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Ohio Trilobites

Ohio Trilobites



Many people have an aversion to “bugs” and other crawly southeastern , and southwestern Ohio is world renowned critters, but when it comes to an extinct “bug,” known for abundant and well-preserved trilobites and other marine as a (TRY-lo-bite), is among the most highly prized fossils. Although trilobites can be found in both and specimens. There is such a fascination with trilobites that in shale in this region, the shale beds have produced the greatest 1985 an trilobite , , was named the number of specimens. offi cial state invertebrate of Ohio (see photo on reverse The most common Ordovician trilobite is . Three side). are recognized: F. meeki, F. retrorsa, and F. granulosa. Trilobites have been found in (not exposed at the Adult specimens are about 2 inches long and are commonly surface in Ohio), Ordovician, , , Mississippian, preserved enrolled. and Pennsylvanian rocks in Ohio. Although a few trilobite spe- Isotelus, the state fossil, is common in Ordovician rocks in the cies survived until the middle of the , when they fi nally Cincinnati area; however, most specimens are fragmentary and became extinct, no Permian species are known from Ohio because complete specimens are rare. Enrolled specimens can be col- by Permian time the sea in which the trilobites lived had with- lected after they have weathered out of a shale bed, but prone drawn from the area that is now Ohio. specimens generally must be excavated from fresh, unweathered Trilobites belong to a of known as Trilobita, a shale. Prone specimens are rare because the shale holding the subdivision of the phylum Arthropoda, which includes , trilobite together quickly weathers when exposed to the elements, , , scorpions, centipedes, and spiders. The name causing the speci men to disintegrate. Fragments and complete “trilobite” is derived from the three-lobed longitudinal division of specimens also are common in limestone beds. Three species of the body (see diagram on reverse side). Trilobites lived ex clusive ly Isotelus are currently recognized in Ordovician rocks in Ohio: in marine seas from their fi rst appearance at the begin ning of I. maximus, I. brachycephalus, and I. gigas. Isotelus maximus the Cambrian Period (542 million ago) to the end of the is the most common species. However, I. brachycephalus and Permian Period (251 million years ago). The majority of trilobites I. maximus may prove to be the same species when they are crawled on or burrowed through soft, muddy, sea-fl oor sediments more thoroughly studied. All species of Isotelus are character- and obtained nutrients by ingesting mud, somewhat like an earth- ized by a low, fl at profi le, smooth cephalon and , eight worm. They are thought to have hatched from eggs, as this is thoracic segments, and a broad axial lobe. Isotelus is one of the the reproductive method employed by all living . Also largest trilobites known—some specimens are more than 18 like living arthropods, trilobites grew by shedding (molting) the inches long. hard outer body covering known as the . Fragments of tessellatus, known as the “lace collar” trilobite shed exosk el e tons of trilobites are common fossils. In fact, most be cause of a distinctive fringe on the cephalon, and Ceraurus spec imens are fragmentary. Less commonly, trilobites are found milleranus, which has a winglike cephalon, are rare trilobites in as complete specimens, either outstretched (prone) or enrolled. the oldest Ordovician rocks in the Cincinnati area. Cryptolithus It is thought that trilobites enrolled for protection, much like reached a length of about 3/4 inch; Ceraurus reached a length of modern pill bugs do. about 2 inches. Some of the more common trilobites found in Ohio rocks are dis cussed below. The term “common,” however, should not be SILURIAN TRILOBITES confused with “easy to fi nd.” In this discussion “common” refers to the fact that if one fi nds a trilobite in Ohio it is likely to be one of the species discussed below.



Silurian rocks of Ohio contain several trilobite genera. However, unlike the well-preserved Ordovician trilobites, most Silurian specimens are preserved as casts or molds in and Although no rocks of Cambrian age are exposed in Ohio, a dolomites of western Ohio. Cambrian trilobite was identifi ed in rock cored from the Cambrian , which attained a length up to 3 inches, is similar rocks underlying Warren County. The trilobite was identifi ed in appearance to Flexicalymene, of Ordovician age, but has a as Cedaria, cf. C. prolifi ca (Hansen, 1992). Species of Cedaria more semicircular cephalon and very rounded genal angles. reached a length of 2 inches. Calymene niagarensis is the most common species of this genus found in Ohio. ORDOVICIAN TRILOBITES Trimerus has a triangular cephalon, rectangular glabella, and triangular pygidium that has a small terminal spine. It reached lengths of 8 inches and is the largest Silurian trilobite in Ohio. One of the more common species, T. delphinocephalus, is particularly abundant in Silurian rocks in south-central Ohio. Bumastus has a smooth cephalon and pygidium similar to that of Isotelus; however, it has a more infl ated (thicker) profi le Flexicalymene Isotelus Cryptolithus Ceraurus than does Isotelus. Bumastus reached a length of 6 inches. An unusual aspect of this trilobite is that its length is almost equal At least 20 species of trilobites have been collected from Ordovi- to its width, giving it a nearly oval outline. Bumastus niagarensis cian rocks in Ohio. The tristate area of north-central , is the most common species of this genus in Ohio. granular texture. genal spines.Thesurfaceoftheentireexoskeletonhasa semicircular cephalon,prominentmedium-sizedeyes,andstout Ohio, only for trilobites.OftheeightMississippiangeneraknownfrom . Thisenvironmentwasnotparticularlysuitable that weredepositedoffshorefrom and ately wideborder . Thepygidiumisdistinct fromthethoraxandhasamoder- The cephalonhasstoutgenal spinesandlargecrescent-shaped spec Odontocephalus perforations alongtheanteriormarginandlonggenalspines. Each ofthesespeciescanreach alengthof1 gid Ohio. Themostcommongenusis Limestone, ofMiddleDevonianage. pleural spines. eyes andgenalspines.Theborderofthepygidiumhasnumerous reaching lengthsinexcessof8inches.Thecephalonhaslarge quarries arenowclosedtofossilcollecting. Middle DevonianageinquarriesnorthwesternOhio.These rana are lessthan2incheslong.AlmostallOhiospecimensof Sevillia bites areveryrareanddiminutive,threegenera— sea overthecoal-swampdeltas.AlthoughPennsylvanian trilo- of marinelimestoneandshalerepresentbriefincursions ofthe had periodicallynearlyexcludedtheseafromOhio.Thin beds from sedimenterodedtherisingAppalachianMountains, hence thespeci rana by thethousandsfromaquarrynearSylvania,westofToledo. the 1920’sto1970’s,specimensofthisspecieswerecollected and probablythebestknownOhiotrilobite,is and well-preservedtrilobites.ThebestknownDevoniantrilobite, Phacops Odontocephalus Mississippian rocks in Ohio are typically sandstones and shales Mississippian rocksinOhioaretypicallysandstonesandshales Ditomopyge The distinctivecephalonof Coronura By Pennsylvaniantimethegreatcoal-swampdeltas,formed Devonian rocksofOhioareworldfamousfortheirabundant i um. Theglabellarregioniscovered by D. decurtata i havebeencollectedfromshalesoftheSilicaFormation mens commonlyexceed2inchesinlength. , and The Divisionof GeologicalSurveyGeoFacts Seriesisavailableonthe World WideWeb: Paladin isthelargestDevoniantrilobitefoundinOhio, Ameura PENNSYLVANIAN TRILOBITES hasanin hasasemicirculartoparabolic cephalonandpy- MISSISSIPPIAN TRILOBITES fi c nameof Coronura isknownfromtheColumbusLimestoneand , areknownfromPennsylvanian rocksinOhio. DEVONIAN TRILOBITES Paladin fl ange. Twospeciesof canbecon —are knownfromPennsylvanianrocksin fl atedglabellaandlarge,froglikeeyes, isfoundprimarilyintheColumbus Ditomopyge reachedalengthof1 rana Paladin • ThisGeoFacts compiledbyDouglasL.Shrake •RevisedMay2005 • Odontocephalus sid , Latinforfrog.Mostspecimens Ditomopyge ered common. fl uvial depositstothe eastin

Ditomopyge, D.scitula fi netocoarsegranules. 1 hasaseriesofoval / 4 . inches. 1 / 2 Paladin inches. Ditomopyge, Phacops hasa . From . From fi ne,

Stewart, G.A.,1927,FaunaoftheSilicaShaleLucasCounty:Ohio ______1992,ExcursiontoCaesarCreekStateParkinWarren Shrake, D.L.,1990,CommontrilobitesofOhio:OhioDivisionGeologi- La Rocque,Aurèle,andMarple,M.F.,1955,Ohiofossils:Division Hansen, M.C.,1992,Ohio’soldestfossils:DivisionofGeologicalSurvey, Feldmann, R.M.,andHackathorn,Merrianne,eds.,1996,FossilsofOhio: Davis, R.A.,ed.,1985,Cincinnatifossils:anelementaryguidetothe Brezinski, D.K.,Sturgeon,M.T.,andHoare,R.D.,1989,Pennsylva- Division ofGeologicalSurveyBulletin32,76p.(Outprint.) Ohio DivisionofGeologicalSurveyGuidebook12,18p. County, Ohio:aclassicUpperOrdovicianfossil-collectinglocality: cal Survey,OhioGeology,Summer1990,p.1-5. of GeologicalSurveyBulletin54,152p.(Outprint.) Ohio Geology,Spring1992,p.6. Ohio DivisionofGeologicalSurveyBulletin70,577p. Museum ofNaturalHistoryPopularPublicationSeriesNo.10,61p. Ordovician rocksandfossilsoftheCincinnati,Ohio,region:Cincinnati In nian trilobitesofOhio:OhioDivisionGeologicalSurveyReport ves ti was namedthestateinvertebratefossil ofOhioin1985. Isotelus maximus Feldmann andHackathorn,1996). Generalized morphologicalfeaturesofatrilobite(from ga pygidium thorax cephalon tions 142,18p. pleural lobe FURTHER READING 1 cm/0.4inch isamongthelargestknowntrilobites and cranidium axial lobe pleural www.OhioGeology.com lobe glabella facial librigena segments fixigena lateral glabellarfurrow thoracic occipital ring lateral glabellarlobe genal spine genal angle cheeks