Role of development in the evolution of the scapula of the giant sthenurine kangaroos (Macropodidae:...

Role of Development in the Evolution of the Scapula ofthe Giant Sthenurine Kangaroos (Macropodidae:Sthenurinae)Karen E. Sears*

Division of Developmental Biology, Department of Pediatrics, University of Colorado Health Sciences Center,Aurora, Colorado 80045

ABSTRACT Extinct giant sthenurine kangaroos pos-sessed scapulae morphologically distinct from those of allother extant and extinct adult macropodids, but qualita-tively resembling those of newborn macropodids. The sim-ilarity between adult sthenurine and neonatal macropodidscapulae suggests that a developmental process, such asheterochrony, might have been behind the evolution of theunique sthenurine scapular morphology. By incorporatingadult and ontogenetic data, this study examines the evo-lution and development of the sthenurine scapula. Thisstudy quantitatively upholds the previous qualitativemorphological observations of macropodid scapulae andfinds that ontogenetic and evolutionary morphologicalchanges are correlated in macropodids. The similarity ofscapula morphology in sthenurines and newborn macropo-dids, the correlation between ontogenetic and evolution-ary morphological change, and information from othersources (i.e., sthenurine evolutionary history) suggeststhat pedomorphic shifts in morphology, most likely due toneotenic processes, occurred within the development ofthe scapula of giant sthenurines. J. Morphol. 265:226–236, 2005. © 2005 Wiley-Liss, Inc.

KEY WORDS: heterochrony; Macropodidae; pedomorpho-sis; ontogeny; scapula; Sthenurinae

The subfamily Sthenurinae (Macropodoidea, Dip-rotodontia) is a largely or completely extinct clade ofgiant kangaroos. The sthenurines diverged fromtheir sister-taxon, the Macropodinae (other kanga-roos and wallabies), by the end of the middle Mio-cene (Flannery, 1989; Murray, 1991; Westerman etal., 2002; Prideaux, 2004), and then radiated to be-come Australia’s most abundant medium-to-largebrowsers in the Pliocene and early Pleistocene (Fig.1). However, by the end of the Pleistocene mostmembers of the clade were extinct, with only a singlepotential sthenurine genus, Lagostrophus, survivingto the present day (Tedford, 1966; Flannery, 1983,1989; Wells and Tedford, 1995; Westerman et al.,2002; Prideaux, 2004).

Extinct sthenurines are unique among kangaroosin their large body size (some species stood over 2meters), short muzzles (the group is often referred toas the “short-faced” kangaroos), and in the morphol-ogy of their postcranial skeleton (Fig. 2). Both the

fore- and hindlimbs of sthenurines are morphologi-cally specialized relative to those of other macropo-dids. For example, the sthenurine forelimb is pro-portionally different, with greatly elongated centraldigits implying considerable prehensile ability, andhindlimbs exhibiting monodactyly. Sthenurine scap-ulae also are highly dissimilar from both those ofother macropodines and all other marsupials (Fig.3), being set apart by their relatively short andbroad appearance, which is largely a result of theirlarge infraspinous fossae, and their large acromionprocesses. This distinctive scapula has been func-tionally linked to the hypothesized feeding behaviorof the extinct sthenurines, which are thought tohave been browsers that used their highly mobileforelimbs to gather food, in some cases from abovethe head, in a manner distinct from other macropo-dids (Wells and Tedford, 1995). Wells and Tedfordcite several morphological features of the sthenurinedentition, forelimb, and scapula to support this con-clusion. Within the scapula the long and well-developed acromion process and large infraspinousfossa are both potential adaptations for reachingand pulling with the forelimbs, given that they arethe origins for muscles involved in the abduction ofthe forelimb (deltoid), and in the external rotation ofthe scapula (infraspinatus), respectively. Further-more, the acromion is so long that the clavicle canpass over the coracoid process when the forelimb israised, thereby increasing the range of movement ofthe forelimbs. No other kangaroos exhibit this kindof specialization in the acromion.

Contract grant sponsor: National Science Foundation; Contractgrant number: 0104927 (doctoral dissertation improvement grant);Contract grant sponsors: Society of Integrative and Comparative Bi-ology (grant in aid of research); Women’s Board of the Field Museumof Natural History (fellowship).

*Correspondence to: Dr. Karen E. Sears, UCHSC at Fitzsimmons,Mailstop 8322, P.O. Box 6511, Aurora, CO 80045.E-mail: kesears@alumni.uchicago.edu

Published online 23 June 2005 inWiley InterScience (www.interscience.wiley.com)DOI: 10.1002/jmor.10353

JOURNAL OF MORPHOLOGY 265:226–236 (2005)

© 2005 WILEY-LISS, INC.

Although the feeding hypothesis provides a poten-tial adaptive explanation for the unique morphologyof the sthenurine scapula, it does not specify themechanism by which this evolutionary modificationoccurred. However, the morphology of the sthenu-rine scapula suggests a guiding role for developmen-tal processes in its evolution. Although distinct fromall other adult macropodids, the unique scapularmorphology of adult, extinct sthenurines does re-semble, at least superficially, the morphology of thescapulae of macropodine newborns (Fig. 3). Thescapular attributes shared among the giant sthenu-rines and the macropodine neonates include a smallsupraspinous fossa, large infraspinous fossa, andextremely long acromion process. These similaritiessuggest that the scapulae of extinct sthenurinesmay have evolved by means of the heterochronicprocess of pedomorphosis, in which adult descen-dants resemble juvenile rather than adult ancestors(Alberch et al., 1979), in a manner similar to thatproposed for human evolution (e.g., de Beer, 1930,1958; Gould, 1977).

By incorporating morphological data from ontoge-netic and adult sources, this study will quantita-tively test the primary hypothesis that the evolution

of the unique morphology of the adult giant sthenu-rine scapula is directly linked to development pro-cesses (i.e., heterochrony).

MATERIALS AND METHODSData Collection

The linear measurements used in this study are depicted inFigure 3b and are described in Table 1. Measurements wereselected to capture the shape and function of the scapula. Quan-titative data were gathered from disarticulated adult osteologicalspecimens and digital photographs of cleared and stained juve-nile specimens.

The osteological specimens examined for this study are housedat the Field Museum of Natural History, the U.S. National Mu-seum of Natural History (Smithsonian), the South AustralianMuseum, the Australian Museum, and at the University of NewSouth Wales. Measurements smaller than 150 mm were takenwith Mitutoyo (Aurora, IL) absolute digimatic calipers, thosefrom 150 mm to 30 cm with Fowler (Des Plaines, IL) verniercalipers, and those greater than 30 cm with a metric tape mea-sure. Measurements were taken three times and averaged tominimize measurement error. Adult data were obtained fromrepresentatives of 39 macropodid species (32 extant and 7 ex-tinct). Representative species from every modern macropodid ge-nus were examined. To allow broader comparisons of adult scap-ula shape, the same measurements were also obtained fromrepresentatives of every modern eutherian family and every mar-supial genus housed in the museums visited.

Fig. 1. Phylogeny of the Macropo-doidea. A composite phylogeny from sev-eral studies, including Cooke and Kear(1999), Kear (2002, pers. commun.), Kearand Cooke (2001), Kear et al. (2001a,b),Prideaux (2004), and Westerman et al.(2002). Groups included in this studyare indicated by boldface type and anasterisk.

227HETEROCHRONY IN GIANT KANGAROOS

For extinct species, the balungamayine Ganguroo bilamina,the balbarine Nambaroo sp. nov., and five giant extinct sthenu-rines, Hadronomas puckridgi, Sthenurus stirlingi, S. tindalei,Simosthenurus occidentalis, and Procoptodon goliah were exam-ined (see Fig. 1 for a phylogenetic placement of the extinct sthe-nurines). These species were selected to capture the evolution ofthe unique morphology of the sthenurine scapula. The balungam-ayines are reconstructed as having been primarily quadrupedal,although capable of bipedal locomotion at higher speeds (Kear etal., 2001a,b). Although the classically defined Balungamayinae isnow commonly accepted as paraphyletic (see Fig. 2), the subfam-ilial grouping will be used throughout this study in the mannerfollowing Prideaux (2004) for lack of a more appropriate classifi-catory term. Although the unequivocal designation of ancestrallineages is difficult among fossil taxa, both sthenurines andmacropodines most likely derived from balungamyine ancestors(Kear et al., 2001a; Kear, 2002; Prideaux, 2004). However,whether balungamyines are ancestral to, or are the immediateoutgroup of the rest of macropodids, their proximity in time to theancestral node of this group suggests that their scapular mor-

phology is at least similar to the ancestral condition, and for thisreason the term ancestral will be used to describe them. In regardto limb function, the balungamayine commonly cited as ancestralto the sthenurines, Rhizosthenurus flanneryi (see Fig. 1), hasbeen hypothesized to have used its forelimb for browsing in asimilar manner as that speculated for the sthenurines (Kear etal., 2001a; Kear, 2002). However, because scapular material hasnot yet been recovered for R. flanneryi, this taxon cannot aid inthe reconstruction of the evolution of the sthenurine shouldergirdle. Scapular material is available for a closely related bal-ungamayine, Ganguroo bilamina (Kear et al., 2001b), which hasbeen reconstructed as the sister taxon to the clade comprisingsthenurines and macropodines (Kear et al., 2001b) (see Fig. 1).The morphology of the G. bilamina scapula resembles that ofother primarily quadrupedal macropodines such as Dendrolagus,not that of the highly specialized sthenurines (pers. obs.). Withinthe Sthenurinae, H. puckridgi is traditionally viewed as basal(Fig. 1) because of its status as the oldest (the first H. puckridgifossils date to the late-Miocene) and most plesiomorphic memberof the clade (Prideaux, 2004). The cranial and postcranial mor-

Fig. 2. Skeletal comparison ofsthenurine and macropodine kanga-roos. Macropodines are representedby (a) a male Macropus giganteus,South Australian Museum specimenP22533, and sthenurines by (b) amale Sthenurus stirlingi, based onthe holotype South Australian Mu-seum specimen P22533. Scale bar �20 cm; M. giganteus and S. stirlingiare shown at the same scale. Notethe greater absolute size of the sthe-nurine relative to the macropodine,and the sthenurine’s flatter face anddivergent postcranial morphology(i.e., relatively short and broad scap-ula). Reconstructions from Wells andTedford (1995).

228 K.E. SEARS

phology and associated dietary and behavioral reconstructions ofH. puckridgi appear to be intermediate between that of the bal-ungamayines and other more derived sthenurines (Murray, 1991;Prideaux, 2004). For example, H. puckridgi did not possess theelongated and highly mobile forelimb, or the associated modifica-tions of the scapula, of the more derived sthenurines (e.g.,Sthenurus, Simosthenurus, Procoptodon). A complete list of themacropodid species examined and the specimen numbers of thefossils is provided in the Appendix.

Pedomorphosis can arise by many developmental processes,including delayed onset (postdisplacement), early offset (progen-esis), or slower rate (neoteny) of development (Gould, 1977; Al-berch et al., 1979; Klingenberg, 1998). Distinguishing amongthese processes is often difficult, and is perhaps best achieved byapplying a phylogenetic framework to the analysis of shapechange through ontogeny and over evolutionary time (Gould,

1977; Fink, 1982). Unfortunately, a strict phylogenetic approach(i.e., via ancestor–descendant comparisons) cannot be applied tothe study of the development and evolution of the scapula ofextinct giant sthenurines because embryonic material is rarelypreserved and is therefore unknown for this group. In addition,Lagostrophus fasciatus, the only potential extant member of thesthenurine clade, not only possesses an adult scapula that doesnot resemble those of extinct forms, but is endangered, therebyeliminating any chance of its use as a model for the scapulardevelopment of early sthenurines. Furthermore, the position of L.fasciatus as a sthenurine has recently been questioned (Prideaux,2004).

The closest relative of sthenurines for which ontogenetic mate-rial is readily available is the tammar wallaby, Macropus eugenii.M. eugenii, the smallest of the wallabies, is a member of theMacropodinae, the sister clade to the Sthenurinae (Flannery,

Fig. 3. Scapulae of macropodids atbirth and in adulthood. Adult giantsthenurines are represented by (a)Simosthenurus occidentalis, otheradult macropodids by (b) Thylogalebrunii and (d) Macropus agilis. Neo-natal macropodids are represented by(c) M. eugenii. A graphical depictionof the scapula variables used in thisstudy is shown in b.

229HETEROCHRONY IN GIANT KANGAROOS

1983, 1989; Westerman et al., 2002) (Fig. 1). Molecular datasuggest that the Sthenurinae and Macropodinae diverged�19–20 million years ago (Westerman et al., 2002). M. eugenii isbred in captivity and has been used extensively for studies ofreproduction and development (e.g., Renfree et al., 1995; Renfreeand Blanden, 2000). In this study, ontogenetic material from M.eugenii will be used to investigate the hypothesis that the sthe-nurine scapula evolved via the developmental process of pedo-morphosis.

A potential problem with taking this approach is that patternsof ontogenetic change in the extant macropodid taxon Macropuseugenii are not necessarily the same as those present in theextinct giant sthenurines. Encouragingly, Sears (2004) found asignificant correlation between patterns of morphologic variationthrough the ontogeny of M. eugenii and morphologic variationamong adult macropodids and sthenurines. This finding suggeststhat the ontogenetic patterns of individual taxa within theMacropodidae are indeed appropriate for use in testing the rela-tionship between development and evolution in other familymembers. Therefore, although it should be noted that they areprobably not identical, the ontogenetic series of extant macropo-dines can be used as proxies for the ontogenetic series of theirextinct and endangered sthenurine relatives.

Data from 30 juvenile Macropus eugenii specimens ranging inage from 0 days (birth) to 110 days were obtained. Specimenswere generously donated by Dr. Marilyn Renfree of the Univer-sity of Melbourne in Australia. The juvenile specimens werecleared and stained (with Alizarin Red staining bone and AlcianBlue staining cartilage, according to the protocol of Hanken andWassersug, 1981) and photographed using a Nikon 995 digitalcamera with microscope attachment connected to a dissectingmicroscope. Measurements were taken on the digitized imagesusing the computer program Scion Image 1.62c (available viahttp://www.scioncorp.com). All data were log-transformed prior toanalysis to standardize the variance of each measurement. Un-less otherwise mentioned, statistical analyses were performed inStatView 4.5 (SAS Institute, Cary, NC).

Analyses

To compare the shapes of sthenurine scapulae to those of allother macropodids, as well as to those of eutherians and all othermarsupials, a principal components analysis (PCA) was per-formed on the combined scapular data of these groups. PCA is amultivariate statistical technique that summarizes the variationpresent in a dataset on a series of orthogonal axes, or principalcomponents (PCs) (Reyment and Joreskog, 1993). The first PCsummarizes the majority of the variation in the data, and, as aresult, is commonly highly correlated with body size. The subse-quent PCs usually correspond to notable variations in shape thatare less correlated with size. Plotting the resultant second andthird PCs produced a morphospace that summarized therianscapula shape. An additional PCA was performed on the adultmacropodid (including the sthenurines) and neonatal Macropusdata (from specimens aged 0, 2, and 8 days) to graphically depictthe similarity of scapula shape in sthenurines and neonatalmacropodids. The similarity of scapula shape in newborn

macropodids and derived sthenurines, and the dissimilarity be-tween these groups and other adult macropodids, was statisti-cally tested using a series of Mann-Whitney U-tests on the PC2values of derived sthenurines (Sthenurus stirlingi, S. tindalei,Simosthenurus occidentalis, and Procoptodon goliah), intermedi-ate taxa (Ganguroo bilamina, Nambaroo sp. nov., and Hadrono-mas puckridgi), other adult macropodids, and newborn macropo-dids. The heavy loadings of several variables on PC2, as detailedin Results, indicate that it is a good proxy for general scapulashape, and therefore appropriate for use in this analysis.

Residual analysis (Sokal and Rohlf, 1995) was performed toquantitatively assess differences in the relative size of individualscapula variables between representative adult sthenurines(Sthenurus stirlingi, S. tindalei, and Simosthenurus occidentalis)and other adult macropodids. Using data from all adult macropo-dids, each individual variable was regressed on the caudal-to-cranial length of the scapula. Residuals were then calculated foreach individual scapular variable of S. stirlingi, S. tindalei, andS. occidentalis. This residual analysis method is quantitativelysimilar to examining the total multivariate distance from theoverall macropodid centroid. However, a centroid size approachwas not used here because it is likely to be sensitive to the sizedifferences between the larger sthenurines and other macropo-dids. Caudal-to-cranial length was used as a proxy for body sizefor this study because it scales consistently and evenly amongmacropodids (data not shown). Least-squares (LS) regressionanalysis was used in this situation to allow the calculation of theregression residuals (Sokal and Rohlf, 1995). A positive residualindicates that the sthenurine variable in question is larger thanexpected based on the regression calculated from all adultmacropodids, whereas a negative residual value indicates theopposite. Because scapular shape and the associated residualsare virtually identical in the three species, only the results of theS. occidentalis analyses are presented as representative of alltaxa. To further test the similarity of sthenurine and macropod-ine scapular morphology, the absolute values of the residuals ofall variables were summed for every macropodid taxon. The re-sidual sums of the derived sthenurines, intermediate taxa, andother adult macropodids were then statistically compared using aseries of Mann-Whitney U-tests. The logic behind this test is asfollows: the higher the sum of the absolute residuals, the furtherthe taxon lies from the macropodid regression line, and thereforethe greater the difference in shape of its scapula from that of theaverage macropodid.

Similarity between newborn and adult macropodid (includingsthenurines) scapula shape was assessed using PCA and Mann-Whitney U-tests, described previously, and UPGMA (unweightedpair group method with arithmetic mean) cluster analysis. Re-siduals, calculated from LS regressions of the individual scapulavariables on caudal to cranial length, were used as the raw datafor the UPGMA to reduce the effect of body size. UPGMA is amultivariate technique that, using a sequential clustering algo-rithm, iteratively joins the two most similar taxa (or groups oftaxa) until only one cluster remains (Sneath and Sokal, 1973).UPGMA results in a tree-like branching pattern that summarizesthe phenetic similarities among taxa.

TABLE 1. Scapula measurements

Measurement Measurement Description

Total Length from mid-glenoid fossa to point on outer margin nearest to end of scapular spineDorsal Length length of dorsal edge of scapula from edge of glenoid to caudal angleGlenoid Fossa Length from first point of concavity to lastAcromion Process Length from body of scapula to tipScapular Spine Length measured at connection to scapulaInfraspinous Fossa Width measured at maximum widthSupraspinous Fossa Width measured at maximum widthCaudal to Cranial Angle Length distance from caudal angle to cranial angleNeck Width at level of scapular notch

230 K.E. SEARS

An additional analysis was performed to determine if the evo-lution of scapular morphology from a balungamayine (the grouphypothesized to have been ancestral to sthenurines) to a sthenu-rine mirrors that of the change in scapula shape during macropo-did ontogeny. To calculate morphological change from balungam-ayines to sthenurines, residuals for each scapula variable (whenregressed on caudal-to-cranial length using the LS method) werecalculated for both the balungamayine Ganguroo bilamina andthe sthenurine Simosthenurus occidentalis. The residuals of thebalungamayine were then subtracted from those of the sthenu-rine. The magnitudes of the resulting values are proxies for thechange in scapula shape that occurred during sthenurine evolu-tion. To calculate morphological change during ontogeny in Mac-ropus eugenii, a measure of allometric shape change was used.The reasoning behind this metric is as follows; a scapula whosevariables are growing isometrically (i.e., scaling with size)through ontogeny does not change shape during development.However, if its variables were growing allometrically (i.e., in amanner deviating from isometry), then the scapula’s overallshape would be changing. When linear variables are scaling iso-metrically, the line formed by regressing them (logarithmically)against a proxy for size (i.e., caudal-to-cranial length) has a slopeof one (Jungers and German, 1981; Wayne, 1986; Klingenberg,1998). Therefore, an appropriate measure of a variable’s shapechange through ontogeny is the relative magnitude of the differ-ence between its slope (when regressed logarithmically againsttotal scapula length) and the slope of isometric change (in thiscase, one). When calculated by this method, more positive allom-etries indicate variables that are relatively larger than expectedby isometry, and more negative allometries variables that arerelatively smaller. The similarity between balungamayine tosthenurine shape change and allometric change in M. eugenii wasthen determined using the rho and associated P-value calculatedby a Spearman Rank Correlation Analysis (Sokal and Rohlf,1995). Spearman’s rho is a measure of the linear relationshipbetween the rank order of two variables, with a negative rhoindicating a negative relationship and a positive rho indicating apositive relationship. If the morphological change in shape frombalungamayines to sthenurines is the opposite of that incurredduring macropodid ontogeny, then the rho value for a comparisonof these sets of data should be significantly negative.

RESULTSScapular Morphology of Adult GiantSthenurines Is Quantitatively DifferentFrom That of Other Adult Macropodids

The qualitative morphological differences in scap-ular shape between adult giant sthenurines andother adult macropodids are upheld quantitativelyby the results of the principal components and re-sidual analyses.

The uniqueness of scapula shape in sthenurines,in comparison with all other marsupials, and evenmost eutherians, is highlighted by the results of theadult-only PCA (Fig. 4). On both principal compo-nents 2 and 3, the derived sthenurines Simosthenu-rus occidentalis, S. stirlingi, Sthenurus tindalei, andProcoptodon goliah group together at the margins ofthe centrally located cluster formed by the othermarsupial taxa. Acromion process length has themost significant loading on PC2; its high positiveloading indicates that specimens with more positivePC2 values, like the sthenurines, have a relativelylonger acromion process. Many variables have sig-nificant loadings on PC3: those with negative load-ings include total length, scapular spine length, and

supraspinous fossa width. These loadings indicatethat taxa with more positive PC3 values, such as thesthenurines, have shorter scapulae, scapular spines,and very short supraspinous fossae. The balungamay-ine Ganguroo bilamina, the balbarine Nambaroo sp.nov., and the basal sthenurine Hadronomas puckridgigroup with the main marsupial cluster on both prin-cipal components axes, supporting the conclusion thattheir scapulae are not as specialized as those of thederived sthenurines, and that they can be character-ized as intermediate taxa. In this analysis, PC2 andPC3 accounted for 4% and 3% of the variation in thedata, respectively. The placental taxa that cluster withthe sthenurines are the great apes that were sampledfor this analysis, specifically Homo sapiens (human)and Pongo pygmaeus (orangutan). This result is per-haps not surprising given that the scapula of humansand orangutans is also hypothesized to have evolved toprovide a wide degree of freedom for extension of thearm above the head, similar to that proposed for sthe-nurines, as a result of locomotor and functional factors(Ashton and Oxnard, 1963; Oxnard, 1963; Ashton etal., 1965; Oxnard, 1967). Most notably in terms of bothfunctionality and overall morphology, the scapulae ofhominoids and advanced sthenurines share a largeinfraspinous fossa and an extremely large and robustacromion.

The results of the residual analysis are consistentwith the qualitative observations of adult macropo-did scapulae and, to a large extent, with the resultsof the PCA. Residual results indicate that, in com-parison with the scapulae of other adult macropo-dids, sthenurine scapulae have statistically signifi-cantly longer acromions and wider infraspinous fossaeand narrower supraspinous fossae (Table 2). The con-

Fig. 4. Scapula morphospace for therian mammals (euther-ians and marsupials) generated from a PCA on adults. Key: EEutherians, ✚ Ganguroo, } Nambaroo, ✖ Hadronomas, Œ Sthenu-rus, ■ Simosthenurus, � Procoptodon, F � All other marsupials.The sthenurines, with the exception of Hadronomas, group togetherfar outside the cluster formed by all other marsupials. Ganguroo, amember of the balungamayines, Nambaroo, a balbarine, and Had-ronomas group with the main marsupial cluster.

231HETEROCHRONY IN GIANT KANGAROOS

cordance of the residual and PCA results is furthersupported by the statistical comparisons of thesummed absolute residuals of derived sthenurines, in-termediate taxa, and other adult macropodids. Thesummed residuals of derived sthenurines were signif-icantly different from those of both the intermediatetaxa (P � 0.001) and other adult macropodids (P �0.02), whereas the values of the intermediate taxa andother adult macropodids were statistically indistin-guishable (P � 0.51). This analysis provides furthersupport for the hypothesis that the scapula shape ofderived sthenurines is quantitatively different fromthat of other adult macropodids.

The residual and PCA results differ in some de-tails, but both agree that giant sthenurines differfrom other adult macropodids, most notably in theshape of their acromion processes, but also in theshape of their supraspinous fossae. Not surprisingly,results of the residual analysis are more consistentwith qualitative observations of adult macropodidscapulae than are those of the PCA. Residual anal-ysis directly measures the difference between theobserved giant sthenurine morphology and that pre-dicted by the morphology of the other adultmacropodids for every individual variable. PCAloadings, in contrast, are the products of patterns ofvariation of multiple variables, which are them-selves determined by a combination of both the sthe-nurines and other macropodid data, and as a resultare more difficult to interpret on an individualvariable-to-variable basis. Therefore, although PCAresults can be used to compare general scapulashape in adult giant sthenurines with that of otheradult macropodids and neonatal macropodids, resid-ual results should be used to more finely assess themorphological differences between the adult groups.

Scapular Morphology of Adult GiantSthenurines Is Quantitatively Similar toThose of Neonatal Macropodids

The results of the PCA performed on the combinedadult and juvenile macropodid dataset support theassertion that the scapular morphology of adult giant

sthenurines and juvenile macropodids (at least of thespecies Macropus eugenii) are similar. The second PCresulting from this analysis (Fig. 5) separates the de-rived sthenurines (Procoptodon goliah, Sthenurusstirlingi, S. tindalei, and Simosthenurus occidentalis)and neonatal M. eugenii (from 0, 2, and 4 days post-birth) from the basal sthenurine Hadronomas puck-ridgi, the balungamayine Ganguroo bilamina, the bal-barine Nambaroo sp. nov., and all other adultmacropodids examined in this study. Interestingly, thePC2 score of H. puckridgi, the basal sthenurine, placesit near the bottom of the range encompassed by mod-ern adult macropodids, and therefore closer to thederived sthenurines and juvenile macropodids thanare most other macropodid taxa. The graphically ob-served differences and similarities between the de-rived sthenurines, intermediate taxa, other adult

TABLE 2. Comparison of evolutionary and ontogenetic morphological change

Balungamayine to SimosthenurusShape Change Morphological Variable

Ontogenetic Shape Change(Macropus eugenii)

�0.251 Supraspinous Fossa Length 0.380�0.103 Spine Length 0.141�0.058 Total Length 0.175�0.056 Dorsal Length 0.213�0.001 Infraspinous Fossa Length 0.002

0.002 Glenoid Fossa Length 0.0520.030 Neck Length 0.0390.224 Acromion Length 0.023

The evolutionary morphological changes from balungamayines to sthenurines (as measured by thedifference in the shape of their scapula elements) is quantitatively opposite (Spearman’s Rho � –0.762,P � 0.04) of that observed during macropodid ontogeny (as measured by allometric change).

Fig. 5. Scapula PC2 for macropodids, including three neonatalMacropus eugenii. Key: ✚ Ganguroo, � Nambaroo, ✖ Hadronomas,Œ Sthenurus, ■ Simosthenurus, � Procoptodon, � neonatal Macro-pus eugenii (0, 2, and 8 dpc), � All other macropodoids. Thebalungamayine, the presumed sthenurine ancestor, and Hadrono-mas PC2 values fall within the range of the non-sthenurinemacropodids. The PC2 scores of the other sthenurines and neonatalmacropodids are very low, which, based on the PC2 loadings, sug-gests that their scapulae have relatively long acromion processes,and short supraspinous fossae, scapular spines, and dorsal lengths.

232 K.E. SEARS

macropodids and newborn macropodids are upheldstatistically by a series of Mann-Whitney U-tests. Theshape of the scapula (as measured by PC2) in derivedsthenurines is significantly different from that of in-termediate taxa (P � 0.01) and other adult macropo-dids (P � 0.001), but not significantly different thannewborn macropodids (P � 0.18). The shape of thescapula in “intermediate” taxa is statistically indistin-guishable (P � 0.30) from that seen in other adultmacropodids. As was the case in the adult-only PCA,acromion process length has the most significant load-ing on PC2; however, in this case its high negativeloading indicates that taxa that have low PC2 scores,such as the derived sthenurines and juvenile macropo-dids, have relatively longer acromion processes. Incontrast, supraspinous fossa length has a relativelyhigh positive loading on PC2, indicating that the low-scoring taxa on PC2 also possess relatively small su-praspinous fossae. Scapular spine length and dorsallength also have relatively high PC2 loadings, indicat-ing that these elements are relatively short in derivedsthenurines and juvenile macropodids. These resultsare consistent with those observed in the previousanalyses regarding the shape of the sthenurine scap-ula. In this analysis, PC2 comprised 3% of the totalvariation in the data.

The results of the UPGMA cluster analysis per-formed on the scapula of adult and juvenilemacropodids corroborate those of the PCA analysis.The sthenurines, with the exception of Hadronomaspuckridgi, cluster with the neonatal Macropus euge-

nii, confirming the morphological similarity of theirscapulae (Fig. 6). Hadronomas puckridgi and thebalungamayine Ganguroo bilamina cluster with theother adult macropodids and potoroids. Nambaroosp. nov., the balbarine, clusters with Hypsiprymn-odon moschatus, the musky rat-kangaroo, sepa-rately from other macropodids.

These results indicate that, despite differences inbody size, adult giant sthenurines are more similar inscapular shape to neonatal macropodines than theyare to other adult macropodids. The similarity be-tween the scapulae of adult giant sthenurines andneonatal macropodids supports the assertion thatsome type of heterochronic process (i.e., pedomorpho-sis) could have played a causative role in the evolutionof the giant sthenurine scapula. However, without anontogenetic series for extinct sthenurines or their im-mediate ancestors, any assertion made about hetero-chrony’s role in the evolution of the giant sthenurinescapula is best viewed as a hypothesis.

Evolutionary Morphological Changes FromBalungamayines (Presumed Ancestor of theSthenurines) to Sthenurines Are the Oppositeof the Ontogenetic Morphological ChangesIncurred During Macropodid Ontogeny

The differences in scapular morphology betweenthe balungamayine, Ganguroo bilamina, and thederived sthenurine, Simosthenurus occidentalis (asmeasured by the difference in shape of their scapulaelements), are negatively correlated to those in-curred during scapular development in Macropuseugenii (as measured by allometric shape change),as indicated by Spearman Rank Correlation (rho �–0.762; P � 0.04) (Table 3). Therefore, variablesthat get relatively larger during macropodid ontog-eny get relatively smaller during sthenurine evolu-tion, and vice versa. This result, taken together withthe morphological similarity between adult derivedsthenurines and newborn macropodids, suggests

Fig. 6. UPGMA cluster analysis based on the scapular mor-phology of macropodids, potorids, sthenurines, and neonatal Mac-ropus eugenii. The sthenurines, with the exception of Hadrono-mas, cluster together with the neonatal Macropus, furthersupporting their morphological similarity. The balungamayinesand Hadronomas cluster with most of the Macropodidae, indicat-ing that their scapulae do not show the extreme morphologicalspecializations of most sthenurines.

TABLE 3. Simosthenurus residuals (from a regression linegenerated from all adult macropodids on total length)

Variable

Sthenurus Residuals(from all adult macropodoid

regression line)

Acromion Process Length 0.318*Infraspinous Fossa Width 0.085*Neck Width 0.087Scapular Spine Length 0.080Total Length 0.079Glenoid Width 0.067Dorsal Length 0.006Supraspinous Fossa Width �0.209*

Positive residuals indicate that the scapula variable in question islarger than expected in Simosthenurus based on other adultmacropodoids, negative residuals indicate the opposite. Residualsindicated by an asterisk (*) are those that fall outside the 95%confidence interval for all macropodids.

233HETEROCHRONY IN GIANT KANGAROOS

that the morphological evolution of the specializedsthenurine scapula occurred along the pathway cir-cumscribed by ontogenetic change, albeit in the op-posite direction, until the adult sthenurine scapulaclosely resembled that of the newborn macropodids.In doing so, this test further supports the assertionthat the specialized morphology of the derived sthe-nurine scapula arose via the heterochronic processof pedomorphosis.

DISCUSSION

Sthenurines form a distinct kangaroo clade thatdiverged from the lineage leading to today’s abun-dant macropodid subfamily, the Macropodinae, be-fore the Middle Pliocene (Murray, 1995). Using acombination of adult and juvenile data, this studytested the hypothesis that the evolution of theunique morphology of the adult giant sthenurinescapula was directly linked to development pro-cesses (i.e., heterochrony).

The putative ancestors of the sthenurines andmodern macropodines, the paraphyletic balungam-ayines, were examined in this study and found tohave scapulae that resemble those of modern adultmacropodines. The scapulae of the earliest-appearing, and most basal member of the sthenuri-nae, Hadronomas puckridgi, are also similar tothose of modern adult macropodines. In contrast,the scapulae of the more derived sthenurines exam-ined in this study (Sthenurus stirlingi, S. tindalei,Simosthenurus occidentalis, and Procoptodon go-liah) are not similar in shape to those of modernadult macropodines, and are instead extremely sim-ilar to those of newborn macropodines. Further-more, the change in the scapular morphology frombalungamayines to sthenurines was shown to benegatively correlated with that observed duringmacropodid ontogeny.

Taken together, these results suggest that derivedsthenurines underwent a pedomorphic shift in thetiming of scapular development shortly after thedivergence of Hadronomas puckridgi in the LateMiocene. However, without a sthenurine ontoge-netic series it is impossible to determine exactlywhich pedomorphic process or processes (i.e., de-layed onset [post-displacement], early offset [pro-genesis], or slower rate [neoteny]) might underliethe evolution of the adult sthenurine scapula. Nev-ertheless, given the results of this study, and theevolutionary history and morphology of sthenurines,it is possible to hypothesize about the mechanismsbehind the evolution of the giant sthenurine scap-ula.

Gould (1977) introduced a clock model for hetero-chrony that uses the relationship between size andshape to distinguish between two of the pedomor-phic processes: neoteny and progenesis. According tothe model, morphological features that are pedomor-phic in shape and similar in size relative to the

ancestral state evolved via neoteny, whereas fea-tures that are pedomorphic in shape and relativelysmall in size evolved via progenesis. Although Gould(1977) did not directly consider the condition of pe-domorphic shape and relatively larger size in theconstruction of his clock model, he analyzed a testcase in which just such a state occurs, that of humanevolution, and determined that several aspects ofhuman morphology (e.g., “flat” face, high relativebrain weight, etc.) arose via the pedomorphic pro-cess of neoteny. Gould’s conclusions concerning hu-man evolution were consistent with the work of ear-lier authors (e.g., Bolk, 1926; de Beer, 1930, 1958),but some have been challenged more recently (e.g.,Dean and Wood, 1984; Shea, 1989; McKinney andMcNamara, 1991; Wood, 1996; McKinney, 1997).

Adult giant sthenurines are larger than their sis-ter group (the balungamayines) but several aspectsof their morphology (i.e., scapular shape and faceorthognathy) resemble those of the juveniles of theirclosest relatives for which ontogenetic material isreadily available (Macropus eugenii). Therefore, it ispossible to hypothesize that neotenic shifts in mor-phology have also occurred within, at least, the de-velopment of the scapulae and possibly also the faceof giant sthenurines.

Unfortunately, Gould’s model does not provide amechanism for testing between postdisplacement(delayed onset of development) and the other pedo-morphic patterns. However, the marsupial mode ofreproduction, and that of macropodines in particu-lar, makes postdisplacement an unlikely mechanismfor evolutionary change in the sthenurine scapula.Like most marsupials, macropodines give birth afterextremely short gestation times to immature neo-nates that must immediately crawl from the birthcanal to the teat, a distance �10 times their bodylength (Beeck, 1955; Gemmell et al., 2002). Neonatalmacropodines are able to complete this crawl, inpart, because of the great precociality and highlyspecialized morphology of their scapula (Walker andRose, 1981; Klima, 1987; Sanchez-Villagra andMaier, 2003). In fact, Sears (2004) found that themacropodine scapula is so critical to the crawl thatits morphology, along with that of all other marsu-pials that complete a crawl, has been developmen-tally and evolutionarily constrained by it. The factthat the macropodine scapula is highly constrainedto be sufficiently developed at birth to provide thenecessary structural support for the crawl makes ithighly unlikely that the onset of its developmentcould be delayed, as would be the case if postdis-placement was the driving mechanism behind thepedomorphic patterns documented in advancedsthenurines. The scenario required for neoteny, inwhich the rate of development of the sthenurinescapula slows after birth, is much more compatiblewith the mode of macropodine reproduction and de-velopment. Therefore, taken together, the results ofthis study and the reproductive biology of marsupi-

234 K.E. SEARS

als suggest that the unique morphology of the sthe-nurine scapula, an adaptation for a browsing life-style, likely evolved by means of changes in thetiming of its development, specifically, a slowing ofits developmental rate after birth (i.e., neoteny).

ACKNOWLEDGMENTS

I thank the following institutions for access tospecimens: Zoology Department of the Field Mu-seum of Natural History, U.S. National Museum ofNatural History (Smithsonian), South AustralianMuseum, the Australian Museum, and the Univer-sity of New South Wales. I especially thank M. Ren-free (University of Melbourne, Australia) for hergenerous donation of juvenile Macropus eugenii. B.Kear, J. Hopson, J. Marcot, N. Shubin, and twoanonymous reviewers critically reviewed the manu-script, and I greatly appreciate their assistance.

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235HETEROCHRONY IN GIANT KANGAROOS

APPENDIX. Species list

Family Subfamily Genus Species Location Specimen # Source

Potoroidae Potorinae Potorous tridactylus multiple specimensBalbaridae Balbarinae Nambaroo sp. nov. NSW QMF 34532 specimenPotoroidae Potorinae Bettongia penicillata multiple specimensHypsiprymnodontidae Hypsiprymnodontinae Hypsiprymnodon moschatus multiple specimensMacropodidae Macropodinae Aepyprymnus rufescens multiple specimensMacropodidae Macropodinae Dendrolagus goodfellowi multiple specimensMacropodidae Macropodinae Dendrolagus inustus multiple specimensMacropodidae Macropodinae Dendrolagus matschiei multiple specimensMacropodidae Macropodinae Dorcopsis luctuosa multiple specimensMacropodidae Macropodinae Dorcopsulus vanheurni multiple specimensMacropodidae Macropodinae Lagorchestes hirsutus multiple specimensMacropodidae Macropodinae Macropus agilis multiple specimensMacropodidae Macropodinae Macropus antilopinus multiple specimensMacropodidae Macropodinae Macropus dorsalis multiple specimensMacropodidae Macropodinae Macropus fuliginosus multiple specimensMacropodidae Macropodinae Macropus giganteus multiple specimensMacropodidae Macropodinae Macropus parryi multiple specimensMacropodidae Macropodinae Macropus parma multiple specimensMacropodidae Macropodinae Macropus robustus multiple specimensMacropodidae Macropodinae Macropus rufogriseus multiple specimensMacropodidae Macropodinae Macropus rufus multiple specimensMacropodidae Macropodinae Onychogalea unguifer multiple specimensMacropodidae Macropodinae Petrogale brachyotis multiple specimensMacropodidae Macropodinae Setonix brachyurus multiple specimensMacropodidae Macropodinae Thylogale brunii multiple specimensMacropodidae Macropodinae Thylogale billardierii multiple specimensMacropodidae Macropodinae Thylogale stigmatica multiple specimensMacropodidae Macropodinae Wallabia bicolor multiple specimensMacropodidae “Balungamayinae” Gangaroo bilamina NSW QMF 40114 specimenMacropodidae Sthenurinae Hadronomas puckridgi SGM SGM P894 specimenMacropodidae Sthenurinae Procoptodon goliah UCMP UCMP 45475 Tedford (1967)Macropodidae Sthenurinae Simosthenurus occidentalis SAM P17476 specimenMacropodidae Sthenurinae Sthenurus stirlingi SAM SAM P22533 Wells and Tedford (1995)Macropodidae Sthenurinae Sthenurus tindalei AMNH AMNH 117499 Wells and Tedford (1995)

The specimen numbers for fossil specimens are indicated. AM � Australian Museum, AMNH � American Museum of Natural History,QMF � Queensland Museum, NSW � University of New South Wales, SAM � South Australian Museum, SGM � Museum of CentralAustralia, UCMP � University of California Museum of Paleontology.

236 K.E. SEARS