % reliable and can give misleading results.
Reference: Li, W., Hide, W., Zharkikh, A., Ma, D. and Graur, D. (1992) 'The molecular taxonomy and evolution of the guinea pig.' Journal of Heredity, 83 (3): 174-81.

November 1993
In a review, a brief statement was made claiming that "No morphologist has yet come forward in support of the new order Caviomorpha, and other molecular data hardly favours it." Although negative, this statement was not based on any singular piece of research and is highly subjective. The debate continues.
Reference: Patterson, C., Williams, D. and Humphries, C. (1993) 'Congruence between molecular and morphological phylogenies.' Annual Review of Ecology and Systematics, 24: 153-188.

1994
Taking on previous research by the Graur team, Japanese scientists examined the caviomorph phylogeny via a slightly different method. Perhaps not surprisingly, the results they obtained contradicted Graur et al.'s research, stating that caviomorphs were not of a separate order to Rodentia. They state the importance of using the most appropriate method for such analysis, and point out that because Caviomorpha is a relatively small group, the data set may be unreliable.
Reference: Cao, Y., Adachi, J., Yano, T. and Hasegawa, M. (1994) 'Phylogenetic place of guinea pigs: No support of the rodent-polyphyly hypothesis from maximum-likelihood analyses of multiple protein sequences.' Molecular Biology and Evolution, 11: 593-604.

October 1994
Using the same method as the first Japanese researchers, two more Japanese biophysicists analysed caviomorph phylogeny. They also discovered that Graur's research was not supported, and concluded that caviomorphs did belong to Rodentia. They even went further and analysed the lagomorphs, and the results supported the theory that Lagomorphs are in in their own order as they are actually more closely related to primates. They suggest the branching sequence progressed from Rodentia-Lagomorpha-Primates-Carnivora-Artiodactyla.
Reference: Kuma, K. and Miyata, T. (1994) 'Mammalian phylogeny inferred from multiple protein data.' Japanese Journal of Genetics, 69 (5): 555-66.

June 1996
In response to the Graur et al. research, a group of Italian molecular biologists took on the debate. They claim that since then, several findings have been reported both for an against the 1991 argument, bringing the validity of their research into question. Taking this on, the Italian group analysed the complete mitochondrial sequence of the guinea pig, which they found strongly contradicted their rodent classification. These researches therefore support the debate that caviomorphs should not belong to the order Rodentia, but may in fact be closer to the order Lagomorpha!
Reference: D'Erchia, A., Gissi, C., Pesole, G., Saccone, C. and Arnason, U. (1996) 'The guinea-pig is not a rodent.' Nature, 381 (6583): 597-600.

January 1997
In response to D'Erchia et al.'s conclusions, the researchers Cao and Hasegawa pointed out some drawbacks of their research. In a letter to Molecular Biology and Evolution, they claim that the research did not adequately resolve the phylogeny debate. They said that while the research should not be dismissed, D'Erchia's argument was too weak to completely exclude the rodent monophyly hypothesis. According to them, the most likely argument is still that caviomorphs belong to Rodentia.
Reference: Cao, Y., Okada, N. and Hasegawa, M. (1997) 'Phylogenetic position of guinea pigs revisited.' Molecular Biology and Evolution, 14 (4): 461-464.

April 2000
In his attempts to contribute to the debate, a biology student has submitted a thesis on the topic. His research compared the anatomy of the skull of the rat with that of a primitive species of caviomorph (Proechimys trinitatus). He concluded that the caviomorph was "probably not a rodent". However, his research only compared one member of the caviomorph sub order to one member of Rodentia, and even he agreed that this result was not enough to base a classification descision on.
Reference: Marcin, R. (2000) 'Comparative cranial anatomy of Rattus norvegicus and Proechimys trinitatus.' Undergraduate honours thesis, City University of New York.

May 2000
This time a French research team has made a contribution. By analysing only one particular gene code, which has previously been used in determining rodent phylogeny, these researchers found that Rodentia is monophylytic. This supports the argument that caviomorphs are not a separate order.
Reference: Robinson-Rechavi, M., Ponger, L. and Mouchiroud, D. (2000) 'Nuclear gene LCAT supports rodent monophyly.' Molecular Biology and Evolution, 17: 1410-1412.

December 2000
Some of the Italian molecular biologists have released a paper. Using amino acid sequencing, the phylogeny of a whole group of rodents was assessed. They discovered that Rodentia was indeed polyphyletic, and not only that but the caviomorphs were in a separate order with species such as doormice and squirrels!
Reference: Reyes, A., Pesole, G. and Saccone, C. (2000) 'Long-branch attraction phenomenon and the impact of among-site rate variation on rodent phylogeny.' Gene, 259 (1-2): 177-87.

January 2001
A team of Swedish molecular geneticists researching hystricomorphs (including caviomorphs) release a paper. They analysed the complete mitochondrial genome of the hystricomorph Thryonomys swinderianus, the cane rat. They concluded that this research supported the theory that Hystricomorpha and Caviomorpha should not belong to Rodentia. However, they also appeared to state that this research did NOT support the theory that Lagomorpha was distinct from Rodentia!
Reference: Mouchaty, S., Catzeflis, F., Janke, A. and Arnason, U. (2001) 'Molecular evidence of an African Phiomorpha-South American Caviomorpha clade and support for Hystricognathi based on the complete mitochondrial genome of the cane rat (Thryonomys sinderianus).' Molecular Phylogenetics and Evolution, 18 (1): 127-135.

July 2002
The discovery of a new fossil, an early Oligocene hysticognath rodent Bugtimys zafarullahi, leads French researchers to revise phylogeny assessment for hystricognaths based on dental findings. Their research supported monophyly of the hystricognath clade.
Reference: Marivaux, L., Vianey-Liaud, M., Welcomme, J-L. and Jaeger, J-J. (2002) 'The role of Asia in the origin and diversification of hystricognathous rodents.' Zoologica Scripta, 31: 225.

October 2004
Prolific degu researcher, Dr. Gerd Poeggel, published a paper on degus. In this latest paper, Poeggel and colleagues (also renowned for their work on degus- L. Nowicki and K. Braun) referred to degus as "a precocious lagomorph". This was highly unusual as Poeggel had previously only referred to degus as caviomorph rodents. Had there been a sudden reclassification of degus as belonging to the rabbit family, or was this purely speculation? Degutopia contacted Dr. Poeggel to ask about this issue, but we did not receive a reply- we should probably assume that this was in fact a mistake.
Reference: Poeggel, G., Nowicki, L. and Braun, K. (2005) 'Early social environment interferes with the development of NADPH-diaphorase-reactive neurons in the rodent orbital prefrontal cortex.' Journal of Neurobiology, 62: 42-6.