Analysis of function in the absence of extant functional homologues: a case study using mesotheriid notoungulates (Mammalia)
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Paleobiology, 33(2), 2007, pp. 227–247
Analysis of function in the absence of extant functional
homologues: a case study using mesotheriid notoungulates
(Mammalia)
Bruce J. Shockey, Darin A. Croft, and Federico Anaya
Abstract.—We use two approaches to test hypotheses regarding function in a group of extinct mam-
mals (Family Mesotheriidae, Order Notoungulata) that lack any close extant relatives: a principle-
derived paradigm method and empirically derived analog method. Metric and discrete morpho-
logical traits of mesotheriid postcranial elements are found to be consistent with the morphology
predicted by a modified version of Hildebrand’s paradigm for scratch diggers. Ratios of in-force
to out-force lever arms based on skeletal elements indicate that the mesotheriids examined had
limbs modified for high out-forces (i.e., they were ‘‘low geared’’), consistent with the digging hy-
pothesis. Other mesotheriid characters, such as cleft ungual phalanges, a curved olecranon, and a
highly modified pelvis (with extra vertebrae incorporated into the sacrum and fusion between the
ischium and the axial skeleton) are regarded as being functionally significant for digging and also
occur in a variety of extant diggers. Analog methods indicate that mesotheriids share numerous
traits common to a variety of extant diggers. Principal component analyses of postcranial elements
indicate that mesotheriids consistently share morphometric space with larger extant fossorial
mammals: aardvark, anteaters, wombats, and badger. Likewise, discriminant function analyses cat-
egorized mesotheriids as fossorial, though imperfectly analogous to the extant diggers analyzed.
Thus, both theory-driven and empirically derived methods of estimating function in these extinct
taxa support a digging hypothesis for the mesotheriids examined. Adaptations for digging in both
the forelimb and sacropelvic functional complexes of mesotheriids provide independent support
for the fossorial hypothesis.
Bruce J. Shockey. Department of Vertebrate Paleontology, American Museum of Natural History, New York,
New York 10024. E-mail: bshockey@amnh.org
Darin A. Croft. Department of Anatomy, Case School of Medicine, Case Western Reserve University, Cleve-
land, Ohio 44106-4930. E-mail: dcroft@case.edu
Federico Anaya Daza. Facultad de Ingenierı́a Geológica, Universidad Autónoma ‘‘Tomás Frı́as,’’ Potosı́, Bo-
livia
Accepted: 27 September 2006
Introduction rived from the paradigm method. Theory-
driven and empirically derived hypotheses
Functional reconstruction of life in the past
serve as at least quasi-independent tests for
is challenging, especially for extinct taxa that
one another (Rudwick 1964). Likewise, func-
lack extant functional homologues. To be con-
fident in functional interpretations for such tional hypotheses based on metric variables
taxa, any proposed hypothesis must somehow (lengths of elements, and ratios of these
be tested. In this work, we provide an example lengths) may be compared with those based
that uses principles from functional morphol- upon discrete (presence-absence) morpholog-
ogy to predict the morphology for a given be- ical characters. When both types of data are
havioral complex and then test that prediction compatible with the hypothetical function,
by empirically derived means. In this case, a then confidence in the hypothesis is much
modified version of the ‘‘scratch digging’’ greater.
paradigm (sensu Hildebrand 1974, 1985) is Our example here is from the extinct, en-
used to predict morphological patterns in a demic, South American notoungulates (Order
mammal that habitually digs (see ‘‘Meth- Notoungulata Roth, 1904) and includes three
ods’’). Comparing morphologies of the puta- taxa in the family Mesotheriidae Alston, 1876.
tive fossorial fossil taxa and a variety of un- Mesotheriids were rabbit- to mostly sheep-
related extant mammals in order to find close sized notoungulates distinguished by their
functional analogs tests the hypothesis de- ever-growing, gliriform, chisel-like incisors,
䉷 2007 The Paleontological Society. All rights reserved. 0094-8373/07/3302-0004/$1.00228 SHOCKEY ET AL.
and hypsodont to hypselodont cheek teeth. theres with toxodonts and various other ex-
Two subfamilies are recognized, Trachytheri- tinct, endemic, South American ungulates in
inae Ameghino, 1894, and Mesotheriinae Al- his Order Notoungulata, a scheme that has
ston, 1876. Trachytheriines include basal been stable (with minor revisions of included
forms that may constitute a paraphyletic as- taxa) for a hundred years.
semblage of taxa (Reguero and Castro 2004); Various hypotheses regarding the biology
they occur in faunas referable to the late Eo- of mesotheriids have been proposed. From his
cene (Divisaderan South American Land comparison of the forelimb of Mesotherium
Mammal ‘‘Age’’ [SALMA]; but see Cerdeño et with those of beavers (Castor), river otters (Lu-
al. 2006 for an alternative interpretation of the tra), and seals (Phocidae), Serres (1867) inter-
Divisaderan trachytheriine) to late Oligocene preted Mesotherium as an aquatic animal.
(Deseadan SALMA). Mesotheriines constitute Loomis (1914) characterized the typotheres
a monophyletic group recognized by numer- (including Trachytherus) of the Deseado fauna
ous synapomorphies (see Croft et al. 2004), (Deseadan SALMA) as ‘‘running and hopping
the most conspicuous being the loss of the up- animals (p. 53)’’ and implicitly characterized
per second incisor through second upper pre- Trachytherus, by way of its ever-growing inci-
molar and lower third incisor to third pre- sors, as ‘‘gnawing the bark and eating the
molar (I2–P2/i3–p3) and the hypselodont twigs and leaves of bushes (p. 61).’’ Heidi Sy-
(ever-growing) condition of all teeth. The ear- dow (1988), noted many morphological char-
liest well-preserved mesotheriines are from acteristics of Trachytherus that are associated
the early Miocene (Santacrucian SALMA) of with fossoriality in modern mammals. She in-
northern Chile (Croft et al. 2004) and the clade terpreted Trachytherus as being a ‘‘scratch dig-
persisted until the Pleistocene (see below). ger’’ (sensu Hildebrand 1974 and 1985), an in-
The Pleistocene Mesotherium Serres 1867 terpretation tested and supported by the pres-
was the first known mesotheriid and for sev- ent study. Apparently because of the similar-
eral decades was widely known by its junior ities between mesotheriids and the semi-aquatic
synonym Typotherium Gervais, 1867 (see rodent Hydrochaeris (Family Hydrochaeridae),
Simpson 1940 for details of this somewhat Bond et al. (1995) used the capybara as a mod-
complicated taxonomic problem). For several el for Pseudotypotherium and Mesotherium and
decades, Typotherium served as the type genus suggested that these mesotheriids were ‘‘cur-
for the family Typotheriidae Lydekker, 1884 sorial y semiacuático’’ (Bond et al. 1995: p. 264)
(⫽ Mesotheriidae Alston, 1876) and it remains and probably grazers.
the namesake of the currently recognized sub- In this study, we attempt to gain some un-
order Typotheria Zittel, 1892. derstanding of mesotheriid biology. Because
mesotheriids—indeed all notoungulates—are
‘‘. . . le Mesotherium est réellement un animal extinct, and because the interordinal relation-
paradoxal . . . .’’—Serres 1867: p. 6. ships of notoungulates are unresolved, it is
Serres (1867) christened the beast Mesother- impossible to gain any useful functional in-
ium (‘‘middle beast’’) because he believed the sights by using an extant phylogenetic bracket
animal was an intermediate between two dis- (sensu Bryant and Russell 1992; Witmer 1995):
tinct orders of mammals. Noting an animal such a bracket would be so large as to be
with rodent-like incisors and an ungulate-like meaningless for our purposes. Therefore, we
body, but with a clavicle (absent in most un- rely upon a principle-to-practice approach
gulates), he suggested that Mesotherium was in known as the ‘‘paradigm method’’ (Rudwick
transition from a rodent to a ‘‘pachyderm’’ 1964; see also Gould 1970) and we test the re-
(Serres 1867: p. 7). The odd suite of characters sults via empirical methods (i.e., we compare
of Mesotherium and other typotheres inspired the morphology of the extinct taxa with extant
a variety of hypotheses regarding their phy- taxa whose behavior is known).
logenetic relationships, including being relat- For the principle-based paradigm method,
ed to prosimian primates (Ameghino 1891, we use a modified form of Hildebrand’s
1906). Ultimately, Roth (1903) united typo- scratch-digging paradigm (1974, 1985) as an aMESOTHERIID MORPHOLOGY 229
priori ‘‘prediction’’ of the morphological char- paradigm the observation that the pubic sym-
acter states to be found in an animal that ex- physis is weak or absent in most diggers. No
cavates by ‘‘scratch digging’’ (i.e., extending functional principle predicted that such
the forefeet to the substrate, breaking the sub- should occur in diggers and, indeed, its func-
strate, and pulling the material under the tion is not understood. Because the function is
body by flexion of digits and wrist, elbow ex- unknown and is not predicted from principle,
tension, and forelimb retraction). Scratch dig- we do not include it in our version of the par-
gers must be capable of directing great force adigm. Conversely, from lever mechanics we
against the substrate in order to excavate. would predict a high greater tubercle of the
Thus, their forelimbs must be able to produce humerus, because it would provide a mechan-
large out-forces (Fo ), which may be expressed ical advantage (high Li) for protracting the hu-
as: merus. We therefore include this feature in our
version of the scratch-digging paradigm even
Fo ⫽ Fi Li/Lo (1)
though it is not common among living diggers
where Fi is the in-force, Li the in-lever, and Lo (see Larson and Stern 1989 for discussion of
the out-lever. By inspection of the equation greater tubercle functions).
above, one can see that there are three ways to In this study, we examine the postcranial re-
increase Fo: (1) increase Fi by increasing the mains (and to a lesser extent, cranial remains)
cross-sectional area of the muscle(s) that act of mesotheriids of both subfamilies, Trachy-
upon the lever; (2) increase Li by increasing the theriinae and Mesotheriinae, and compare
distance between the point of muscle insertion them with the scratch-digging paradigm (de-
and the point of rotation (generally requiring tailed below). As a quasi-independent confir-
lengthening the bone acting as Li), or (3) de- mation (Rudwick 1964), we also compare me-
crease Lo by shortening the bone that acts as sotheriid morphologies with those of extant
the out-lever. All three of these approaches are scratch diggers and other fossorial animals.
seen in extant scratch diggers. For example, The scratch-digging hypothesis should be re-
the armadillo Dasypus has huge triceps brachii garded as falsified should the extinct taxa
muscles with extensive areas of origin and in- have fewer modifications of the postcranial
sertion (large Fi), a relatively long olecranon skeleton than extant scratch diggers. Of
(large Li), and a short ulnar shaft (small Lo) course, such a falsification would not imply
(Hildebrand 1985). that the animals did not dig (dogs dig, but
Evaluating such features in fossil specimens would fail this test), but rather that the fos-
can be problematic; direct measures of cross- sorial hypothesis would not be robust and that
sectional muscle areas are almost never avail- digging may not have been a specialization for
able for fossils, and osteological evidence re- the animal. To fail to falsify the hypothesis re-
garding musculature may be unreliable (Bry- quires that the fossils show a greater number
ant and Seymour 1990). Many of the muscu- of and/or more extreme adaptations for dig-
loskeletal adaptations of fossorial animals are ging than some extant diggers. This method,
so extreme, however, that they frequently of course, will generate more false negatives
leave conspicuous muscle scars, crests, and tu- than false positives, but we desire difficult
berosities on the bones, thus often rendering tests for functional hypotheses regarding ex-
general interpretations of major muscles un- tinct animals lacking close extant relatives. We
ambiguous (see examples below and Fig. 1). must be content with our ignorance about life
Also, evidence of the levers of the musculo- in the past, when appropriate, but robust
skeletal system, the bones themselves, is com- functional hypotheses need not be ignored.
monly available to vertebrate paleontologists.
In its ideal form, the paradigm method de- Materials and Methods
rives a predicted morphology from biome- Abbreviations. Institutional abbreviations
chanical principles. In some cases, however, it are as follows: CMNH, Cleveland Museum of
may be more empirically based. For example, Natural History, Ohio; FLMNH, Florida Mu-
Hildebrand (1985) included in his digging seum of Natural History, Gainesville; UF, Ver-230 SHOCKEY ET AL. FIGURE 1. Left forelimb elements of selected fossorial mammals and the generalized ambulatory marsupial Di- delphis compared with those of the mesotheriid Trachytherus. Humeri are illustrated in anterior view above, ulnae in anterior view below. Elements are from the following specimens: Didelphis virginiana (UFm 21682); Trachytherus spegazzinianus (UF 91933); Orycteropus afer (CMNH 18504); Taxidea taxus (UFm 6734); Tamandua tetradactyla (UFm 10119); Vombatus ursinus (CMNH 18946). Scale bars, 3 cm.
MESOTHERIID MORPHOLOGY 231 tebrate Paleontology Division of FLMNH, and dylar processes. Fossorial animals almost in- UFm, Mammalogy Division of the FLMNH, variably have DHW ⬎30 owing to an enlarged both of the University of Florida, Gainesville; medial epicondylar process for the attachment MNHN-Bol, Museo Nacional de Historia Nat- of enlarged wrist and digit flexor muscles. The ural, La Paz, Bolivia; MUSM, Museo de His- golden mole, Amblysomus, is an extreme ex- toria Natural, Universidad Nacional Mayor de ample, having its medial epicondylar process San Marcos, Lima, Peru. extended to such a degree that its distal width Other abbreviations are as follows: SALMA, is nearly the same as the entire humeral length South American Land Mammal ‘‘Age’’; PCA, (DHW ⫽ 98) (Hildebrand 1985). We note here principal components analysis; DFA, discrim- that high DHW values in diggers can also re- inant function analysis. Abbreviations for le- sult from relative shortening of the humerus; ver mechanics are Fo, out-force, Fi, in-force, Lo, a long humerus would be mechanically dis- out-lever, Li, in-lever, Vo, out-velocity, and Vi, advantageous, reducing Fo while increasing in-velocity. Indices (defined below) are BI, Lo. Cursors have low DHW values. brachial index; CI, crural index; DHW, distal Deltopectoral Crest Index (⌬PC) is 100 ⫻ humeral width index; ⌬PC, deltopectoral (length of the deltopectoral crest/length of crest index; MtFI, metatarsal index; OI, olec- humerus). This is examined as a means of es- ranon index. timating the relative out-force for humeral Indices. Various ratios useful for estimat- protraction. ing functional abilities were calculated from Olecranon Index (OI) is 100 ⫻ (olecranon the specimens we examined and were supple- length/ulnar shaft length); the length of the mented by published data for Mesotherium olecranon is measured from the tip of the olec- (Serres 1867) and other taxa (e.g., Coombs ranon to the midpoint of the trochlear notch 1983; Hildebrand 1985; Van Valkenburgh (sensu Hildebrand 1985 and Van Valkenburgh 1987; Garland and Janis 1993). Such indices 1987) and the ulnar shaft is measured from taken by themselves have limited value in pre- the midpoint of the trochlear notch to the dis- dicting function, but are useful when consid- tal tip of the ulna, ignoring any stylar process. ered in the context of an animal’s phylogenetic OI here is not directly comparable to that of relationships and body size (see Garland and Coombs 1983, which offers greater precision Janis 1993). Indices used in this study are ra- but does not estimate the ratio of the levers. tios expressed as percentages (i.e., ⫻100). Fossorial mammals have olecranon indices They are calculated for mesotheriids in ⬎25, usually ⬎30 (Hildebrand 1985; Van Val- Appendix 1 (http://dx.doi.org/10.1666/pbio kenburgh 1987), and at times much higher 05052.s1) and are defined as follows: (e.g., 95 in the giant armadillo, Priodontes, ac- Brachial Index (BI) is the ratio of the length cording to Hildebrand’s [1985] data). of the radius to the length of the humerus Crural Index: (CI) is 100 ⫻ (tibia length/fe- (measured from the head to the trochlea, ne- mur length). Cursorial and saltatory animals glecting the height of the greater tuberosity) tend to have elongated distal limb elements times 100 (100 ⫻ [radius length/humerus and almost invariably have CI values ⬎100, length]). Mammals specialized for digging al- whereas diggers and many climbers have CI most invariably have a radius that is shorter values ⬍100 (Hildebrand 1974). than the humerus (BI ⬍ 100; Hildebrand 1974; Metatarsal/Femur Index (MtFI) is 100 ⫻ Coombs 1983). Exceptions are few and include (metatarsal III length/femur length). Fossorial diggers that also are good runners (e.g., some animals tend to have MtFI values ⬍30, cur- rabbits [e.g., Lepus]; see Coombs 1983) and the sorial carnivores have MtFI values of 35–50, terrestrial anteater Myrmecophaga (Coombs and most cursorial ungulates—though no 1983; Taylor 1985). faster than cursorial carnivores—have ex- Distal Humerus Width (DHW) is 100 ⫻ (dis- treme MtFI values, 50–150 (Garland and Janis tal humeral width/humeral length); distal 1993). humeral width is measured over the trochlea Specimen Information. We examined the and includes the distal portions of the epicon- skeletal remains of mesotheriids from two dis-
232 SHOCKEY ET AL.
tantly related genera; one is a trachytheriine phaga), animals that excavate social insects
(Trachytherus spegazzinianus) and the other is a from a variety of substrates. We also sampled
mesotheriine (Plesiotypotherium sp.). The ma- taxa that exhibit other locomotor functions, in-
terial studied is as follows: cluding those that are known to be semiaquat-
Trachytherus spegazzinianus (late Oligocene, ic, cursorial, or generalized. The taxa consid-
Deseadan SALMA, Salla, Bolivia): UF 91933, ered (specimens and references) are listed in
skull, mandibles, and associated postcrania Appendix 1.
including left and right forelimbs, mostly Paradigm Method. We use the scratch-dig-
complete; UF 90960, cranium, left scapula, left ging paradigm of Hildebrand (1974, 1985),
humerus, left and right ulnae, left radius, left though in a modified form to exclude his em-
metacarpals (Mc) III, IV, and V, and right Mc pirically derived characters. Our version of
III and IV, several vertebrae, left innominate, the scratch-digging paradigm (1–5 below)
left and right femora and tibiae, and proximal lists the biomechanical demands of scratch
tarsus (UF 91933 and 90960 were described in digging and the morphological characters (in
Sydow 1988); UF 172437, right astragalus, left italics) related to meeting those demands. The
tibia; UF 173257, right astragalus; UF 172514, major requirement of scratch digging is that
right calcaneum; MNHN-Bol field #94-02, left the forelimb generates significant out-forces
astragalus (Shockey 1997a: Fig. 6.6a). (Fo) against the substrate. Morphological fea-
Plesiotypotherium sp., ?Pliocene, Casira, Bo- tures (sensu Bock and von Wahlert 1965) that
livia: MNHN-Bol-3724, partial skeleton in- would facilitate the generation of Fo are as-
cluding partial cranium, mandible, left hu- sessed in terms of putative functional com-
merus (in articulation with the ulna and ra- plexes or faculties (1–6). These include (1)
dius, including a radial sesamoid), partial shoulder (i.e., glenohumeral) joint for humeral
left? manus, several vertebrae, and a partial retraction and (2) shoulder joint for humeral
pelvis (including extra fused sacral [fused extension; (3) elbow joint; (4) wrist and digits.
caudal] vertebrae, partial left ilium, left pubis The earth opposes these significant out-forces,
and ischium). This specimen was collected at which in turn pass back into the body and
Casira by one of us (F.A.D.) and Pierre-An- need to be opposed by (5) reinforcing/sup-
toine Saint-André, formerly of the Muséum portive structures in the hands. As the forces
National d’Histoire Naturelle, Paris. pass through the body, the digger must be
Postcranial data for Mesotherium cristatum firmly ‘‘planted’’ in the substrate via the pos-
were obtained from the descriptions of Serres terior limbs (or, in some cases, the tail); thus,
(1867) and the plates for this work, which were there should be (6) reinforcing/supportive
published later by Gervais (1869). Other de- structures in the posterior appendicular skel-
scriptions of Tertiary notoungulates used for eton.
comparison included those of the isotemnid Some of these characters are discrete and do
Thomashuxleya (Simpson 1967); the notohippid not require quantification (i.e., they may be as-
Eurygenium pacegnum (Shockey 1997a,b); the sessed as present or absent) whereas others
leontiniid Scarrittia canquelensis (Chaffee are relative and are quantified via the mea-
1952); the toxodontids Nesodon and Adinother- sures indicated in Appendix 1 and defined
ium (Scott 1912); and the typotheres Protypo- here:
therium, Interatherium, and Hegetotherium (Sin-
1. High Fo generated through the shoulder
clair 1909).
joint for humeral retraction in order to pull
We also examined osteological specimens
substrate under the body. This high Fo may be
and reviewed the literature regarding a vari-
generated via high Fi as evidenced by:
ety of extant fossorial animals and behavior-
ally and functionally similar ‘‘tearing’’ ani- Acromion enlarged (extends to or beyond the
mals (sensu Coombs 1983), also known as glenoid) and/or spine of scapula raised to
‘‘hook and pull’’ diggers (sensu Hildebrand accommodate a well-developed acrom-
1985); this latter group includes the myrme- iodeltoid muscle;
cophagid anteaters (e.g., Tamandua, Myrmeco- Deltopectoral crests enlarged and distinctive toMESOTHERIID MORPHOLOGY 233
provide significant surface area for en-
larged deltoid and pectoralis muscles;
Posterior angle of scapula extended, forming a
distinct postscapular fossa for the origin
of an enlarged teres major. (Also increas-
es Li by way of its more distant location
from the shoulder joint.)
High Fo for humeral retraction may also be
generated through the shoulder joint by way
of an increased Li and would be evidenced by:
Deltopectoral crests positioned distally (mid-
way down the shaft or further) for inser-
tion of deltoid and pectoralis muscles far
from the shoulder joint.
2. High Fo for humeral protraction in order
to break the substrate or to inhibit (oppose)
the actions of retraction (noted above). This
high Fo may be generated by high Fi for hu-
meral protraction as evidenced by:
Scapular spine high (height of spine above
blade ⬎ dorsoventral diameter of glen-
FIGURE 2. Forelimb elements of Trachytherus spegazzin-
oid) to accommodate massive supraspi- ianus (UF 91933) of Salla, Bolivia (late Oligocene). A,
natus and infraspinatus muscles. Left humerus in anterior (left) and lateral (right) views.
B, Left manus (dorsal view). C, Left ulna with pisiform
High Fo for humeral protraction may also be in anterior (above) and medial (below) views, distal to
accomplished by way of an increased Li as in- right. (Scale bar applies to all.)
dicated by:
provide greater surface area for attach-
Greater tuberosity higher than humeral head to
ment of enlarged pronator teres and wrist
increase Li for supraspinatus.
flexors, thus generating increased Fi;
3. High Fo generated through the elbow for Olecranon inflected medially for enlarged ori-
flexion necessary for substrate removal as in- gin of wrist and digit flexors (Figs. 1, 2);
dicated by: Pisiform elongate and robust to increase Li for
the flexor carpi ulnaris (e.g., the badger,
Olecranon long (OI ⬎ 30) to provide greater
Taxidea [Hildebrand and Goslow 2001]);
moment arm (Li) for extension and great-
Metacarpals short, creating small Lo, thereby
er insertion area for triceps muscles and
increasing Fo.
other elbow extensors (anconeus, tensor
fasciae antebrachii); 5. Resistance to ground forces. As the large
Radius short (BI ⬍ 100), shorter than humer- Fo generated by the forelimb acts upon the
us, thus decreasing Lo and thereby in- substrate, the substrate will have an equal and
creasing (Fo). opposite force upon the animal. Adaptations
in response to these forces of the earth will be
4. High Fo generated through the wrist (car-
evident by:
pal), metacarpophalangeal, and interphalan-
geal joints: Ungual phalanges fissured to secure claw, nail,
High Fo for digit and wrist flexion is nec- or hoof to ungual phalanx;
essary to break and remove the substrate and Bony dorsal ‘‘stops’’ on phalanges to prevent
will be evident by: overextension of digits.
Entepicondylar process large (DHW ⬎ 30) to 6. Transfer of force through body. Forces234 SHOCKEY ET AL.
transferred to the posterior body from the categories, we acknowledge that these ani-
earth via the forelimb during excavation are mals have locomotor repertoires that may in-
opposed by: clude several types of behaviors (see ‘‘Discus-
sion’’). The multivariate analyses were execut-
Supernumerary fused sacral vertebrae (or
ed using SPSS 11.0.2 (SPSS Inc.) on an Apple
‘‘pseudosacrals’’ [e.g., fused caudal ver-
PowerBook G4 computer.
tebrae]),
Qualitative and discrete morphological fea-
Additional sacropelvic contacts to reinforce the
tures of functional significance were also not-
pelvic girdle and/or
ed and compared among the extant taxa and
‘‘Low-geared’’ hind limbs (CI ⬍ 100 and MtFI
the mesotheriids. Examples of such characters
⬍ 40)
relevant to scratch digging in mesotheriids in-
Analog Method. We tested the results of the clude bifid ungual phalanges, curved olecra-
paradigm method by comparing the mor- non, supernumerary fusion of vertebrae at the
phologies of mesotheriids (Trachytherus and sacrum, and sacroischial fusion.
Mesotherium) with those of 30 species of mam-
mals of known locomotor function including Comparative Functional Anatomy
scratch diggers, runners, frequent swimmers This section is devoted primarily to an ele-
(i.e., semiaquatic mammals), and generalists ment-by-element description of the forelimb,
(Appendix 1). The notohippid Eurygenium pa- pelvic girdle, and hind limb of Trachytherus
cegnum Shockey 1997b was also included in spegazzinianus and Plesiotypotherium sp.; these
the analyses as a putative example of an un- are compared to Serres’s description of Me-
specialized notoungulate. Nine measure- sotherium cristatum (Serres 1867) and to a va-
ments were recorded for each specimen: (1) riety of other taxa including notoungulates
humeral shaft length (head to trochlea, ne- and extant taxa whose general ecomorpholo-
glecting any extension of the greater tubercle gy is known. Metric data are provided in Ap-
beyond the head), (2) distal humeral width pendix 1.
(across and including the condyles), (3) length Scapula. UF 90960 preserves most of the
of deltopectoral crests (from the humeral head left scapula of Trachytherus spegazzinianus. The
to the point where they meet and terminate), spine is high, relatively higher than the spine
(4) olecranon length (tip to midpoint of troch- of Protypotherium (see Sinclair 1909), rising to
lear notch), (5) ulnar shaft length (midpoint of a height (22.8 mm) that exceeds the diameter
trochlear notch to distal end of ulna), (6) total of the glenoid fossa (20.6 mm). The acromion
length of radius, (7) femoral shaft length is fairly well developed, reaching a point
(head to distal end, neglecting any extension above, but not beyond, the coracoid process.
of greater trochanter), (8) tibial shaft length, Its length is much greater than that of Proty-
and (9) metatarsal (Mt) III length. potherium and the basal toxodontids Nesodon
The nine measurements described above and Adinotherium (see Scott 1912); although
were analyzed using principal components the tip is broken, it does not appear to be ex-
analysis (PCA) and discriminant function tended to the degree exhibited by many xe-
analysis (DFA). The PCA was performed in or- narthrans. There does not appear to have been
der to assess variation among the taxa exam- a metacromion process or a ventrally directed
ined and included nine appendicular vari- spinal process as in Nesodon or Adinotherium
ables. As a means of objectively assigning the (see Scott 1912). Damage to the scapular blade
fossil taxa to functional groups based on limb does not permit an assessment of the presence
bone measurements, we also performed a or absence of an extension of the posterior an-
DFA using the same nine variables; the prior gle.
probabilities of the four locomotor categories Serres (1867) described the spine of the
(generalized, fossorial, cursorial, semiaquatic) scapula of Mesotherium as being quite high
were considered equal, and the fossil taxa above the scapular body. The figure of the
were coded as unknown. Although we clas- scapula (Gervais 1869: Plate XXV.1) shows a
sified the extant taxa in discrete locomotor well-developed, single metacromion; that ofMESOTHERIID MORPHOLOGY 235
the toxodontid notoungulate Nesodon has two
metacromia (Scott 1912). There is no signifi-
cant hypertrophy of the posterior angle.
Humerus. UF 90960 and UF 91933 include
complete humeri of Trachytherus spegazzini-
anus. The most conspicuous features of the hu-
merus are its robust nature and well-defined
crests and muscle scars (Figs. 1, 2). The crests
for the deltoid and pectoralis muscles are pro-
nounced and extend two-thirds the length of
the element (Appendix 2, http://dx.doi.org/
10.1666/pbio05052.s2). A medial epicondylar
process is present and well developed. It is
perforated by a small entepicondylar foramen.
The supinator crest is also well developed and
extends proximally over one-third the length
of the shaft, such that it overlaps the distal re-
gions of the deltopectoral crests, lying caudal
to them. The olecranon fossa is fairly deep, but
apparently did not perforate the humerus, as
it appears to do in Figure 2 (Sydow [1988] re-
ported that the hole is due to breakage.) The
capitulum and trochlea are distinct, with the
capitulum being convex and the trochlear area
being quite concave. The medial ridge of the
FIGURE 3. Forelimb elements of mesotheriids. A, Right
trochlea is high, forming a well-defined area humerus of Mesotherium cristatum in anterior view
of articulation with, and buttress for, the cor- (modified from Gervais 1869). B, Left elbow joint of Ple-
onoid process of the ulna. siotypotherium sp. (MNHN-Bol-V-3724) of Casira, Boliv-
ia, in an oblique anterolateral view. C, D, Line drawings
The humeri of Plesiotypotherium archirense of left manus of Trachytherus spegazzinianus (C) and right
(see Villarroel 1974: Text and Fig. 12) and Ple- manus (shown as left) of Mesotherium (modified and re-
siotypotherium sp. of Casira, Bolivia, resemble versed from Ameghino, 1891) (D). Scale bar applies to
C and D. Abbreviations for B: hum, humerus; rad, ra-
that of Trachytherus. The most notable differ- dius; rad ses, radial sesamoid; uln, ulna. Those for C:
ences are that the humerus of Plesiotypotherium Cu, cuneiform; Lu, lunar; Mg, magnum; Sc, scaphoid;
is more gracile than the particularly robust Td, trapezoid; Tm, trapezium; Unc, unciform.
humerus of Trachytherus (UF 91933) and that
the perforation of the olecranon fossa is nat- Antebrachium. The ulnae of Trachytherus
ural in Plesiotypotherium. Serres (1867) likewise and Plesiotypotherium sp. are similar (see Figs.
described the humerus of Mesotherium as ro- 1, 2), having unreduced distal ends and a
bust (Fig. 3A), with well-defined deltoid and well-developed, long (but not excessively so)
pectoral crests extending two-thirds the olecranon (damaged in MNHN-Bol-V-3724;
length of the humeral shaft (Serres 1867: p. 743 but see Villarroel 1974: Fig. 13). The olecranon
and the unnumbered table, p. 747; Gervais of Trachytherus (UF 91933; Figs. 1, 2) curves
1869: Plate XXV.3) and as having a well-de- medially, providing a large surface area for at-
veloped epicondylar region (the DHW from tachment of carpal and digital flexors (as in
his data is 0.36). The olecranon process of the extant scratch diggers like Taxidea, Tamandua,
right humerus was perforate (Fig. 3A), but the and the subterranean pocket gopher Geomys).
left was not (Gervais 1969: Plate XXII.4). Ser- The shaft is broadly excavated on the lateral
res noted similarities between the humerus of surface, likely providing space for massive
Mesotherium and those of several sloths (My- carpal and digital extensor muscles. An ex-
lodon, Scelidotherium, and Megalonyx) and the cavation for carpal and digital flexors is pres-
beaver (Castor). ent on the proximomedial surface of the shaft,236 SHOCKEY ET AL. resulting in an I-beam cross-sectional mor- similar elbow sesamoid only in the pocket go- phology in this region. The pisiform (fused by pher Geomys pinetus (UFm 12358), though matrix to the distal ulna of UF 91933; Fig. 2C) some fossorial animals (e.g., Dasypus and is a robust element that provided insertion for Manis) have a bony articular process of the hu- the ulnar carpal flexor muscle (compare with merus (the HuRl discussed and figured by Taxidea in Hildebrand and Goslow 2001: Fig. Szalay and Schrenk [1998]) that shields the lat- 25.6). eral surface of the radius. Serres (1867) described the antebrachium of The distal radius of Trachytherus has well- Mesotherium as consisting of two distinct and developed and distinct facets for the scaphoid robust bones that allow liberal movements. He and lunate. Distinctive grooves for the ten- also described the ulna of Mesotherium as hav- dons of the extensors are also present in Trach- ing a large, curved olecranon, although that ytherus and Plesiotypotherium (see Villarroel shown by Gervais (1869: Plate XXV.5) is not as 1974), as well as in Mesotherium (Gervais 1869: curved as the olecranon of Trachytherus. The Plate XXV.4). shaft is similarly excavated, with a ‘‘gutter’’ Manus. The manus of Trachytherus (Fig. inside and out (‘‘et parcouru en dedans et en de- 2B) is remarkably similar to that of Mesother- hors par une vaste gouttière’’ p. 745). He noted ium (Fig. 3C,D; see also Ameghino 1891: Fig. that the trochlear groove is broad, allowing 10). Both are pentadactyl, but with a reduced considerable movement at the elbow joint. first digit. The remaining four digits are sub- The proximal end of the radius is sub-rect- equal in size with the axis of symmetry run- angular in T. spegazzinianus and Plesiotypo- ning between the third and fourth digits. The therium sp. (see also Villarroel 1974: Fig. 13), second metacarpal (Mc II) has the most prox- not rounded as in the Deseadan ‘‘notohippid’’ imal origin, which overlies the base of Mc III; Eurygenium pacegnum of Salla (Shockey 1997a: this, in turn, overlies the base of Mc IV, which Fig. 6.2; Shockey 1997b). Thus, we judge the overlies Mc V. Mc III is the longest of the meta- forearm of these mesotheriids to have been carpals, but extends no further distally than less capable of supination than that of E. pa- Mc IV (owing to the more proximal position cegnum. of its base). These central digits, III and IV, are While preparing Trachytherus specimen UF only slightly more robust and longer than dig- 90960, Sydow (1988) discovered a large sesa- its II and V. Ungual phalanx III is the best pre- moid in articulation with the proximal radius. served and is moderately flattened and bifur- Such an unusual element is also present and cated. fused by matrix to the proximal radius in Ple- The carpometacarpal joints deviate little siotypotherium sp. (MNHN-Bol-V-3724; Fig. from a serial articulation, with most of the in- 3B). Scott (1912: Plate 25.8) reported an elbow terlocking occurring between the metacar- sesamoid in the toxodontid notoungulate Ne- pals. The phalanges are relatively short. The sodon. It clearly articulated with the proxi- proximal phalanges have a small palmar molateral surface of the radius, leaving a dis- notch for articulation with the poorly devel- tinctive facet just proximal to the bicipital tu- oped metacarpal keels. The interphalangeal bercle of the radius in Trachytherus and Plesi- joints are simple, lacking palmar notches or otypotherium, as well as Nesodon (Scott 1912; the dorsal ‘‘stops’’ seen in many extant dig- see also Croft et al. 2004: Text and Fig. 6). Pre- gers (Hildebrand 1985). sumably, this elbow sesamoid developed in The ungual phalanges are distinctive, es- the tendon of the wrist and digit extensors, pecially (or best preserved) in Mesotherium; originating on the well-developed distolateral the distal ends are fissured as in many extant side of the humerus. The function of such a fossorial animals (e.g., moles, golden moles, sesamoid of the elbow is unknown to us, but and pangolins [see Hildebrand 1985]) and as it likely helped stabilize the joint and reduce in some Tertiary notoungulates (Homalodo- the chance of dislocation. It might also have therium [see Scott 1912]; Scarrittia [Chaffee reduced wear on the tendons themselves. 1952]; Eurygenium [Shockey 1997b]). These Among extant animals, we have observed a phalanges are blunt, like the distal phalanges
MESOTHERIID MORPHOLOGY 237
moles, marsupial moles, pocket gophers, ant-
eaters, and armadillos (Hildebrand 1985; Rose
1999). Sacroischial or other sacropelvic rein-
forcements are also found in a variety of fos-
sorial mammals, including moles, pangolins,
and armadillos (Hildebrand 1985; Rose and
Emry 1993), as well as pocket gophers. Rose
and Emry (1993) noted that wombats (Vom-
batus) and aardvarks (Orycteropus) also have
sacroischial contact, but that it is not ossified;
rather, the firm contact is maintained by
strong ligaments.
The only pelvis of Trachytherus we are aware
of is that of UF 90960. It includes all three pel-
vic bones, but the sacral region is not pre-
served. The area of attachment of the ilium to
the sacrum is preserved, but breakage of the
ischial spine precludes our determination of
whether this process reached and contacted
the axial skeleton or only supported connect-
ing ligaments.
FIGURE 4. Pelves of mesotheriids. A, Pelvis of Plesioty- Femur. The femur of Trachytherus (UF
potherium sp. (MNHN-Bol-V-3724) of Casira, Bolivia 90960) has a greater trochanter that extends
(left lateral view). B, Pelvis of Mesotherium cristatum
(dorsal view, anterior to left), adapted from Gervais proximally to about the same level as the head,
1869: Plate XXIV.9. Arrow indicates fusion of ischium to which lies at the end of an obliquely oriented
the sacral complex. Scale bar applies to both A and B. neck. The lesser and third trochanters are con-
spicuous. The center of the third trochanter
lies about one-third the length down the shaft
of Eurygenium (Shockey 1997b), not clawlike and terminates at nearly midshaft. The femur
as in Homalodotherium (Scott 1912; Coombs of Trachytherus is quite similar to that of Eur-
1983). ygenium pacegnum (Shockey 1997a: Fig. 6.4;
Sacropelvic Complex. The pelvic region of Shockey 1997b), the major difference being
Plesiotypotherium sp. of Casira is most distinc- that the shaft of Trachytherus is more dorso-
tive in that it preserves, posterior to the ilium, ventrally flattened in cross-section. The femur
five vertebrae that are solidly fused to one an- of Plesiotypotherium achirense is also similar to
other (Fig. 4A). (These five do not include the that of Trachytherus, the most notable differ-
more anterior sacral vertebrae, as they were ence being the slightly more proximal location
not preserved.) The transverse processes of of the lesser and third trochanters (see Villar-
the penultimate vertebra contact, and are sol- roel 1974: Fig. 14). The lesser and third tro-
idly fused to, the ischium. This distinctive chanters of Mesotherium (Gervais 1869: Plate
morphology is similar to that described by XXV.20) are reduced compared to those of
Serres (1867) for Mesotherium (Fig. 4B). Serres Trachytherus and Plesiotypotherium).
indicated that a total of nine vertebrae are Crus. The tibia and fibula are not fused in
fused in the sacral region of Mesotherium and any of the three mesotheriids studied. The
that the seventh is ‘‘soldered’’ to the ischium smoothness of the tibiofibular facets suggests
(‘‘on voit l’ischion, se souder d’une manière très- that movement occurred between the two
intime avec une vertèbre sacrée, la septième.’’ p. bones, implying that rotation of the upper an-
13). kle joint occurred.
Supernumerary fused vertebrae in the sa- Pes. Ameghino (1905) described and com-
cral region are found in a variety of fossorial pared the astragalus of Trachytherus (referred
mammals including talpid moles, golden to by its junior synonym, Eutrachytherus) and238 SHOCKEY ET AL.
6.6a), but is fairly short in others (Fig. 5B). The
head is subspherical and articulates within a
deep concavity of the navicular. In contrast to
the condition in many toxodonts (e.g., Eury-
genium, Nesodon, Adinotherium) and interath-
eriids, both plantar facets lie roughly in the
horizontal plane, such that the astragalus
overlies the calcaneus (Shockey and Anaya
2007).
The calcaneum of Trachytherus (Fig. 5A) has
a poorly developed, obliquely oriented fibular
facet. The ectal facet is more horizontal than
that of toxodonts and interatheriids. The lat-
eral surface of UF 172514 (Fig. 5A) has a
groove for the tendon of the peroneus longus
and a distal peroneal process that is not di-
rectly adjacent to the distal region of the pe-
roneal groove. The apex of the tuber calcanei
is rugose and none of the specimens we ex-
amined had a well-developed groove for the
FIGURE 5. Photos (above) and line drawings (below) of Achilles tendon, as occurs in most ungulates
proximal tarsals of Trachytherus spegazzinianus. A, Right and cursorial carnivores. The cuboid facet is
calcaneum (UF 172514) in lateral (left) and dorsal (right)
views; B, Right astragalus (UF 172437) in dorsal, medial, slightly concave.
plantar, and distal (clockwise from upper left). The ar- The transverse ankle joint of mesotheriids
row (plantar view) indicates the groove for the tendon (i.e., Trachytherus, Plesiotypotherium, and Me-
of the flexor hallucis longus (discussed in text). Scale
bar, 2 cm. Abbreviations: ect f, ectal (lateral) facet; fg, sotherium) is composed of a ball-and-socket ar-
flexor (flexor hallucis longus) groove; fib f, fibular facet; ticulation between the astragalus and navicu-
lat tib f, lateral tibial facet; med tib f, medial tibial facet; lar combined with a modified sliding articu-
nav f, navicular facet; pf, peroneal fossa; pp, peroneal
process of calcaneum; ppa, peroneal process of astrag- lation of the calcaneocuboid joint. This kind of
alus; sus f, sustentacular facet. transverse ankle joint permits not only exten-
sion-flexion but also a considerable degree of
supination-pronation of the pes.
that of Mesotherium (Ameghino 1905: Figs. 72– From the well-developed flexor groove of
74). He noted these were remarkably similar the astragalus of Trachytherus and Mesotherium,
to the astragalus of Orycteropus, except that Ameghino (1905) correctly predicted the pres-
the trochlear foramen was absent in the me- ence of a great toe in Mesotherium (confirmed
sotheriid specimens (Ameghino 1906). Villar- by him [Ameghino 1906]) and Trachytherus
roel (1974: Fig. 16) figured an astragalus from (confirmed by a recent discovery of a speci-
Plesiotypotherium that is also very similar to men of Trachytherus [MUSM 668] in the late
that of Trachytherus (Fig. 5) and Mesotherium. Oligocene [Deseadan] of Moquegua, Peru
The three nearly identical mesotheriid astra- [Shockey et al. 2006]).
gali are distinctive in their asymmetric troch-
lear keels; the lateral keel is much larger than Results
the medial and approaches the extreme asym-
Paradigm Analysis
metry seen in some extinct sloths (e.g., Mylo-
don, Megatherium; see Owen, 1840). The troch- All three mesotheriids (Trachytherus, Plesi-
lear groove is shallow and a separate groove otypotherium, and Mesotherium) showed dis-
is present for the passage of the tendon of the tinctive morphological characters consistent
flexor hallucis longus, visible in ventral view with fossorial habits in all six categories of our
(Fig. 5). The neck is constricted and elongated version of Hildebrand’s scratch-digging par-
in some specimens (see Shockey 1997a: Fig. adigm (see ‘‘Methods: Paradigm Method’’).MESOTHERIID MORPHOLOGY 239
The functional indices calculated for meso- sorial mammals, especially the larger diggers
theriids (see Appendix 2) were all within the such as Orycteropus, Taxidea, and Vombatus.
predicted ranges for scratch diggers. Such characteristics are qualitatively evident
Evidence for high out-forces (Fo) for humer- in the forelimb (Fig. 1) and in the manus, pel-
al retraction at the shoulder joint (Part 1 of the vis, and hind limbs. Some discrete features re-
paradigm) include the relatively high spine lated to digging that are found in mesother-
and elongated acromion of the scapula and ex- iids and various extant diggers include the fis-
ceedingly well developed humeral deltopec- sured ungual phalanges (as seen in pangolins
toral crests. The distal extension of the delto- [Manis]), elbow sesamoids (as in pocket go-
pectoral crests provides a long in-lever (Li) re- phers [Geomys]), and supernumerary ‘‘sa-
sulting in high Fo for humeral protraction (the crals’’ and fusion between the ischium and the
deltopectoral crests extend 54–71% the length sacrum (as in dasypodids, myrmecophagids,
of the humeral shaft in Trachytherus and 67% and Geomys) .
in Mesotherium). Mesotheriids, however, do not compare fa-
Evidence for high Fo for humeral protrac- vorably with agile, fast-moving extant mam-
tion (Part 2) includes the tall greater tubercle mals such as cursorial carnivores and ungu-
of the humerus and the high scapular spine. lates; they lack the elongated distal limb ele-
High Fo at the elbow joint (Part 3) is seen in ments (e.g., metapodials, ulna, radius, tibia)
the moderately elongated olecranon (OL: 37– typical of such taxa. Additionally, mesother-
46) with Fo increased by the somewhat short iids exhibit no reduction in digit numbers or
forearm (BI: 90–95). fusion of metapodials, unlike extant cursorial
High Fo for wrist and digit flexion (Part 4) perissodactyls and artiodactyls.
is evident in a modestly broad distal humerus Besides the discrete morphological charac-
(HW: 32–38), medially inflected olecranon, ters that mesotheriids share with extant dig-
and well-developed pisiform. gers, they also closely resemble modern dig-
Evidence of resistance to forces of the sub- gers in the proportions of their skeleton. This
strate acting upon the animal (Part 5) is sug- resemblance is evident in the multivariate
gested in the fissured ungual phalanges of the analyses comparing mesotheriids with vari-
forelimb. No mesotheriid, however, has the ous extant taxa of known function.
‘‘bony stops’’ seen in the phalanges of some Principal Component Analysis. The first
scratch diggers (e.g., Dasypus). principal component (PC-1) explained most of
Hind-limb support for resistance to the the variation in the data (86.69%), but there
forces of the substrate acting upon the animal was little variation among the eigenvector co-
(Part 6) is remarkable; Plesiotypotherium and efficients of the nine variables (Table 1). All
Mesotherium have supernumerary fused ‘‘sa- were positive and ranged from 0.827 to 0.987.
cral’’ vertebrae and the ischium is strongly Thus, PC-1 is heavily influenced by body size,
fused to the axial skeleton. Hind-limb larger animals having higher scores than
strength is also suggested by relatively low CI smaller ones (Fig. 6A).
and MtF values (CI ⫽ 93 in Trachytherus and Though PC-2 explained much less of the
83 in Mesotherium; MtF ⫽ 33 in Trachytherus overall variation (7.69%), there was consider-
and 25 in Mesotherium). Our digging para- able variation among the eigenvector coeffi-
digm did not predict the mobile ankle joint cients of the nine variables. The width of the
seen in these mesotheriids, but we note that distal humerus and length of the deltopectoral
this morphology is similar to that of the fos- crests had the highest eigenvector loadings
sorial aardvark. (0.456 and 0.333, respectively) and the length
of the olecranon had a modest influence on
Analog Analysis PC-2 (eigenvector coefficient ⫽ 0.164). These
Qualitative Results. The three mesotheriids three morphological variables are associated
examined (Trachytherus, Plesiotypotherium, and with forelimb strength, thus high positive
Mesotherium) have numerous morphological PC-2 scores are abstracted as high forelimb
characteristics seen in a variety of extant fos- out-forces. Conversely, the most negative240 SHOCKEY ET AL.
TABLE 1. Summary statistics of multivariate analyses. (PCA and DFA scores for individual taxa given in Appendix
3).
Summary statistics for Principal Component Analysis of variables (log transformed [ln]) used in the study.
PC-1 PC-2 PC-3
Eigenvalues 7.802 0.692 0.281
% Variance 86.688 7.693 3.124
Variables
Radius (L) 0.967 ⫺0.142 ⫺0.133
Humerus (L) 0.981 0.033 ⫺0.107
Humerus (W) 0.862 0.456 0.059
Deltopectoral (L) 0.911 0.333 ⫺0.161
Olecranon(L) 0.882 0.164 0.427
Ulna (L) 0.976 ⫺0.131 ⫺0.125
Mt III (L) 0.827 ⫺0.513 0.155
Femur (L) 0.987 0.021 ⫺0.033
Tibia (L) 0.972 ⫺0.210 ⫺0.026
Summary statistics for Discriminant Function Analysis of the nine variables used in the study.
DF-1 DF-2 DF-3
Eigenvalues 8.838 0.555 0.430
% Variance 90.0 5.6 4.4
Variables
Radius (L) ⫺0.617 ⫺1.951 0.668
Humerus (L) ⫺1.615 ⫺1.622 3.608
Humerus (W) 1.813 1.152 ⫺1.529
Deltopectoral (L) 1.503 ⫺1.897 ⫺0.577
Olecranon(L) 1.574 ⫺0.662 1.453
Ulna (L) ⫺1.597 3.378 0.609
Mt III (L) 0.146 1.356 ⫺0.367
Femur (L) 1.219 5.472 ⫺0.316
Tibia (L) ⫺2.262 ⫺4.695 ⫺3.355
FIGURE 6. Principal component analysis of nine variables for 30 extant taxa of known function, two mesotheriid
notoungulates (Trachytherus and Mesotherium) and a notohippid notoungulate (Eurygenium). A, Plot of PC-2 versus
PC-1; B, Plot of PC-3 versus PC-1. Symbols for functional groups are as follows: x, generalist; box, fossorial; triangle,
cursorial; circle, semiaquatic; and diamond, notoungulates of unknown function. (See Table 1 for summary statistics
and Appendix 3 for individual PC scores.)MESOTHERIID MORPHOLOGY 241 FIGURE 7. Discriminant function analysis of nine variables for 30 extant taxa of known function, two mesotheriid notoungulates (Trachytherus and Mesotherium) and a notohippid notoungulate (Eurygenium). A, Plot of DF-2 versus DF-1. B, Plot of DF-3 versus DF-1. Symbols for functional groups as in Fig. 6. (See Table 1 for summary statistics and Appendix 3 for individual DF scores.) PC-2 eigenvector coefficients were found extant species analyzed, 27 (87%) were clas- among distal limb elements (Mt III ⫽⫺0.513; sified correctly. Two generalists (the opos- Tibia ⫽⫺0.210; ulna ⫽⫺0.131). Thus, more sum, Didelphis, and the ‘‘false paca,’’ Dinomys) negative PC-2 scores serve as a proxy for high were misclassified as semiaquatic, and two out-velocity. All fossorial taxa had positive fossorial (both species of Marmota) and one PC-2 scores (low out-velocity) and nearly all semiaquatic species (the capybara, Hydro- cursorial taxa had negative scores. Unspecial- chaeris) were misclassified as generalists (Ap- ized and semiaquatic taxa had both positive pendix 3). Scores on Discriminant Function 1 and negative values. All three notoungulates (DF-1) clearly distinguished cursorial and fos- had positive PC-2 scores and shared morpho- sorial taxa (Fig. 7); this function accounted for metric space with fossorial taxa, though Me- 90.0% of the variation in the data and was sotherium had a higher PC-1 value (i.e., was dominated by high negative loadings for elon- larger) than any of the extant diggers (Fig. gated limb elements (especially the tibia, but 6A, Appendix 3, http://dx.doi.org/10.1666/ also the humerus and ulnar shafts) and by pbio05052.s3). positive loadings for characters associated PC-3 accounted for only 3.12% of the vari- with high out-forces (distal width of humerus, ance. Positive values were influenced most by length of deltopectoral crest, and olecranon olecranon length (eigenvector coefficient ⫽ length). Cursorial taxa thus had low scores on 0.427), and a variety of limb elements had just DF-1, whereas fossorial taxa had high DF-1 weak influence on the negative values (Fig. scores; generalists and semiaquatic taxa were 6B). Trachytherus and Mesotherium had positive intermediate. PC-3 values and shared morphometric space There was little discrimination on DF-2 with fossorial taxa, whereas the PC-3 value for (5.6% of the variation in the data) and relative Eurygenium was negative. It shared morpho- lengths of the hind limb had an important ef- space with diggers and the semi-aquatic cap- fect on this function; taxa having long tibiae ybara (Hydrochaeris). Using varimax rotation, relative to their femora scored negatively, log-transforming the data, and adding/sub- whereas the reverse occurred for those with tracting taxa/variables did little to change the relatively short tibiae. overall pattern of the PCA. The DFA classified Trachytherus and Meso- Discriminant Function Analysis. Of the 31 therium as fossorial with high posterior prob-
242 SHOCKEY ET AL.
abilities (0.980 and 0.995, respectively) but high Fo required for digging. Thus, the obser-
very low conditional probabilities (0.034 and vation of fossorial taxa having short Lo and
0.000, respectively); Eurygenium was classified long Li is consistent with fundamental prin-
as a generalist, also with a relatively high pos- ciples of lever mechanics.
terior probability (0.800) and a low condition- Although there is general concordance be-
al probability (0.162). These disparate proba- tween the real and ideal (i.e., that predicted by
bilities result from the observation that the form and that observed), there were some in-
mesotheriids were much closer to the digger stances of discordance (e.g., ‘‘misclassifica-
group centroid than to any other (posterior tions’’ in the DFA). Some lack of precision in
probability) but that they were positioned the relationship between form and functional
near the periphery of digger morphospace classification results partly from having dis-
(conditional probability). In terms of num- crete categories for continuous phenomena.
bers, the mean squared Mahalanobis distanc- Perhaps the greatest technical challenge is
es (MD)2 from the group centroid for extant when there is little morphological difference
diggers was 1.689 (ranging from 0.422 to between functional categories. For example,
3.267), that of Trachytherus was 8.688, and that the most common misclassifications in our
of Mesotherium was 36.12. This is easily seen DFA were due to morphometric similarities
graphically (Fig. 7). Unlike the PCA, the DFA between generalists and semiaquatic taxa
was not insensitive to changes in parameters; (e.g., the semiaquatic capybara was misclas-
log-transforming the data or deletion of taxa sified as a generalist and two generalists were
included resulted in some changes in classi- misclassified as semiaquatic). This appears
fication. largely to be a function of both generalists and
semiaquatic taxa having intermediate DF-1
Discussion scores. The remaining misclassifications of ex-
Integrated Analysis. A conspicuous result tant taxa were those of two species of fossorial
of the multivariate analyses is that the skeletal groundhogs (Marmota caudata and M. flaviven-
morphology associated with digging is at the tris). They were misclassified as generalists.
opposite end of a continuum with that asso- These relatively small diggers were near the
ciated with running (see also Elissamburu periphery of fossorial morphospace where
and Vizcaı́no 2004). This great difference is small fossorial taxa approached or shared
consistent with fundamentals of mechanics. morphospace with generalists. Size seems to
Whereas high out-forces (Fo) are required matter, at least in regard to digging: larger
for digging, high velocity defines running. In diggers had more extreme modifications for
terms of the lever mechanics involved, velocity digging than the smaller ones, which were
is inversely related to strength (high Fo). This more similar to generalists.
is best illustrated by comparing the equation Most important for our immediate purpos-
for Fo with the following equation regarding es, however, is that no non-fossorial taxa were
the relationship of out-velocity to levers and classified as fossorial by the DFA. Moreover,
in-velocity (see Hildebrand and Goslow 2001 all larger-bodied extant fossorial taxa were
for discussion): correctly classified as diggers, undoubtedly
owing to their distinctive morphologies.
Vo ⫽ Vi Lo /Li (2)
These distinctive morphologies are likely due
where Vo is the out-velocity (the velocity at the to the great physical requirement for digging
out-lever), Vi is the in-velocity, Li the in-lever, in large animals (see Woolnough and Steele
and Lo the out-lever. 2001 for discussion). Mesotheriid postcranial
Note that Vo is proportional to Lo /Li , where- morphology is highly reminiscent of that of
as Fo is proportional to Li /Lo. For example, large extant diggers and they are classified to-
whereas elongated Lo (e.g., relatively long gether in the DFA.
ulna, radius, metacarpals) and short Li (e.g., Morphology as a Test for Morphology? In his
relatively short olecranon) facilitate velocity, important uncoupling of the form-follows-
such a lever ratio fundamentally conflicts with function dogma, Lauder (1995: p. 13) wrote,You can also read
