No. 24 OHIOGeoFacts DEPARTMENT OF NATURAL RESOURCES • DIVISION OF GEOLOGICAL SURVEY
Many people associate paleontology (the study of ancient life) as index fossils (fossils that are diagnostic of a certain age or rock with dinosaurs and other large and spectacular fossils. Although unit). Microfossils can be used for correlations within and between dinosaurs get a lot of attention, and deservedly so, there are many continents in order to show whether or not a body of rock was ﬁ elds of paleontological research that are virtually unknown to the deposited at the same time in different regions. Such information general public but of great importance to geologists. One such ﬁ eld allows reconstructions of the history of the Earth. Rocks that lack is micropaleontology, the study of microscopic fossils, or microfos- larger fossils may, and commonly do, have abundant microfossils. sils, which are small, fossilized remains that can best be studied The microfossils may be the only tool available for determining the using a microscope. Seemingly lifeless pieces of rocks can contain environmental origin and age of a rock. Microfossils are particu- thousands of entombed miniature fossils that are very important larly useful in examining drill cores, which have obvious sample for the understanding of our changing planet. size limitations. Microfossils include a wide variety of minute organisms, including The composition of the microfossil also may have advantages. single-celled plants and animals, small pieces of larger organisms Some groups, such as conodonts (see discussion below), show such as teeth and scales, and pollen and spores. The juvenile forms variations in color when exposed to different temperatures and of many larger animals also may be preserved as microfossils. can be used as a “paleothermometer.” Because oil and gas are The sedimentary rocks of Ohio contain an abundance of microfos- formed within a speciﬁ c temperature range, it is of great impor- sils. These hidden treasures are not only fascinating and beautiful tance to know to what degree a rock has been heated during its but also can be used to solve a wide range of geological problems. geologic history in order to determine whether or not it may be Because most microfossils require good laboratory facilities to be a source rock, and, as such, if it is worth exploring further. Thus, extracted from the rocks and specialized equipment to be studied, microfossils not only have great scientiﬁ c value but are also of it is only during the past 50 years or so that micropaleontology crucial economic importance. has become an important research ﬁ eld and, as discussed below, a highly applicable science. HIGHLIGHTS OF SOME MICROFOSSIL GROUPS
MICROFOSSIL EXTRACTION AND PREPARATION Some of the more common microfossil groups found in the sedi- mentary rocks of Ohio are discussed here. With the exception of most Microfossils are prepared using several different techniques pollen and spores, these groups were selected because they represent depending on the origin and composition of the fossil and on the fossils that may be observed in the ﬁ eld with the naked eye or with chemistry and hardness of the enclosing rock (matrix). Microfossil a hand lens. Other types of microfossils include tentaculitoids (a preparation basically consists of the following steps: breaking up cone-shaped fossil, probably belonging to the mollusks), acritarchs (disaggregating) the sample, removing ﬁ ne sediment through siev- (algal-like, planktonic organisms), dinoﬂ agellates (single-celled ing, drying the residue, and hand-picking under a microscope. algae), foraminiferans (single-celled, shell-bearing organisms), and A general rule is that the harder the sample, the harsher the radiolarians (single-celled organisms with a siliceous skeleton). treatment. For soft and porous sediments, wet or dry sieving is All microfossils discussed below, except for ostracodes, are acid generally the simplest and most efﬁ cient method and is inexpensive. resistant and are easily extracted from carbonate-rich rocks using If the specimens are embedded in harder rock, such as limestone, the acid-dissolution technique. sieving will not be of much use, at least not initially. Instead, the Conodonts are one of the best known and most widely used rock must be dissolved or crushed without damaging the fossils. groups of microfossils. They consist of calcium phosphate and are “Semi-hard” rock, such as shale, can be saturated with water and the jaw elements of an extinct wormlike marine animal, which is then put into a freezer. The formation of ice breaks up the rock. considered to have been a chordate. The animal had a jaw appa- This freeze-thaw method can be repeated several times until the ratus consisting of 12 or 14 individual elements, which were the rock is disaggregated. Shale also can be soaked in kerosene. (Note: only hard parts in the body. Although a few conodont elements kerosene is highly ﬂ ammable and must be used with great care are known that reach a centimeter or more in size, the majority are and only in a well-ventilated location.) After soaking for a few between 0.1 and 2 mm long. The conodont animal appeared in the hours, the kerosene is poured off and the sample is covered with Cambrian Period (c. 530 million years ago) and became extinct at water. The water forces the kerosene out of the pores in the rock the end of the Triassic Period (c. 208 million years ago). Conodonts and in the process disaggregates it. Although some of the fossils are abundant in most sedimentary rocks of marine origin and are may be damaged during these processes, rocks generally break very useful for determining the age of rocks, especially rocks of early at their weakest points, which are the contacts between the fossils and the matrix. Many microfossils are insoluble in acids. If they are embedded in a carbonate-rich rock such as limestone, an acid solution (commonly acetic acid) can be used to dissolve the rock without damaging the B A D fossils. The residue of acid-resistant minerals and fossils is gener- C E ally free of adhering sediment particles. When studying the sieved and dried residue for the ﬁ rst time under a regular reﬂ ected-light microscope, a whole new exciting world of minute fossils appears. If greater magniﬁ cation is required, a scanning electron microscope (SEM) commonly is used. SEM produces beautiful and highly de- tailed images of the fossils. FG H IJK USEFULNESS OF MICROFOSSILS SEM (scanning electron microscope) micrographs of conodonts (A-E) and scolecodonts Microfossils are very useful in many geological investigations. (F-K) characteristic of the Upper Ordovician rocks of Ohio. Specimens are about 0.2-2 Their size and abundance make them invaluable tools for deter- mm. Conodont photographs (A-E) courtesy of J. G. Richardson. Scolecodont photographs mining the age of sedimentary rocks, and they are commonly used (F-K) courtesy of C. F. Bergman; J, K from Eriksson and Bergman (1998). continued ➮ limestones ofOrdovician andMiddleDevonianage. rocks ofOhioandare particularlyabundantandwell-preserved in mining theageofrocks. Theyare widespread inthesedimentary generally smallerthan1mm.Chitinozoansare usefulfordeter- strongly resistant, organic materialknownaspseudochitinandare lion yearsago).Theseexclusivelymarinefossilsare composedof from thelateCambriantoendofDevonian( in anexclusivegroup, theChitinozoa,andoccurinfossilrecord of anas-yetunknownmarineinvertebrate.Thesefossilsare placed Many researchers currently thinkthatchitinozoansare eggcapsules ornamentation. Thebiologicalaf cally shapedlikebottles, scolecodonts. old) insouthwesternOhiocontainanabundanceofwell-preserved of environment. UpperOrdovician rocks ( of themare associatedwithsedimentsdepositedinacertaintype tial indexfossilsandare wellsuitedforecologicalstudiesasmany diverse groups ofinvertebrateorganisms. Scolecodontsare poten- worms are verycommonandcompriseoneofthelargest andmost age ( The oldestscolecodontsare reported from rocks oflateCambrian reach almost10mminlength,butare generally0.1to2mmlong. of 10to15elements.Scolecodontsare darkbrown toblackandcan are equippedwithacomplexjawapparatusgenerallycomposed leeches. Similartotheconodontanimal,jaw-bearingpolychaetes Polychaeta ofthephylum Annelida, whichincludesearthwormsand well-preserved inlimestones. rine sedimentaryrocks inOhioandare especiallyabundantand the rock hasbeenheated.Conodontscanbefoundinmostma- can becompared. Thecolorindicatesthetemperature towhich index, orCAI)hasbeencompiledtowhichanynewconodont as paleothermometers. A colorscale(knownasthealteration because conodontschangecolorwithtemperature, theyare used to middlePaleozoicage( Jansonius, Jan,andMcGregor, D.C.,eds.,1996, Palynology:principlesandapplications: American Association ofStratigraphic Feldmann, R.M.,andHackathorn, Merrianne,eds.,1996,FossilsofOhio:OhioDivisionGeological SurveyBulletin70,577p. Eriksson, Mats,andBergman, C.F., 1998,Scolecodontsystematics exempli Davis, R. A., ed.,1992,Cincinnatifossils:anelementaryguideto the Ordovician rocks andfossilsoftheCincinnati,Ohio,r sylvanian ( Ordovician ( throughout Ohio.Theyare especiallywellknownfrom rocks of rock. Fossilostracodesare foundinmarineandfreshwater rocks used asenvironmental indicatorsandfordeterminingtheage ofa common inallmodernaquaticenvironments. Ostracodescan be are knownfrom theOrdovician Periodtothepresent; theyare very of thevalves(afeature knownassexualdimorphism).Ostracodes to distinguishmalesfrom femalesbytheshapeandornamentation and equippedwithspinesknobs.Insomespeciesitispossible of calciumcarbonateandmaybesmoothtoheavilyornamented are lessthan1mmlong.Ostracodevalvesgenerallyare composed of thephylum Arthropoda, whichincludestrilobites.Mostostracodes Ohio. Specimensare about0.06mm.From FeldmannandHackathorn(1996). SEM micrographs ofMiddleDevonianchitinozoansfrom LucasCounty, northwestern Chitinozoans Scolecodonts Ostracodes v. 1 v. ogy, v. 72,p.477-485. Natural HistoryPopularPublications Series10,61p. c - . 505millionyearsold).Inpresent-day oceans,polychaete 3, 1,330p. The Divisionof GeologicalSurveyGeoFacts SeriesisavailableontheWorld Wide Web: http://www.OhioGeology.com c . 410-290millionyearsold)age. c are bean-shaped,two-valvedmicroscopic crustaceans . 505-440millionyearsold)andDevoniantoPenn- are thejawsofmarinewormsbelongingtoclass are hollow, jet-blacktobrown fossilscharacteristi- • ThisGeoFacts compiledbyMatsEriksson, TheOhioStateUniversity• January2002• ﬂ c asks, orsackswithwithoutexternal . 505-360millionyearsago).Moreover, ﬁ nity ofchitinozoansisdebated. c . 450-440millionyears c . 505-360mil- FURTHER READING ﬁ nd ﬁ ed bythepolychaete remains of sylvanian marineandfreshwater rocks yieldabundantmicroscopic “bone beds”thatwere formedaslagdepositsduringstorms.Penn- 380 millionyearsold),where theycommonlyare concentratedin abundant intheColumbusLimestoneofMiddleDevonianage( potentially usefulfordatingrocks. InOhio,microvertebrates are of surfaceornamentation.Somegroups ofmicrovertebrates are are composedofcalciumphosphateandmayshowahighdegree other microfossils, especiallyconodonts.Thesevertebrateremains range. Microvertebrates maybefoundinsamplesprocessed for of fossilvertebratesthatcommonlyfallintothemicroscopic size and climatechangesinaregion. time. Suchstudiesprovide importantinformationonvegetation an area andto analysis) enablesresearchers toreconstruct plantassociationsin The studyofthedistributionpollengrainsthrough time(pollen years old)ofOhiocantellthetaledramaticclimatechanges. and lakesedimentsotherIce Age deposits( rocks. Perhaps more importantlythough,plantremains from bog of Ohioand,likemostothermicrofossils, canbeusedfordating onward. Pollenandspores are verycommoninsedimentaryrocks are extractedfrom rocks usinghydro tough, organic substancecalledsporopollenin. Pollenandspores extremely resistant todestruction. Theyare composedofavery yield similarmicrovertebrates. from aboutthelateOrdovician Period( and spores havebecomeincreasingly commonthrough time, except forthespecimensinquestion.Plantsthatproduce pollen and hazardous acidthatdissolvesalmostallconstituentsofarock Photographs courtesyofJ.G.Richardson. Early Mississippianspores from northeasternOhio.Specimensare about0.02mm. Specimens are about1.2-1.5 mm.From FeldmannandHackathorn(1996). Middle Devonian(A)andPennsylvanian(B,female,C,male)ostracodesfrom Ohio. Microvertebrates Pollen andspores B ﬁ shesand,rarely, amphibians.Freshwater Permianrocks ﬁ Hadoprion cervicornis nd outhowtheseassociationschangedthrough A are theteeth,scales,bones,andotherremains are reproductive tissuesofplantsthatare (Hinde,1879):JournalofPaleontol- C ﬂ uoric acid,averystrong egion: CincinnatiMuseumof c . 450millionyearsago) PalynologistsFoundation, c . 14,000to24,000 c .
STATE OHIO DEPARTMENT DIVISION OF recycled OF OHIO OF NATURAL RESOURCES GEOLOGICAL SURVEY paper