Octodontoidea, Waterhouse, 1839

MacPhee, R. D. E., 2011, Basicranial Morphology And Relationships Of Antillean Heptaxodontidae (Rodentia, Ctenohystrica, Caviomorpha), Bulletin of the American Museum of Natural History 2011 (363), pp. 1-70 : 38-62

Octodontoidea

Abrocomidae : In Abrocoma bennetti AMNHM 33273 the middle ear is highly inflated and sculpted, with notable septal content, but there is no recognizable posttympanic foramen or canal. Pessulus is present. Chinchilla rats have sometimes been regarded as closely related to Chinchillidae ( Glanz and Anderson, 1990) , but molecular data clearly support positioning them within Octodontoidea ( Huchon and Douzery, 2001). The basicranium shows none of the distinctive features of chinchilloids: the tympanic fenestra is absent and there are only small foramina for meatal innominate vessels, as in octodontoids generally.

Capromyidae : The posttympanic foramen could not be detected and the intratympanic canal is not present. There is no indication that a tympanic fenestra is formed early in ontogeny (fig. 21). The pessulus exists in Geocapromys brownii AMNHM 45156. (fig. 21).

Echimyidae : This is a speciose group, but basicranial characters are relatively uniform in the four taxa investigated—the eumysopines Proechimys guyannensis and Lonchothrix emiliae , the dactylomyine Dactylomys dactylinus , and the echimyine Echimys sp. Although the bullae are primitively bean shaped in all specimens, with no postympanic foramen and an unbreached external acoustic meatus, middle-ear pneumatization is marked, producing near isolation of the crista tympani from the lateral bullar wall in eumysopines in particular. All specimens displayed a single meatal innominate foramen under the meatal lip; the margins of the foramen were ragged, however, especially in the dactylomyines, indicating that the aperture developed by gradual enclosure. However, it is never as large as in Octodon (see below) and therefore the tympanic fenestra is scored as absent, although I recognize this is a judgment call. A pessulus was detectable in the dactylomyine specimens and Proechimys guyannensis AMNHM 92952, but was either absent or indeterminable in the other specimens.

Myocastoridae : In Myocastor coypus AMNHM 80097, a young animal with P1 and M1 erupted but only slightly worn, a foramen just below the lip of the right meatus leads to a sulcus on the shelflike tympanic collar. On the left side the foramen is not in evidence but the sulcus, evidently for meatal innominate vasculature, can be clearly seen (fig. 22A). The only other aperture in the area is the mastoid foramen, clearly leading into the mastoid; there is thus no foramen or tube for the ramus posttympanicus. The pessulus is present in this particular specimen, but is apparently absent on the examined (right) side in M. coypus AMNHM 35626. Whether there is real variability will have to be determined on the basis of a larger sample.

Octodontidae : This family, also markedly speciose, is fairly conservative for the characters under consideration. In a representative sample consisting of Octodon degus AMNHM 242476, Octodontomys gliroides AMNHM 249052, and Spalacopus cyanus AMNHM 33296, the posttympanic foramen and canal are lacking, but a pessulus is present in the first two. In the specimen of Octodon the tympanic fenestra is present as a ragged-edged gash, and is scored as present (C1:1); in the others there is only a pinhole for innominate meatal vasculature.

Ctenomyidae : Ctenomys magellanicus AMNHM 17445 exhibits a highly inflated middle ear with elaborate septal content, like some octodontids (e.g., Octodon ), but there is no posttympanic foramen/canal. There is, however, a pessulus and a restricted foramen for innominate vasculature (C1:0).

Caviidae : Cavia aperea AMHNM 264468 features a well-defined fenestra, completely walled off from the external acoustic meatus, with a distal expansion and a partially closed ventral cleft. This specimen also presents good examples of accessory bones attached to the lip of the moderately developed meatal canal. There is neither posttympanic foramen nor canal in this specimen or Dolichotis patagonum AMNHM 48218, but the pessulus is present in the latter, as is a fenestra similar to that of Cavia . Galea musteloides AMNH 213465 exhibits the same combination of conditions. As noted elsewhere, Aihara (1958) presented a detailed dissection of middle-ear vasculature in Cavia , but did not address the problem of homological equivalents.

In Hydrochoerus hydrochaeris AMNHM 15468 (fig. 23), a young animal, there is a foramen situated immediately posterior to the stylomastoid foramen. However, probing with a bristle reveals that this foramen leads not into the tympanic cavity but instead into the open suture between the petrosal mastoid and the exoccipital (parocccipital process). It is therefore considered the mastoid foramen. The pessulus is absent, but the fenestra is well developed. The ventral cleft is quite ragged with opposing edges widely separated, as in some adult capybaras (e.g., H. hydrochaeris AMNHM 206444). In others ( H. hydrochaeris AMNHM 98634 and 134102, both large adults) the spiculate edges of the cleft are actually in contact, with a slight distal expansion conforming to the tympanic fenestra. As in Dinomys branickii AMNHM 185372 (fig. 12), there is a long cleft formed by nearly approximating bone territories.

Dasyproctidae : The posttympanic foramen and tube are absent in Dasyprocta punctata AMNHM 41394 (fig. 24), a young specimen, but the tympanic fenestra is well developed. In Myoprocta acouchy AMNHM 70198 conditions are essentially identical. In this latter specimen the stapedius muscle appears to be completely enclosed in a thin, translucent shell of bone; its tendon travels to the stapes in its own tiny tube. This tube is clearly not a conduit for the ramus posttympanicus, as it ends as usual on the posterior crus (to which the stapedius tendon normally attaches) and is not directed toward the obturator foramen. The pessulus was absent in the examined specimens of Dasyprocta ; pessular absence in Myoprocta acouchy AMNHM 70198 could not be confirmed but seems probable. A well-developed tympanic fenestra is present in the Deseadan Oligocene species Incamys bolvianus , nominally regarded as a dasyproctid ( Patterson and Wood, 1982).

Cuniculidae : Cuniculus paca AMNHM 2905 and 266567 present conditions similar to those seen in Dasyprocta and Myoprocta with regard to features of interest, except for absence of the fenestra. As in these taxa, the posterior part of the tympanic cavity is very restricted, with no exterior or interior indication of a ramus posttympanicus. A large foramen near the posterior lip of stylomastoid foramen is directed posteriorly, and is thus a mastoid foramen. The pessulus is absent.

DEVELOPMENT, HOMOLOGY, AND CHARACTER ANALYSIS

TYMPANIC FENESTRA AND DEVELOPMENT OF THE LATERAL BULLAR WALL

Although it would be necessary to have access to developmental series in order to thoroughly document fenestral development in different clades of ctenohystricans, major patterns can be plausibly induced from juvenile and adult conditions, as briefly treated in this section and figure 25. The present investigation indicates that the critical factor in fenestral development is the relative growth rates of the crural margins of the primordial U-shaped ectotympanic. Theoretically, if crural growth rates were roughly similar on both the lateral and the medial aspects of the ectotympanic, the bulla would tend to expand in a simple, balloonlike manner, with the primitive external acoustic meatus appearing to reduce in relative size as its floor ossifies. Further, if the floor of the meatus were to evenly ossify and remodel in concert with the rest of the lateral bullar wall, as in humans, the fenestra as a ‘‘potential space’’ separate from the meatal aperture should disappear by the definitive stage of development. By contrast, if growth gradients along the crura were such that, for example, one crus tended to preserve its embryonic proportions while the other became greatly expanded, the effect on the meatal floor would be a persistent defect (ventral cleft) in the presumptive margin of the external acoustic meatus. In later ontogeny the defect might remain completely open or partly close, depending on the degree of approximation of bone territories.

Other than the fibrous membrane and associated dense connective tissues, the only structures in the presumptive meatal area are the tympanic membrane and the vascular elements grouped here as meatal innominate vessels. Meatal innominate foramina are common in the target groups, and are found in taxa that exhibit the tympanic fenestra as well as those that do not. It is unlikely that local blood supply has any influence on the relative speed of ossification. Body size is likewise irrelevant to fenestral persistence. Although megafaunal rodents like Amblyrhiza , Eumegamys , and Phoberomys (?and perhaps Josephoartegasia) all possess a large tympanic fenestra in the adult stage, so do much smaller Dinomys and many cavioids. On the other hand, the size of the fenestra is positively correlated with the length of the external acoustic canal, which is itself a proxy for the hyperdevelopment of the anterior crus. This is especially evident in chinchilloids.

The reconstructed ontogeny of the three character states of C1, each developing from the indifferent or unexpanded anulus of late fetal life, is depicted in highly diagrammatic form in figure 25. The facial nerve, in its standard or primitive position passing immediately behind the posterior crus ( MacPhee, 1981), is provided as a reference point. In order to make character analysis consistent, C1:0 does not distinguish between complete absence of all apertures in the circummeatal area and presence of one or more meatal innominate foramina. C1:1 and C1:2 cover the remaining possibilities, which focus on whether the fenestra is simply a notch or a separate window, and whether an identifiable ectotympano-ectotympanic suture is formed.

The tympanic floor conformation seen most frequently in mammals is C1:0 (fig. 25, top), in which the definitive, smoothly round- ed meatus is the only sizeable opening in the lateral wall of the bulla. Minor variations on these conditions are seen in many caviomorphs (capromyids, echimyids, and cuniculids) and Old World ctenohystricans (e.g., Ctenodactylus gundi USNMM 325852, fig. 15; Thryonomys swinderianus AMNHM 216340, fig. 18). For this pathway the track of a meatal innominate vessel is also shown; it is assumed that such vessels are constantly present, whether or not an evident trackway can be discriminated osteologically.

C1:1 (fig. 25, middle) is possibly a wastebasket grouping, but with the material at hand I am not able to discriminate more than one character state. The recognition criterion for this character state is the notched condition of the ventral margin of the meatus. The apex of the notch is either little or not at all expanded, and shapes range from a narrow slit to a triangular gap in the meatal floor.

The only complication in recognizing this character state is that small projections from the margins of the notch may eventually grow together in some taxa or individuals, yielding a bridged condition with a large opening above (meatus) and a much smaller aperture below (tympanic fenestra). For example, notches are nearly closed bilaterally in Dolichotis sp. AMNHM 48218, an old juvenile, and only just bridged over in Galea musteloides AMNHM 213465. In adult Cavia aspera AMNHM 264468, bridging is complete, and the distance between fenestra and meatus is very wide indeed. It is not known whether the presence of so-called accessory meatal bones have an effect on fenestral development in this taxon. This last arrangement impinges, at least descriptively, on that described under C1:2, although other distinctions also apply. As explained below, the primary difference is that the bony projections of the bridged condition of C1:1 do not ordinarily form sutural tissues at an early developmental age or lead to the lengthy line of sutural fusion seen in taxa expressing C1:2.

Interestingly, some specimens display a notch on one side and a bridge on the other, which points up that these morphologies should be thought of as slightly different outcomes produced along the same general ontogenetic pathway. Thus in the dasyproctid Dasyprocta punctata AMNHM 41394, a very young animal in which only the first upper molars are fully erupted, the meatus is deeply notched (fig. 24). This also applies to a somewhat older D. punctata AMNHM 23461 (upper third molars erupting), but in the dentally mature D. punctata AMNHM 262281 the notch is bridged on the left side and but not on the right.

The hallmark of C1:2 (fig. 25, bottom) is very marked differential growth of the crura and elongation of the meatus into a long canal, as seen in certain chinchillids (e.g., Lagostomus maximus AMNHM 80208, fig.

20). Sutural tissues are inferred to form at an early stage, because in L. maximus AMNHM 70222 (47 mm skull length), a juvenile, the ectotympano-ectotympanic suture is already closed, but still detectable. In the adult stage, a low ridge or seam may be seen running along the line of fusion inside the meatus, marking the line of contact (see Amblryhiza inundata AMNHVP 11842; fig. 5). Chinchilla is essentially the same as Lagostomus , except that pneumatization is so pronounced in this genus that the original form of the crura is entirely masked. However, the closed ventral cleft can easily be distinguished on the lateral bullar wall.

In summary (see also fig. 27), there are no morphological grounds at present for inferring that the fenestra develops, and later disappears, during the ontogeny of most phiomorphs. ( Petromus is the only extant contrary example, although this point should be checked in fossil taxa.) Within Caviomorpha, at the superfamily level the fenestra is absent in adult erethizontoids (i.e., inferior margin never notched, meatal innominate vessels small). In adult stages of extant octodontoids the fenestra is either absent or no more than a small aperture, in many cases bridged off from the meatus by a strap of bone. In most taxa that express a definite fenestra, margins remain rather ragged, as might be expected in an imperfectly ossified region. However, in others the aperture is small and rounded, and functions exclusively as a true foramen for transmitting vasculature (e.g., Spalacopus cyaneus AMNHM 33276; Myocastor coypus AMNHM 35626). In cavioids and chinchilloids, by contrast, the fenestra is usually fairly large relative to the size of the external acoustic meatus.

Amblyrhiza agrees more closely with conditions in Lagostomus and other chinchillids than with Dinomys , in that it possesses a markedly elongated external acoustic canal and a fused ectotympano-ectotympanic suture in the adult in combination with a patent tympanic fenestra. The skull of Eumegamys , although damaged in the critical area, agrees closely with Dinomys in retaining an open ventral cleft and much shorter external acoustic canal. Elasmodontomys resembles the majority of octodontoids in completely lacking a fenestra.

RAMUS POSTTYMPANICUS AS STAPEDIAL RAMUS POSTERIOR

Is the occupant of the posttympanic canal found in some caviomorphs in fact the homolog of the stapedial ramus posterior, a vessel never previously identified in a rodent? From a comparative standpoint, this question is not as unlikely as it may seem. In several basicranially primitive placental taxa ( Erinaceus , Solenodon , and several tenrecs) that preserve an intact, primitive stapedial system, the ramus posterior is a branch released by the parent vessel just before the latter passes through the obturator foramen of the stapes ( MacPhee, 1981). In Erinaceus the ramus posterior is very small, supplies only the area of the stapedius fossa, and evidently involutes in early postnatal life. In tenrecs and, especially, Solenodon , it is a large vessel that travels outside the confines of the middle ear in close relation to the facial nerve ( MacPhee, 1981). Recently, Wible (2008) confirmed that the ramus posterior in Solenodon supplies the area conventionally associated with the posterior auricular artery in other mammals (pinna and related structures in the rear part of the head).

In Homo , a vessel with somewhat similar relations, but lacking its own foramen and individual bony conduit, is the stylomastoid branch of the posterior auricular artery. Whether this vessel may itself be a stapedial remnant has never been properly discussed in the general morphological literature, let alone that oriented toward Rodentia . Thus, Aihara (1958) noted that a branch of the stylomastoid artery—his ‘‘stapedial artery’’—feeds the stapedial muscle in Cavia . However, he did not consider whether both, either, or neither might be homologous with any part of the primitive stapedial system. As Cavia definitely lacks a posttympanic canal (see below), the situation remains as ambiguous in this taxon as in any other.

While the possible equivalency of the ramus posttympanicus and part or all of the stylomastoid artery’s distributary network is worth acknowledging, there are other observations that suggest a different picture for hystricognaths. Although Wible (1984) did not encounter a ramus posterior in any of the taxa he personally examined for his monumental study of mammalian cephalic arterial systems, in his literature review he noted some additional instances of the probable or possible occurrence of this vessel. One such instance is directly relevant here: Struthers’ (1930) study of cephalic arterial development in fetal Erethizon , in which this author described an ‘‘occipital’’ artery transitorily linked to the developing proximal stapedial. Wible (1984) commented that the stapedial ‘‘occipital’’ connection traveled in close relation to the facial nerve and stapedius muscle, just as in verified occurrences of the ramus posterior in nonhystricognaths. Whether the porcupine ‘‘occipital’’ is homologous with the posterior auricular of Homo cannot be determined from the information provided by Struthers, but that is certainly a possibility.

The question that immediately occurs is whether the arrangement seen in fetal erethizontids can be generalized to explain some of the findings presented in this paper. Figure 26 illustrates a possible, although speculative, interpretation. In some unspecified caviomorph (or hystricognath) ancestor the entire primitive stapedial system is inferred to have been present (fig. 26A), fed by the internal carotid stem, and perhaps largely functional in the adult. The ramus posterior might have already possessed the indicated connection with the posterior auricular, or this may have developed later on when the stapedial system as a whole began to involute. With involution, the distalmost segment—now as the ramus posttympanicus—would have crossed the track of the facial nerve and continued to supply the area of the stapedius fossa, possibly via retrograde blood flow from its anastomotic partner as suggested in figure 26B (which purposely mimics conditions in Amblyrhiza , although apart from the ramus posttympanicus there is no conclusive evidence for other possible stapedial vestiges in this taxon). How much of the resultant arterial pathway should be considered equivalent to the original ramus posterior and how much to the neomorphic anastomosis is impossible to say without more evidence. The best course would be to retain the name ‘‘ramus posttympanicus’’ for the whole structure, even though it may well incorporate material ultimately derived from more than one primitive source.

There are other clues that a vestigial stapedial network still exists in extant caviomorphs. One is the inferior tympanic artery identified in Cavia by Aihara (1958), which has some of the characteristics one would expect to find in a vestige of the primitive stapedial stem: it exhibits anastomotic links with both the middle meningeal and the vessel issuing from the stylomastoid artery that Aihara (1958) confusingly named ‘‘the’’ stapedial artery (see above). Another is the ‘‘ramus tympanicus’’ of the maxillary artery that provides a ‘‘supply to the middle ear’’ in Cavia according to Cooper and Schiller (1975: 151). Its position of origin seems too rostral for it to represent one of the meatal innominate vesssels of this paper. Although there are several plausible homologies, it is possible that the ‘‘ramus tympanicus’’ may be yet another remnant of the stapedial system (?ramus inferior)—in this case, one that has been pirated by the maxillary and which now delivers retrograde flow to some part of the tympanic cavity (for parallel cases in primates, see Diamond, 1991). Another is the apparent retention of a foramen of exit for the ramus superior (evidently maintained for meningeal supply) on the cerebral side of the petrosal in Lagostomus (cf. Bugge’s [1974b: fig. 6D] illustration of an injected specimen). It would be of great interest to know whether the vessel traversing this foramen anastomosed with the ramus posttympanicus or with the stylomastoid artery (assuming these are different entities in the plains viscacha).

Also possibly relevant is Tandler’s (1901: 360) alleged detection of a highly reduced internal carotid artery in Hystrix : in his interpretation, the vessel entered the middle ear through a tiny aperture on the posterior side of the bulla, then traveled in a groove transpromontorially in company with the internal carotid nerve toward the cranial cavity. Dierbach (1985) found a ‘‘carotid foramen,’’ but no artery traversing it, in a 25 mm CRL fetus of Cavia ; from this indirect (and admittedly inadequate) evidence he inferred that the internal carotid artery was formed but had already involuted. Although carotid involution occurs during the ontogeny of many placentals, loss need not be complete and some structures may be retained as functionless or near-functionless remnants ( Bugge, 1974b; MacPhee, 1981; Wible, 1984; Diamond, 1991). Cooper and Schiller (1975: fig. 4–15) claim that a highly reduced version of this artery exists in adult Cavia , but this has not been confirmed either. Indeed, whether any nonerethizontid ctenohystrican possesses an intact internal carotid artery at any stage of development remains to be properly demonstrated. By contrast, a much more credible case can be made for the persistence and repurposing of parts of the primitive stapedial system.

SYSTEMATIC SIGNIFICANCE

Despite a certain amount of recent attention, a number of aspects of ctenohystrican phylogeny are still not well resolved (e.g., superfamilial and major-clade relationships within Caviomorpha). In light of this, tracing character distributions on a plausible composite tree is likely to be of more use at present than ‘‘testing’’ systematic hypotheses with tiny character sets. The overall tree topology used in figures 27–29 to present character optimizations ultimately derives from Huchon and Douzery’s (2001) study, with familial and other downtree relationships reflecting more recent analyses ( Rowe and Honeycutt, 2002; Spotorno et al., 2004; Galewski et al., 2005; Morgan, 2009). I have placed the target taxa discussed at length in this paper— Amblyrhiza , Eumegamys , and Elasmodontomys —in what I regard as plausible positions. Although MacClade-style optimizations are not especially worthwhile with so few characters in play, the character traces do bring out some interesting points, as discussed in the figure captions.

The chief result of this study is to demonstrate that, on the basis of basicranial features, the concept of ‘‘Antillean heptaxodontids’’ as a monophyletic group is artificial and its contents need to be reorganized:

1. Relationships of Amblyrhiza . In simplest terms, Amblyrhiza emerges as a chinchilloid with basicranial vascular features that are specifically like those found in extant Dinomys , but with bullar developmental features that are more reminiscent of extant Lagostomus and its allies. If the homological connection between the ramus posttympanicus and stapedial ramus posterior is as intimate as I have proposed, then possession of this vessel at least through early phases of vascular ontogeny is very probably a primitive feature at the level of Caviomorpha, if not higher in the ctenohystrican hierarchy. However, maintenance of a functional vessel into the adult stage is clearly another matter. Possession of a bony tube for this vessel in Dinomys and Amblyrhiza in the comparative set is interesting, but tubes are a variable and often unreliable indication of systematic relationships ( MacPhee, 1981)—witness substitution of the tube by a partial septum in Dinomys AMNHM 70354. If Amblyrhiza is brigaded more closely with chinchillids than dinomyids, then the more Dinomys -like development of its ramus posttympanicus could be regard- ed as a convergence, or as a basal feature of chinchilloids that is no longer present (or at least no longer recognizable) in chinchillids. On the other hand, massive meatal elongation and formation of a substantial ectotympano-ectotympanic suture, features found in both groups of extant chinchilloids as well as Amblyrhiza , do not seem to concur in this particular combination in other caviomorphs. The wisest course at present, then, is not to ally Amblyrhiza exclusively with either family, but place it as the sister group of both. In one way this hearkens back to Anthony’s (1917) original formulation, but with this difference: Amblyrhiza is the only one among the original suite of ‘‘Antillean heptaxodontids’’ that can be placed in Chinchilloidea on the basis of basicranial character evidence (see next paragraph).

2. Relationships of Elasmodontomys . This genus is nondescript basicranially, resembling a wide variety of other taxa— but not Amblyrhiza in particular or chinchilloids in general. It is thus of some interest to note that, in their study, Woods and Hermanson (1985: 533) concluded that Elasmodontomys ‘‘groups most closely with Myocastor coypus and is not very distinct from capromyids. Elasmodontomys stands clearly apart from Dinomys , with which it has been grouped by some authors in the past…. We propose placing the [Antillean] heptaxodontids within the superfamily Octodontoidea , based on the elongated acromion process of Elasmodontomys , and would associate this family adjacent to the West Indian Family Capromyidae with which it may share a common evolutionary history.’’ Woods and Hermanson (1985) did not have evidence of acromial morphology in any of the other Antillean heptaxodontids, but evidently assumed that they would be similar. In the epigraph at the head of this paper, Woods (1982) admits that his ‘‘Capromyidae-Heptaxodontidae clade’’ is exceedingly diverse, but that is only true if the classic organization of Antillean heptaxodontids is retained. Breaking out Elasmodontomys changes things dramatically. There is nothing in the auditory regions of Elasmodontomys , Myocastor , and capromyids/echimyids— notably, if plesiomorphously, similar— which would preclude their close phylogenetic association, despite the assertion of Patterson and Wood (1982: 512) that Elasmodontomys cannot be closely related to echimyids because it replaced its deciduous premolars. If the acromial evidence is correctly interpret- ed to mean that Elasmodontomys is an octodontoid, then any dental resemblances to chinchilloids are spurious. On the other hand, according to Martin (1992), octodontoids are unified by their incisor enamel pattern (Schmeltzmuster), which features low-angle interprismatic matrix not seen in either Amblyrhiza or Elasmodontomys (which show instead the plesiomorphic, acute-angle character state): While this observation is pertinent, the seating of the Puerto Rican rodent remains quite uncertain at this time, and in the tree diagram I prefer to place Elasmodontomys next to Myocastor and capromyids/echimyids without specifying which, if either, is its sister taxon.

3. Current Status of the Problem of Antillean Heptaxodontidae . As noted in the Introduction, the ‘‘problem’’ may be said to have originated with Anthony’s (1917: 186) proposal that, because the ‘‘identical tooth structure’’ of platetooth rodents could not have been due to convergence, then these taxa must in fact be closely related. In fact, the teeth of Amblyrhiza , Elasmodontomys , and Clidomys are far from identical (see Pascual et al., 1990). Given the limited specialist interest in the group, it is unsurprising that most commentators have been content to simply leave ‘‘Antillean Heptaxodontidae’’ as is, or place an asterisk beside the name to imply possible paraphyly (e.g., MacPhee and Flemming, 2003). The issue is now partly resolved, at least to the extent that basicranial evidence does not support a close relationship between the central taxa Amblyrhiza and Elasmodontomys .

Heptaxodontinae remains the appropriate

subfamily name for any monophyletic group

containing Elasmodontomys obliquus (5 Hep-

taxodon bidens ) ( Ray, 1964, 1965). Although

Amblyrhiza can no longer be considered to be

a member of such a group, what about the

other nominal Antillean heptaxodontids?

Quemisia , known only from a a few isolated

teeth, a jaw, and a partial femur, is arguably

the most Elasmodontomys -like (or echimyid/

capromyid-like) of the remaining taxa, as

pointed out in detail by Ray (1964, 1965). So

is Tainotherium ; in fact, the holotype femur

so strongly resembles that of Elasmodont-

omys that here we may have a distinction

without a difference ( Bover and MacPhee,

2007). Clidomys remains a puzzle; while there

are scattered resemblances to Amblyrhiza

( Morgan and Wilkins, 2003), dentally Clid-

omys is as much, or more, like neoepiblemids

(at least as defined by Pascual et al., 1990), suggesting that there might have been one more invasion of the Greater Antilles by a large-bodied caviomorph group than is usually counted. Xaymaca is the most obscure of all: MacPhee and Flemming (2003) made strenuous efforts to show that it might be a diminutive heptaxodontid, but a more realistic positioning for the seriously incomplete holotype of this taxon may be simply Caviomorpha, incertae sedis. This is as far as things can be reasonably taken at present. A more comprehensive answer will have to await analyses of other understudied character sources (e.g., tarsals, other postcranials; MacPhee, 1984; Horovitz et al., 2006), not only with reference to the insular forms discussed here, but also to the confus- ed tangle of continental South American taxa that have been sometimes regarded as ‘‘heptaxodontids.’’

Figures

Fig. 26. Theoretical interpretation of developmental interconnections among internal carotid, stapedial, and posterior auricular arteries in the ctenohystrican Bauplan, based on evidence discussed in the text. All diagrams are idealized for interpretative purposes; actual ontogeny is unknown. A, Hypothetical, fully elaborated arrangement, with proximally intact, medially positioned internal carotid providing blood to stapedial system exhibiting all three rami. Note anastomotic link or continuity between ramus posterior and posterior auricular via ramus posttympanicus. Note also small distributaries to stapedius muscle, inferred to be ramus inferior’s primitive area of supply on basis of conditions in several tenrecs and eulipotyphlans Erinaceus and Solenodon (MacPhee, 1981). Whether, and to what degree, this arrangement is primitive for placentals more generally is not known. B, Example of a derived arterial arrangement, partly based on conditions in Amblyrhiza. For this taxon there is firm evidence only for the presence of the ramus posttympanicus; other parts of the internal carotid/stapedial system may have completely involuted by adult stage, or never formed. Blood supply to ramus posttympanicus here interpreted to have been retrograde, from posterior auricular. Pattern also applies to Dinomys, but may be inferred for Eumegamys and possibly certain other caviomorphs (including chinchillids and erethizontids).