As noted above, the morphological partition alone does not recover most suprageneric clades of the combined-data analysis, and weakly disagrees with many aspects of the GHR tree (Fig. 4). However, it has been demonstrated that a given clade may receive support from a combined dataset even when that clade lacks support from every partition analyzed in isolation [22, 23]. For example, at high taxonomic levels within placental mammals, morphology (among other partitions) has been shown to contribute positively to clade support in a combined analysis, whether or not that clade is present in a tree derived from morphological data alone [24]. For the present dataset, the addition of the morphological partition improves branch support for the majority of nodes present in both the optimal MP and Bayesian topologies of the combined dataset (Fig. 2; Table 1). This observation is consistent with the observation made elsewhere [22–24], based on independent datasets, in which interaction between partitions can increase support beyond the sum of branch supports obtained from partitions in isolation (i.e., "hidden support" of [22]). Hence, we base our discussion on the combined-data tree.
Our results are partly congruent with the suggestion of Bronner and Jenkins [1] that chrysochlorids may be divided into two clades: Amblysominae and Chrysochlorinae, although their content is different according to our results. We place Amblysomus, Neamblysomus, and Carpitalpa arendsi in Amblysominae. The Chrysochlorinae of Bronner and Jenkins [1], excluding Carpitalpa and Chlorotalpa, is a potentially monophyletic clade (Figs. 2B, 3), and is weakly supported as such by our MP analyses of the combined data (Fig. 2B). However, additional data are necessary to address the possibility that the chrysochlorid root falls within this group, possibly near Eremitalpa (Fig. 2A). For ease of discussion, we use the terms amblysomines and chrysochlorines as defined above, but acknowledge that the latter term may be rendered paraphyletic with further phylogenetic scrutiny.
Morphological character evolution
As noted previously, chrysochlorids share a number of features (e.g., three forearm long-bones, hyoid-mandible articulation, hypertrophied malleus) seldom seen elsewhere among living mammals. In the case of ossicular morphology, considerable variation exists across chrysochlorid species [4–6, 25, 26]. Amblysomus, Neamblysomus, and Calcochloris show a relatively small malleus (Fig. 5A), unlike the elongate, club-shaped ossicle seen in Chrysochloris and Cryptochloris (Fig. 5D). The mallear head is enlarged and globular in Eremitalpa (Fig. 5E) and Chrysospalax (Fig. 5F), and is also globular but only slightly enlarged (relative to Amblysomus) in Chlorotalpa (Fig. 5B), Huetia, and Carpitalpa (Fig. 5C). Those taxa with a club-shaped malleus all possess a substantial bulge in the posterior aspect of their orbitotemporal fossa, known as the temporal bulla, which serves as a dorsal continuation of the epitympanic recess for housing the enlarged malleus (Fig. 6). The globular malleus of Eremitalpa is sufficiently large that it too results in an externally visible temporal bulla (Fig. 6C). In the other taxa with a globular malleus (Chlorotalpa, Huetia, and Carpitalpa), the ossicle is smaller and does not result in an externally distinct temporal bulla.
There are a number of other variations on chrysochlorid mallear morphology not accounted for in the current morphological matrix, such as orientation of the manubrium mallei, shared for example in Eremitalpa and Chrysochloris but not in Chrysospalax (Fig. 5; M. Mason pers. commun. and [4–6]). Here, we focus on size and shape of the mallear head, which has figured prominently in previous classifications. This region guided the classification of Simonetta [13], who stated " [mallear] morphology pointed to three divergent evolutionary trends, so that the family could be ... divided into three subfamilies" (p. 33 of ref. [13]). He further implied the intuitive view that the "normal" (i.e., small) malleus of Amblysomus is primitive, and inferred a "morphological sequence" from Amblysomus to the taxa with a globular malleus to the "extreme" club-shape observed in Chrysochloris.
Interestingly, the taxa most frequently inferred as basal by Bayesian analyses possess an enlarged, globular malleus (i.e., Eremitalpa), a slightly enlarged malleus (Huetia), or an elongate malleus (Chrysochloris and Cryptochloris). If such a taxon occupies the base of the chrysochlorid tree, then Simonetta's intuition about the character evolution of the chysochlorid malleus is wrong: taxa with an unenlarged malleus (Amblysomus, Neamblysomus, Calcochloris) occupy relatively nested branches. In contrast, the root supported by four of the eight MP analyses (Fig. 3) shows Calcochloris obtusirostris (with a small malleus) at the base of a monophyletic Chrysochlorinae, which forms the sister taxon of a Chlorotalpa-amblysomine clade. This scenario is potentially consistent with the view that a small malleus (Fig. 5A) characterized the basal-most living chrysochlorids, with enlargement occurring independently within each subfamily. Most phylogenies of Afrotheria support a tenrec-golden mole association [10, 11], and one would expect that the chrysochlorid common ancestor with other afrotherians would have had relatively small ear ossicles. Their scant fossil record [27, 28] indicates that Miocene chrysochlorids lacked a temporal bulla and therefore would not have had a mallear head of the kind seen in Chrysochloris (Fig. 5D), Eremitalpa (Fig. 5E), or Chrysospalax (Fig. 5F).
Previous discussions of mallear evolution in chrysochlorids have also noted the high probability of homoplasy in this region [4, 5, 26]. While we cannot yet resolve the position of the chrysochlorid root, all of our optimal trees agree with recent authors that mallear enlargement has not occurred in a simple progression from small, to globular, to elongate. The paraphyly of chrysochlorids with an enlarged, globular malleus indicates the presence of homoplasy in the occurrence and direction of ossicular enlargement.
Based on the phylogeny in Fig. 2, a few additional morphological characters optimize with relatively low homoplasy across chrysochlorids. The position of the foramen ovale relative to the foramen for the inferior ramus of the stapedial artery (Fig. 7) shows relatively little homoplasy in the optimal MP and Bayesian trees: the two are confluent in chrysochlorines (except Calcochloris) and distinct in amblysomines and Chlorotalpa. The position of foramen ovale relative to the sphenorbital fissure (Fig. 7) also distinguishes most members of the two groups: in amblysomines and Chlorotalpa sclateri they are separated in the ventral part of the temporal fossa, whereas in Chlorotalpa duthieae and most chrysochlorines (but not Huetia or Calcochloris) they are situated close together.
Dentally, most chrysochlorines (except for Chrysospalax) lack talonids on their lower ultimate premolar (Fig. 8), whereas these are present in amblysomines (except for Neamblysomus julianae) and Chlorotalpa. Talonids on the molars are similarly lacking in chrysochlorines (except Chrysospalax and Chrysochloris stuhlmanni), but present in amblysomines (except Neamblysomus) and Chlorotalpa. The overall number of teeth in each jaw quadrant---10 in those taxa with a full complement of three molars and 9 in taxa with just two---has also figured prominently in previous chrysochlorid classifications. However, reduction of molars is restricted to Amblysomus, Calcochloris, and is variable in Neamblysomus. As such, this feature is not diagnostic for the supra-generic clades indicated in this study.