A Possible Heptaxodontine and Other Caviidan Rodents from the Quaternary of Jamaica1

43 Pages·2003·1.7 MB·English

by  R. D. E. Macphee

PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY
CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024
Number 3422, 42 pp., 20 figures, 1 table December 9, 2003
A Possible Heptaxodontine and Other Caviidan
Rodents from the Quaternary of Jamaica1
R.D.E. MACPHEE2 AND CLARE FLEMMING3
ABSTRACT
New World hystricognath rodents (parvorder Caviida)easily qualify asthemostdiversified
members of the nonvolant Quaternary land mammal fauna of the West Indies. This paper
describes three intriguing but problematic representatives of this group from Jamaican cave
deposits. The first is the holotype (and still the only) specimen of Alterodon major from
WallingfordRoadsideCave,ataxonthatcontinuestogeneratecontroversybecausespecialists
disagree as to its placement within Caviida. We reject the argument that it should be placed
in Octodontidae and reaffirm the high probability that it is a clidomyine. The second fossil is
a large proximal femur, apparently recovered from Sheep Pen locality near Windsor (Trelaw-
ney Parish) in the 1960s. Much larger than the femur of Clidomys (previously thought to be
Jamaica’s largest Quaternary mammal), in size and morphology the new fossil somewhat
resembles femora of the eastern Caribbean heptaxodontine Amblyrhiza. Although firm allo-
cation is not possible, the Sheep Pen femur is possibly that of a megafaunal caviidan. The
third fossil described in this paper is the jaw of a previously unknown caviidan from a dated
end-Pleistocene cave context in Portland Ridge (Jackson’s Bay, Clarendon Parish). Xaymaca
fulvopulvis, new genus and species, differs from all West Indian caviidan species presently
known. The jaw is well preserved but retains only the incisor and premolar (the latter in a
very worn state). The few features for which the new species can be usefully analyzed and
compared to caviidan groups represented in the West Indian Cenozoic (capromyids, heterop-
somyines, heptaxodontines, and clidomyines) are largely indecisive from a systematic per-
spective. However, on balance the strongest indicators seem to lie with the ‘‘giant’’ heptaxo-
dontines of the central and eastern Caribbean (the grouping composed of Amblyrhiza, Elas-
modontomys, and possibly Quemisia), and despite its diminutive size Xaymaca is tentatively
placedwithinthatgroup.Itisincreasinglyapparentthatmuchstillremainstobelearnedabout
the origin and history of the land mammal fauna of Jamaica.
1Contribution6 tothe series‘‘Originofthe AntilleanLandMammalFauna’’.
2Division of Vertebrate Zoology (Mammalogy), American Museum of Natural History. e-mail:macphee@amnh.
org
3TheExplorersClub, 46 East70 St,NewYork10021;Associate,DivisionofVertebrateZoology(Mammalogy),
AmericanMuseumofNaturalHistory.e-mail:[email protected]
Copyright(cid:113)AmericanMuseumofNaturalHistory2003 ISSN0003-0082
2 AMERICAN MUSEUM NOVITATES NO. 3422
INTRODUCTION cause most islands have still not been ade-
quately explored paleontologically (Morgan
In this paper we describe and attempt to
and Woods, 1986; Woods, 1989a, 1989b;
interpretthree‘‘problematica’’fromtheQua-
MacPheeandIturralde-Vinent,1995;Iturralde-
ternaryofJamaica.Allthreearemoreorless
Vinent and MacPhee, 1999). This is of in-
clearly caviidan,4 but as they areincomplete-
terest because the rodent fauna of the West
ly or indifferently preserved and their mor-
Indies is now virtually defunct owing to
phologyisstrikinglyunusual,theyarebound
modern-era extinctions: the few surviving
to engender controversy. The first in the se-
lineages provide little in the way of clues to
ries—the holotype of Alterodon major (An-
their former diversity (Woods, 1989a;
thony, 1920)—already has. Spencer (1987)
MacPhee and Flemming, 1999).
has resurrected an earlier claim that Altero-
The new species, described below, is rep-
don is an octodontid sensu stricto, a group
resented by a dentally incomplete hemiman-
not otherwise representedintheinsularNeo-
dible (figs. 7–11). Determining its place on
tropics. We show, with reference to some
the caviidan cladogram has proven difficult,
new character evidence, that Spencer’s
because in the absence ofdefinitiveevidence
(1987)alternativeismuchlesslikelythanthe
any one of severalcaviidancladescouldrep-
one originally proposed by MacPhee et al.
resent its sister-group. This also applies to
(1983), which is that Alterodon is a clido-
the Sheep Pen femur; Alterodon is less of a
myine. The second fossil is a remarkably
problem if our interpretation of its morphol-
large proximal femur—clearly that of a
ogy is accepted. Although alternative place-
megafaunal mammal—which is insecurely
ments are referenced as necessary, we con-
documented as having been found at Sheep
centrate on the likeliest taxonomic home for
Pen, a locality near Windsor, Trelawney Par-
each fossil. Three groups—heteropsomyine
ish (fig. 1). If the femur is indeed that of a
echimyids (Antillean spiny rats),capromyids
rodent, it is safe to say that nothing like it
(hutias and coneys), andaprobablyparaphy-
has been found elsewhere on the island. The
leticgrouphereinformallynamed‘‘Antillean
same can be said about the last member of
platetooths’’ or *heptaxodontids5—are cen-
the series, a diminutive jaw from a cave on
tral to most of our discussions, and therefore
the Portland Ridge near Jackson’sBay(Clar-
warrant a detailed introduction.
endon Parish). These specimens and others
Heteropsomyine echimyids are usually
adduced for comparison are depicted in fig-
placed within Octodontoidea in recent clas-
ures 2–11.
sifications of extinct and extant caviidans
Caviidans are the most diversified of the
(Woods, 1989a, 1993; McKenna and Bell,
nonvolant mammal groups of the West In-
1997). They are well characterized in the lit-
dies, whether this is measured by number of
erature and require no special commentary
species, body size range, presumed ecologi-
except to note that, following Woods (1993),
cal specializations, or any other appropriate
in this paper ‘‘heteropsomyines’’ will refer
index. Despite a certain amount of specialist
only to Antillean spiny rats (Heteropsomys,
interestinAntilleancaviidansduringthepast
Brotomys, Boromys, Puertoricomys) and not
century, many basic aspects of their evolu-
to certain mainland echimyids with which
tionary history remain obscure, in part be-
they are sometimes arrayed (e.g., Proechi-
mys, various Tertiary groups; see de Paula
4Caviida (thus ‘‘caviidan’’) includes the most recent
commonancestorofOctodontoidea,Chinchilloidea,and Couto, 1979; McKenna and Bell, 1997).
Cavioidea, plus all of its descendants (Bryant and Mc-
Kenna, 1995). Erethizontoidea, not part of the caviidan 5Asterisk implies that this family-level group is not
in-group according to McKenna and Bell (1997), is alleged to be monophyletic (contrast with use of Hep-
placed in the more inclusive grouping Hystricognathi taxodontinae). McKenna and Bell (1997) included Am-
(all the foregoing plus Bathyergidae, Thryonomyidae, blyrhiza, Elasmodontomys, Clidomys, and the Patagon-
Hystricidae, Petromuridae, and several wholly extinct ian L. Miocene formsTetrastylomysandPentastylomys
higherleveltaxa).Forotherviewsfoundedonmolecular in this family. The two last-named (and very poorly
phylogenies,seeNedbaletal.(1994)andHouchonand known)taxahavealsobeenconsidereddinomyids(Mo-
Douzery (2001). The informal term ‘‘New World hys- nes, 1981). The conflict essentially disappears if dino-
tricognaths’’ is used here to refer to all post-Tertiary myidsandheptaxodontinesaresistertaxa(seetext),al-
hystricognaths,livingandextinct,fromthishemisphere. though wedo not formallyendorsethisproposalhere.
2003 MACPHEE AND FLEMMING: JAMAICAN FOSSIL RODENTS 3
TABLE1 to be each other’s closest extant relative
Late Quaternary Endemic Nonvolant Land (Woods, 1982; but see Woods, 1993; Woods
Mammals of Jamaica et al., 2001; Nedbal et al., 1994). Jamaica’s
only extant caviidan, the coney (Geocapro-
mys brownii), is a member of this group. So
are two other species that will be mentioned
inthetext,IsolobodonportoricensisandRhi-
zoplagiodontia lemkei from the central Ca-
ribbean. Their higher level systematics pre-
sent no issues in need of comment here.
Antilleanplatetoothsareadifferentmatter.
Inthepast,adiversearrayofextinctrodents,
including several mainland South American
taxa with multilamellar teeth, have been bri-
gaded under the family name *Heptaxodon-
tidaewithoutmuchconcernastowhetherthe
resulting collocation was demonstrably
monophyletic (for commentary see Woods,
1989a, 1993; Woods et al., 2001; Pascual et
al., 1990). Not infrequently, in the caviidan
systematic literature this group is simply ig-
nored as uncharacterized and perhaps un-
characterizable (e.g., Vucetich et al. [1999],
who reviewed the origin times of all main-
land caviidan families except *Heptaxodon-
tidae). The named species of West Indian
*heptaxodontids as traditionally understood
and their known distributions are: Elasmo-
dontomys obliquus (Puerto Rico); Amblyrhi-
za inundata (Anguilla/St. Martin); Quemisia
gravis (Hispaniola); and Clidomys osborni,
C. parvus, and the doubtfully distinct Alter-
odon major (all from Jamaica) (see Ray,
1964; MacPhee, 1984; MacPhee et al.,1989;
Woods, 1989a; McFarlane et al., 1998; Mor-
ganandWilkins,2003;seealsofigs.2–4and
table 1). In a few cases lower level relation-
ships are reasonably clear. Thus Amblyrhiza
inundata and Elasmodontomys obliquus
share derived features of tooth and skeletal
construction (Woods, 1989a; Flemming and
MacPhee, 1996) and may therefore be con-
Thus restricted, Antillean spiny rats are a sidered a monophyletic group for which the
morphologically homogeneous and clearly subfamily name Heptaxodontinae is valid
monophyletic group, and for comparative and appropriate, as noted by Ray (1964)(see
purposes here it will be sufficient to limit also table 1). Similarly, species of Clidomys
most observations to the Cuban genus Bo- (and Alterodon, if different) are obviously
romys. closelyrelatedtoeachotherandmaybecon-
CapromyidaeisanendemicAntilleanfam- sidered to constituteasecondsubfamily,Cli-
ily with no accepted mainland representa- domyinae (Woods, 1989a, 1993). Quemisia
tives of any geological age, although echi- has always been a problem: this very poorly
myids and capromyids are clearly related as known taxon, originally and still frequently
octodontoids and are frequently considered considered to be a heptaxodontine (Miller,
4 AMERICAN MUSEUM NOVITATES NO. 3422
Fig.1. Localitiesofspecimensdescribedintext:WallingfordRoadsideCave(holotypeofAlterodonmajor),
Sheep Pen (caviidan proximal femur), and Drum Cave (holotype of Xaymaca fulvopulvis). MacPhee (1984)
provideddetaileddescriptionsofWallingfordRoadsideCaveandSheepPen.SurveymapofDrumCave(after
Fincham, 1997) shows location of pit #2 in Brown Dust Passage, wheretypejawofXaymacawasrecovered.
2003 MACPHEE AND FLEMMING: JAMAICAN FOSSIL RODENTS 5
1929; Woods, 1989a; McFarlane et al., AMNHM Division of Vertebrate Zoology
2000), has also been identified as a possible (Mammalogy), American Museum of
capromyid (Ray, 1965). Woods (1982) con- Natural History
AMNHP Division of Paleontology (Vertebrate
templated the possibility that heptaxodonti-
Paleontology), American Museum of
nes are derived capromyids (thereby solving
Natural History
the problem of Quemisia indirectly), but ul-
BP radiocarbon years before ‘‘present’’
timately rejected this notion in favor of the
(i.e., radiocarbon datum, AD 1950)
traditional view (see also Woods, 1989a, FLMNH Florida Museum of Natural History,
1993). How heptaxodontines, clidomyines, Gainesville
andQuemisiaarerelatedinterseremainsun- SEM scanning electron microscopy/micro-
resolved. graph
Lack of resolution regarding the affinities UF SystematiccollectionsofFloridaMu-
ofAntilleanplatetoothsextendstotheirhigh- seum of Natural History, University
of Florida, Gainesville
er level relationships (cf. Kraglievich, 1926;
USNM United States National Museum of
Simpson, 1945; Wood and Patterson, 1959;
Natural History, Washington DC
Patterson and Wood, 1982; Woods, 1989a,
1993; McKenna and Bell, 1997). In themost
PRELIMINARY CONSIDERATIONS
recent chapter of this debate, Pascual et al.
(1990) have argued that the clidomyines of For convenience, odontological, histolog-
Jamaica are actually more closely related to ical, and histogenetic terms used in subse-
mainland eumegamyine dinomyids than they quent sections of this paper are briefly dis-
are to Amblyrhiza and Elasmodontomys of cussed and defined here. Several of these
the eastern Caribbean. But according to re- terms have meanings that are well under-
cent higher-level classifications, of which stood in mammalian adult anatomy, but for
McKennaandBell’s(1997)isapertinentex- the particular purposes of this paper an on-
ample, eumegamyines are best considered togenetic perspective on their definition and
cavioids,whileheptaxodontinesare(still)re- application is needed. Tooth-forming tissues
garded as octodontoids. The biogeographical are mineralized dense connective tissues;
consequence of this conclusion is important: they therefore closely resemble one another
if the two major groups of Antillean plate- in structure, histochemistry, and develop-
tooths occupy different limbs of the caviidan ment (Fawcett, 1986; Hancox, 1972; Swin-
cladogram, then there had to have been a dler, 2002). Definitions concentrate on diag-
minimumoftwoinvasionsoftheWestIndies nostic features of each tissue type that can
by progenitors which (?convergently) pos- be easily identified by microscopy or inpho-
sessed platelike molars. This last point is in- tomicrographs. They are not meant to be ex-
triguing, because the new species described haustive and in any case are limited to con-
elsewhereinthispaperexhibitssomenotable ditions encountered in the caviidans used in
similarities to heptaxodontines, yet it comes this study.
from the only island known to have sup-
ported clidomyines. TOOTH ROOTS, SECONDARY DENTINE, AND
THE SIGNIFICANCE OF APICAL HYPERTROPHY
ABBREVIATIONS In the dictionary definition, ‘‘tooth root’’
ANATOMICAL is the imbedded or basal portion of a tooth
containing the pulp chamber and its feeder
dp4 deciduous mandibular premolar
canals. Also by definition, at least in Homo,
(used only when tooth is known to
the root lies apical to a defined cervix and is
be replaced)
I/i1 maxillary/mandibular incisor normally composed of dentine only (no
M/m1, 2, 3 maxillary/mandibular molar 1, 2, 3 enamel), with a minor external wrapping of
P/p4 maxillary/mandibular premolar cementum. But in many other mammals the
imbedded portionsof teetharedifferentlyor-
INSTITUTIONAL ganized, which requires that a somewhatdif-
AAHS Anguilla Archaeological and Histori- ferent vocabulary be used to describe them.
cal Society Our analyses of root categories are derived
6 AMERICAN MUSEUM NOVITATES NO. 3422
from those adopted by Mones (1982), al- viously change as well. This is exactly what
though we explicitly concentrate on ontoge- happens in hypselodonty (q.v.), in which
ny and histology of dental tissuesratherthan teeth cease to differentiate new tissues at the
on general appearances in the adult. same rate and in the same proportion.
Teeth develop in relation to primitive ep- Although hypsodonty may be the most ef-
ithelial sheets possessing the competency to ficient way to produce ever-growing cheek-
form new dental tissues (Fawcett, 1986). teeth, under certain conditions both brachy-
Dentaltissues,likebonetissues,donotgrow odont and hypselodont teeth may add new
interstitially. In species with brachyodont material to apical ends, well after the com-
cheekteeth(e.g.,Homo),thesesheetsarepro- pletion of early ontogeny and root closure.
grammed to shut down relatively early.Thus We name this process apical hypertrophy
at a certain stage of development, amelo- (q.v.), as it involves the apical deposition of
blasts on the inner enamel epithelium at a secondary dentine at a rate well above any
given locus will cease to differentiate,there- conceivable maintenance level and (at least
by fixing the position of the cervix, and no in the cases of interest here) has the notable
new enamel will be formed thereafterexcept effect of increasing useful tooth life. This
pathologically. Odontoblasts typically con- process has not been well understood in the
tinue to differentiate and produce primary paleontological literature, often being con-
dentine in an aboral direction after eruption, fused with the hyperdevelopment of cemen-
thus ‘‘completing’’ the root, but eventually tum, a process which is at most incidentalin
production of new odontoblasts rapidly de- apicalhypertrophy.Althoughmaterialisadd-
clines. Thereafter, only minor maintenance ed to the tooth, apical hypertrophy does not
activitiestakeplace.Forexample,rootcanals normally result in tooth elongation:justasin
may be narrowed by the deposition of sec- hypsodonty, new growth at the root end is
ondary dentine, and cementum will be elab- offset by attrition at the crown end. In both
orated adjacent to the periodontal membrane instances,thephysiologicalgoalistodynam-
toensurethatteethremaintightlyseated.But icallypreserveanearlyconstanttoothlength.
from an ontogenetic standpoint, a human Preservation of tooth length is important
tooth is essentially at the end of its devel- foranother,functionalreason:socketedteeth,
opmental pathway shortly after it erupts and especially ones subjected to heavy chewing
its roots close, except, of course, for the ef- forces, need to be securely fastened to alve-
fects of attrition and, in the case of milk olar walls by means of Sharpey’s fibers de-
teeth, replacement. veloped in the periodontal membrane to pre-
This sequence should be contrasted with ventdrift,unevenwear,orevulsion(Fawcett,
thatfoundinmammalspossessinghypsodont 1986). Apical hypertrophy ensures that teeth
teeth. Hypsodont teeth are sometime de- do not become too short before the end of a
scribed as ‘‘ever-growing’’ because dental normal lifespan, because the ever-diminish-
epithelia remain competent to differentiate ing crown end is perched on a growing stub
thesametissuetypesinthesameproportions of secondary dentine. From these consider-
throughout the individual’s lifespan.Typical- ations it is predictable that apical secondary
ly although not universally, no matter what dentine will progressively form more and
therateofwear,ahypsodonttoothwillretain more of a tooth’s shaft as an animal possess-
more or less the same tooth pattern (q.v.) ing this adaptation grows older. Eventually,
throughout ontogeny (e.g., Equus). This is if the crown end becomes completely con-
understandablebecausepatternisdetermined sumed, apical dentine will appear at the oc-
by the unaltered original three-dimensional clusalsurface.Thesepreciseexpectationsare
arrangement of epithelial sheets. Another met with in several of the Antillean rodent
feature of hypsodont teeth is that the apical groups discussed in this paper.
end generally remains widely open, which
also makes sense because this is thegrowing
TERMS AND DEFINITIONS
end of the tooth where new tissues are elab-
orated.Ifthegrowingendbecomescontorted Apical end (also root or aboral end) is the
or loses competency, tooth pattern will ob- growing end of a tooth, which may be open
2003 MACPHEE AND FLEMMING: JAMAICAN FOSSIL RODENTS 7
sessing teeth thatare‘‘low-crownedandpro-
vided with well-developed roots’’ (Peyer,
1968: 191), with ‘‘abrasion or any damage
being of permanent character’’ (Mones,
1982: 110). Brachyodont teeth typically ex-
hibit a definite enamelodentine or cemento-
dentine junction (crown and root discrete).
Typically,rootscloseatarelativelyearlyon-
togenetic stage, although apical hypertrophy
(q.v.) may still occur (see text).
Cementum is ‘‘a layer of modified bone
covering the dentin of the root and neck of
a tooth’’ (Basmajian et al., 1976: 250). Sec-
ondary cementum is by definition cemen-
Fig. 2. Cheektooth morphology in Puerto Ri- tum which forms on root surfaces aftererup-
can heptaxodontine Elasmodontomys obliquus tion.Diagnostically,itcontainscementocytes
(after Anthony, 1918); despite orientation, both (maintenance cells equivalent to osteoblasts,
specimens are maxillary molars. In A, from a
characterized by possession of long cellular
younganimal,rootisopenandapicalhypertrophy
processes). Primary cementum is typically
hasnotbegun.InB,fromanagedanimal,attrition
noncellular and may lack lacunae, haversian
has substantially reduced original tooth length.
systems, and lamellae. For thisreasonitmay
However,attritionallossispartlycompensatedby
apical hypertrophy at root end, so crown appears be difficult to distinguish from secondary
to be perched on stub of secondary dentine (ar- dentine, but dentinal tubules should never be
row). present in true cementum. Most descriptions
of cementum are based on conditions in pri-
mates, which are not representative of mam-
or closed. Open roots represent the ontoge- malsgenerally.Incaviidanscementumisfre-
netically prior condition for all loci, inas- quently a component of the tooth pattern
much as the root sheath of the developing (q.v.) and therefore appears in the crownand
tooth is in broad contact with the capillary on occlusal surfaces (cf. ‘‘root cementum’’
bed of the dental papilla, the surface cells of vs. ‘‘crown cementum’’ [Peyer, 1968: 214]).
which are induced to form odontoblasts Dentine ‘‘resembles bone in structure and
(Warwick and Williams, 1973: 1227–1235). chemical composition’’ (Fawcett, 1986:
We specifically definean‘‘open’’rootasone 602), but differs atthegrosshistologicallev-
still capable of continuing longitudinal el in various ways. Primary dentine is laid
growth through odontoblast differentiation down during primordial tooth development.
and matrix mineralization. A ‘‘closed’’ root It differs diagnostically from bone and ce-
is one in which such differentiation has ter- mentum in that its matrix is punctuated by
minated or slowed to insignificance. Closure swarms of relatively parallel, evenly spaced
does not affect neurovascularbundles,which dentinaltubules(givingthedentinearadially
must remain functional to feed the pulp. striated appearance in longitudinal ground
Apical hypertropy (also root hypertro- section). The tubules are the conduits forcy-
phy; incorrectly, ‘‘cement extension’’ of toplasmic processes of odontoblasts (Tomes’
some authors) refers to significant posterup- fibers) whose cell bodies are stationed near
tion deposition of secondary dentine(q.v.)at the enamelodentine junction. Secondary
theapicalendofbrachyodontorhypselodont dentine (also ‘‘irregular’’ dentine) is
cheekteeth, evidently a physiological re- ‘‘formed after tooth eruption as a result of
sponse to increase useful tooth life (e.g., fig. irritation from caries, abrasion, injury, or
2). In the species examined, secondary den- simply due to age’’ (Basmajian et al., 1976:
tine is morphologically continuous with pri- 373). From the experimental literature it is
mary dentine at the root end and may bedif- clear that (unlike ameloblasts) lineages of
ficult to separate from it histologically. odontoblast cells remain permanently com-
Brachyodonty is the condition of pos- petent.Dean(1993)founddepositionratesof
8 AMERICAN MUSEUM NOVITATES NO. 3422
3 and 4 (cid:109)m per day in permanent teeth of sodonty and hypselodonty merely represent
Macaca, suggesting a consistent rate of den- different positions on a developmental gra-
tine formation. This, however, is a mainte- dient. In any given species with high-
nance-level activity and should be distin- crowned cheekteeth, if root closure is so de-
guished from apical hypertrophy (q.v.), a layed relative to average lifespan that it does
specialized condition. In general, in second- notnormallyoccur,thedescriptiveresultwill
ary dentine the tubules depart from straight behypsodonty.Alternatively,ifclosureisac-
lines, are less numerous per unit area, and celerated relative to lifespan, the teeth will
are distributed unevenly compared to pri- be classed as hypselodont. At the histoge-
mary dentine, although contrasts with pri- netic level, hypselodonty occurs because tis-
mary dentine are often not very marked. In sues lose their competencies.Suchteethmay
human teeth it has been observed that areas continue to grow longitudinally via apical
of irregular dentine may lack tubules alto- hypertrophy (q.v.), but the result isnolonger
gether, implying that they consist solely of the original tooth pattern but somethingelse.
fibrillar calcified matrix (see Sicher, 1971). InAntilleanheptaxodontines,pulpcavities
Also in humans, a resting line or sudden re- are relatively large and open in young ani-
orientation of tubule direction may differen- mals. Later in ontogeny, pulp cavities pro-
tiate secondary from primary dentine. The gressively narrow, owing to deposition of
degree to which this occurs in nonhumans secondary dentine, until only tortuous chan-
has not been documented. nels remain (root canals for neurovascular
Enamel‘‘ismadeupoforientedrodseach bundles).
of which consists of a stack of rodlets en- Hypsodontyistheconditionofpossessing
cased in an organic prism sheath’’ (Basma- high-crowned cheekteeth that are truly ever-
jian et al., 1976: 456). ‘‘Enamel formationis growing(i.e.,tissuecompetencydoesnotter-
initiated along the dentinoenamel junction minate, so tooth pattern is continually gen-
between the ameloblasts (enamel-forming erated in much the same way for the life of
cells) and odontoblasts (dentine-forming the animal) (also euhypsodonty of Mones
cells) when the latter commence to secrete [1982]).Althoughinoldanimalsrootclosure
predentine, which, in turn, almost immedi- of descriptively hypsodont tooth is certainly
atelystimulatestheameloblaststosecretethe possible, apical hypertrophy would not be
enamel matrix’’ (Swindler, 2002: 14). expected to occur.
Enamelodentine junction is the devel- Tooth pattern refers to the specific ar-
opmentalrestinglineofenamelformationon rangement (folds) of a tooth’s primordial tis-
the exterior of a tooth. ‘‘During the late bell sues (cementum, enamel, primary dentine),
stage, the sheet of cells comprising the inner as seen at the initiation of wear (i.e., imme-
enamel epithelium folds in a genetically de- diately after eruption). The pattern, however,
terminedwaytotakeupthedefinitiveoutline is laid down early in histogenesis. The pro-
of the future enamel-dentine junction.... duction of tooth pattern may be said to ter-
[T]his folding process determines the ulti- minate with loss of epithelial competency
mate shape of the tooth’’ (Warwick and Wil- (see above).
liams, 1973: 1230). Junction usually found
on brachyodont teeth only. STATUS OF ALTERODON MAJOR
Hypselodonty is the condition of possess-
ing high-crowned, but not ever-growing, ThedubiousnotionthatJamaicasupported
cheekteeth (also protohypsodonty of Mones an endemic octodontid during the late Qua-
[1982]). The essential contrast between this ternary may be traced to Anthony’s (1920)
condition and hypsodonty (q.v.) liesnotwith exceedingly brief description of the holotype
crown height but with ontogenetic program- of Alterodonmajor,aspeciesbasedonasin-
ming.ModernEquusisclassedaspossessing gle, apparently hypsodont cheektooth
hypsodont teeth, but, occasionally, in very (AMNHP 17638) from Wallingford Road-
old animals the root ends of the teeth finally side Cave (Manchester Parish) in central Ja-
close (Sisson and Grossman, 1938; see also maica (figs. 3, 4; see MacPhee [1984] for
Peyer, 1968: 194). This suggests that hyp- descriptionanddatingoffaunule).Inlightof
2003 MACPHEE AND FLEMMING: JAMAICAN FOSSIL RODENTS 9
Fig.3. AlterodonmajorAMNHP17638(holotype),stereopairs:(A)?distaland(B)?mesialaspects.
We argue (see text) that this tooth is broken; in the intact state it would have closely resembled cheek-
teeth of Clidomys in being composed of separate or conjoined lamellae connected by interlamellar
plaques of cementum (indicated by dense stippling in fig. 4). Spencer (1987) argued instead that the
plaque identified as feature 1 in A is all that is left of a thick layer of cementum that originally
enwrappedtheentiretooth(thusmakingthespecimen,inhisview,muchmoreoctodontid-like).Feature
2 is a plaque that does not continue to the lateral surface because it fills a space formed by conjoined
lamellae,butthisisnotunusualinClidomyscheekteeth(cf.fig.4C,E,I,J).Feature3isadevelopmental
interruption in enamel formation, also occasionally encountered in Clidomys (fig. 4B) and thus adding
to the sense of similarity. For tooth pattern see figure 4E.
the new fossils described elsewhere in this cementum lining its apparent distal aspect,
paper, it is appropriate to briefly reconsider which in our view issecureevidencethatthe
the status of Alterodon, with special refer- tooth originally possessed at least one more
ence to Spencer’s (1987) arguments in favor lamella. This would make it unlike any ac-
of an octodontid affiliation for this genus. cepted octodontid but suggestively like Cli-
Anthony (1920) was impressed bythefact domys, abundant remains of which occur in
that the holotype of Alterodon bore, in his Wallingford Roadside Cave (MacPhee,
view,acertainresemblancetothecheekteeth 1984). However, as the partial tooth pattern
of Octodontidae, which are characteristically of the Alterodon holotype did not precisely
bilobateordumbbell-shapedincross-section. match that of any teeth assigned toClidomys
However, Anthony’s remarks amount to no (cf. fig. 4), MacPhee et al. (1983) concluded
more than the mere recognition of vague thatAlterodonmightrepresentadifferentbut
similarity: in his only other published re- closely related species of clidomyine. In a
marks on the matter, Anthony (1926: 206) subsequent revision, MacPhee (1984) recon-
simply noted that Alterodon may have un- sidered this point and placed Alterodon ma-
specified ‘‘affinities to Octodontidae?’’. jor in synonymy with C. osborni, arguing
Reference to Octodontidae was enough to that despite its shape the former was just an-
get Alterodon into the biogeographical liter- other dental variant of the latter.
ature as an insular member of that family Spencer (1987) disagreed with each of
(e.g., Darlington, 1957), but not enough to these actions, arguing that MacPhee et al.
getanyoneinterestedinre-examiningtheho- (1983) misinterpreted the amount of damage
lotype in print until MacPhee et al. (1983) to the Alterodon holotype. Far from being
did so. These authors concluded that An- extensively damaged, he argued, the tooth
thony’s interpretation was fundamentally haslostnothingbutthegreaterpartofathick
flawed, because the holotype tooth of Alter- outer covering of cementum. That is, it is
odon is incomplete: AMNHP 17638 as pre- nearly complete as preserved, and therefore
served (fig. 3) has a fracture surface on the cannot have been multilamellar. Spencer
10 AMERICAN MUSEUM NOVITATES NO. 3422