Animal skulls provide a wealthof information to the scientist. Be-sidesthe important identificationofthe species, the skull can show theanimal’s eating preferences, size,gender, brain development, health,cause of death, classification levels,and much more. Investigators ofwildlife crimes who need to matchaparticularanimalorspeciesvictimto the evidence can also run DNAtestsfromthecellsoftheskull.
What Does the Forensic Scientist Need to Know?The forensic scientist must knowthe parts of the skull and the dif-ferences among skulls of animalgroups. Using dichotomous keysandotherresourcestoexamineandmeasure,theylookfordistinguish-ing characteristics to identify thespeciesoftheskull.
How Can a Skull Be Identified?The types, shapes, and patterns ofteeth give the quickest clues abouttheowner,butiftheteetharemiss-ing,scientistscan identify theskull
from other characteristics. For in-stance,theshapeofafelineskull isalways round when viewed fromthe topcompared to the shapeofacanineskull,whichisoblong.Otherclues about the species of the skullarefoundinthesizeandpositionofthe eye sockets and nasal passage,theshapeofearbullaeortubes,andthesizeofthebraincase.Somespe-ciesareidentifiedbydistinctsuturepatternsontheskull.
Types of TeethIncisors—Locatedacrossthefrontofthemouth;used for cutting (Figure7.1).
Canines—Onecaninecanbelocatedbehindeachsideoftheincisors(fouratthemost).Theyworklikedaggersandareusedtograbandholdontoprey. Clues to what an animal eatsaregivenbythepresenceorabsenceofthecanine,aswellasitslength.
Molars and premolars—These cheekteetharelocatedbehindthecaninesandcontinuetothebackofthejaw.Theyarewideteethusedforgrind-ing,crushing,orcutting.
Teeth Patterns Tell Eating TalesHerbivores Examples of herbivores are deer, elk,bison,moose,goats,sheep,peccaries,musk ox, horses, and cattle. Theseplant-eating animals have wavy-topped cheek teeth (molars and pre-molars) to grind apart tough plantparts.Insomeherbivores,theseteethlook like a geologic cut through amountainside because they show al-ternatinglayersofhardwhiteenamelandsofterdarkerdentine.Astheani-maleats,thedentinewearsawayfast-er than theenamel to createa sharpedge good for grinding tough plantparts(Figure7.2).
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Side and top views of a mammal skull (coyote)A: Incisors B: Canines C: Molars
Mostherbivoresdonothavecanines.Exceptions are male horses, withsmallcaninesusedfordefense,andanimalsinthepigfamilywithtusks.Onegroupofherbivores, theNorthAmerican artiodactyls, are missingbothtopincisorsandcanines.Theseanimals—deer, goats, sheep, cattle,andmuskox—chewwithonlycheekteeth(peccariesaretheexception).
Gnawing HerbivoresExamplesofgnawingherbivoresareprairie dogs, beavers, porcupines,squirrels,andrabbits.Theserodentsandrabbitsarealsoplanteaters,butthey have specialized, long, curvedincisorstocracknuts,ripaparttoughplant parts, or chew through wood(Figure7.3).These teetharequicklyworn down from gnawing, so theymust grow continually throughouttheanimal’slifetime.
Theseincisorsmuststaysharptocutthrough tough plant parts. What’sthetrick?It’sallintheenamel.Theouterfaceoftheincisorshasanex-tralayerofenamelthatstrengthensand protects the teeth, but the in-nerfaceiscoveredwithsofterden-tine. When the animal gnaws, it isconstantly sharpening its teeth byshavingofflayersoftheinnerden-tinefasterthantheouterenamel.
Theotherteethofgnawingherbivoresare like those of other herbivores—wavy-topped cheek teeth for grind-ing,andnocanines.
Carnivores Examples of carnivores are the catfamily,wolves,ferrets,mink,badgers,andriverotters.Sincecarnivoreshuntandeatotheranimals,theirteetharecompletely sealed and protected byhardwhiteenamel.Carnivoreshave
Handout 7-A
Teeth Tell All
142 National Science Teachers Association142 National Science Teachers Association
longpointedcaninestograbandholdonto prey, and sharp-edged incisorstocutthroughthetoughmuscleandbody parts (Figure 7.4). The cheekteeth aredifferent sizes and shapes,withmosthavingdeepgroovesandsharp points that resemble a clusteroftinycanines.Thisshapeisbestforcrushingandcuttingprey.
OmnivoresExamples of omnivores are foxes,coyotes,raccoons,bears,andskunks.Omnivore teeth are a mix of herbi-vore and carnivore teeth since om-nivoreseatbothplantsandanimals.Theirsharp-edgedincisorsandlongcanineslooklikethoseofcarnivores,thoughthecaninesarenotassharp(Figure 7.5). The cheek teeth are a
blend of herbivore and carnivorecheek teeth—they do not have thetall, sharp points of the carnivore,butdohavemoregroovesandbluntpoints(e.g.,seehumanmolars)thantheflatterherbivore teeth.All teetharesealedinhardwhiteenamel.
Insectivores Examples of insectivores are batsandshrews.Theseanimalslookliketheyhaveamouthfulofcanines(Fig-ure7.6).All teeth (incisors, canines,and cheek teeth) are small, sharpdaggers. In bats, the incisors aresmaller than the canines. All teetharesealedwithhardenamelforpro-tectionastheinsectivorescatchandcrushhard-shelledinsects,otherar-thropods,andsmallanimals.
Insectivore teeth (shrew)
7.6
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What information can an animal skull tell about the animal?Can an animal skull be identified if all the teeth are missing?
Key to Skulls of North American MammalsThiskeyisintendedasafirststepinidentifyingskullsofsomerepresenta-tiveNorthAmericanmammals.
Thisisa“dichotomouskey”;thatis,youidentifyaspecimenbyworkingthroughthekeyandmakingaseriesof“either/or”(dichotomous)choices.Choices are arranged in “couplets,”or pairs of statements. From eachcouplet, choose the statement thatbest describes your specimen. Thiswillleadyoutothenameofamam-malorgroupofmammalsoritwilllead you to another couplet fartherdownthekey.Simplyworkthroughthestepsinsequenceuntilyouhaveatentativeidentification.
Check your tentative identificationagainst published pictures or otherdescriptions. Suggestions for fur-ther reading are provided on page154. There are excellent resources ontheWeb.Forexample, theUniversityof Michigan’s Museum of Zoologymaintains an excellent site that pro-videsphotographsofskullsofmostofthemammalslistedhere:http://animal-diversity.ummz.umich.edu/site/accounts/specimens/Mammalia.html
This key uses features of the skullsandteethonly.Thatisbecausesuchcranial and dental remains are theremains that are usually found inthefield,inowlpellets,andthelike.Some basic vocabulary is needed tousethiskey.Termsthatarelikelytobe unfamiliar are defined in paren-thesesor labeledontheaccompany-ingdiagrams.
Adentalformulaisashorthandmeth-od to indicate the number and vari-etyof teeth inaparticularmammal.Dentalformulasfrequentlyappearinkeys. Here is the dental formula forthegenusCanis:I=3/3,C=1/1,P=4/4,M=2/3,Total=42.
If a particular kind of tooth is notpresent inaspecies,azeroappearsintheformula.Forexample,rodentslackcaninesandmanyspecies(suchastheNorwayrat,Rattus norvegicus)lack premolars. The dental formulaoftheratistherefore:
1/1,0/0,0/0,3/3=16.
The creator of the “Key to Skulls of North American Mammals “ on pages 148–152 is David M. Armstrong, Department of Ecology & Evolutionary Biology and University Museum, University of Colorado-Boulder. Reprinted with permission.
24 Antlerswithprominentbranchovereye(browtine);uppercaninespresent;lengthofskullfromsocketsofincisorstoforamenmagnum(canalforspinalcord)greaterthan350mm........................ Wapiti, or Elk
Burt,W.H.,andR.P.Grossenheider.1976 Field guide to the mammals: Field marks of all North American species found north of Mexico. PetersonFieldGuideSeries.Boston:HoughtonMifflin.
Fitzgerald,J.P.,C.AMeaney,andD.M.Armstrong.1994.Mammals of Colorado.Denver,CO:DenverMuseumofNaturalHistoryandBoulder,CO:Univer-sityPressofColorado.
Glass,B.P.,andM.L.Thies.1997.A key to the skulls of North American mammals,3rded.Privatelyprinted.Seethe“classifieds”inarecentnumberoftheJournal of Mammalogyforavailability.ThisdiffersfromJonesandManning(1992)inthatillustrationsarelinedrawingsratherthanphotographs.
Hall,E.R.1946.Mammals of Nevada.Berkeley:UniversityofCaliforniaPress.Thisclassicreferencehasbeenre-issuedbytheUniversityofNevadaPress(1995);itisparticularlyvaluableforitsillustratedglossary.
Jones,J.K.,Jr.,andR.W.Manning.1992.Illustrated key to skulls of genera of North American land mammals.Lubbock,TX:TexasTechUniversityPress.
Martin,R.E.,R.H.Pine,andA.F.DeBlase.2001.A manual of mammalogy with keys to families of the world, 3rded.Dubuque,IA:W.C.Brown.
Roest,A.I.1991.A key-guide to mammal skulls and lower jaws. A nontechnical intro-duction for beginners.Eureka,CA:MadRiverPress.
4. ReadyourcommentsonskullsinyourInvestigatorNotebook(What do I know about skulls? What do I want to know about skulls?).Thenrespondtothefollowingquestion:What have I learned about skulls?
