Logs of wood may have become partly compressed prior to silicification ( or calcification) .sx Wood tends to be compressed parallel to the cell files .sx Autochthonous roots , upright stems and trunks are generally preserved as impressions of the bark , and compressions of what tissues remained ( mainly xylem ) after decay and infilling of the space by sand and mud .sx In a lycopod ( Figs 4.7 , 4.11 ) , the woody outer cortex becomes compressed , and the small woody stele is often displaced .sx Particularly difficult to unravel in fossil plants are the different appearances that can occur between different modes of preservation of the same species .sx Most aerial parts of fossil plants are found as fragmented and dispersed remains .sx How can these be reassembled as a reconstruction of the original plant ?sx Palaeobotany is much concerned with this detective work , and it is important to realize , in this respect , the significance of any material one comes across in the field .sx There are three methods of reconstruction :sx 1 .sx Locating material where , by good chance , two or more parts ( organs ) are joined , e.g. root and stem , cone with pollen , and dispersed pollen , leaves and stem , leaves and reproductive organs .sx This is the only positive method , and it depends mainly on field observation .sx 2 .sx Identifying particular anatomical features that are common to two or more organs .sx Frequently-cited characters are particular glands , type of stomata , cell outline :sx features that cannot readily be identified in the field .sx 3 .sx Commonly occurring associations of organs ( e.g. on bedding surfaces ) that would be difficult to explain if not from the same plant .sx 4.2.2 Coal and oil shales .sx Coal is described in terms of rank ( thermal grade - Fig. 4.12 ) and type ( composition) .sx The end members ( anthracite and peat ) can be fairly readily identified in the field .sx Very bright , splintery coal is likely to be of high rank ( low volatile bituminous coal-to-anthracite) .sx The tectonic - sedimentary setting of the site , and the grade of the associated mudrocks , will provide a clue to rank .sx Is there a root bed present ?sx Is the coal autochthonous or drifted ?sx How 'pure' is the coal ?sx It may be no more than a carbonaceous mudstone .sx Try to identify the components of a bituminous coal :sx Vitrain , representing coalified logs or pieces of bark , is glassy ( vitreous ) and usually closely jointed .sx Fusain generally forms lenses and has a silky sheen , but is soft and leaves a black mark .sx Durain is dull and tough , with megaspores often evident in Palaeozoic coals .sx Clarain ( attrital coal ) is finely-laminated vitrain and durain .sx This classification of coal is tedious to follow for logging .sx Try using mm , cm or dm thicknesses of vitrain , fusain or attrital coal .sx Further detail can be introduced by making estimates of vitrain :sx attrital ratio .sx What one is after is an indication of the overall vitrain :sx fusain ratio , and the percentage of mud present .sx figure&caption .sx Oil shales and cannel coals have formed in aquatic anaerobic environments .sx Cannel coal ( dull with conchoidal fracture ) represents drifted , finely-divided terrestrial plant material .sx It is often found at the top of a coal , following a flooding event .sx Torbanite , lamosite , tasmanite and marinite are oil shales distinguished by their principal components ( identifiable microscopically) :sx torbanite , Botryococcus and allied algae ; lamosite , planktonic algae ( both are lacustrine ) ; tasmanite with Tasmanites ( marine ) ; and marinite with various planktonic marine algae ( Hutton , 1986) .sx 4.2.3 Calcareous algae and stromatolites .sx The undulose , arched or planar laminations of stromatolites ( Fig. 4.13 ) are generally readily recognized in the field .sx The laminations reflect discontinuities in formation ( e.g. storm action ) rather than short ( e.g. diurnal ) periodicities .sx Nodular stromatolites ( oncolites ) are distinguished by their irregular lamination around a shell or lithic clast .sx The clotted texture of thrombolites is not easy to prove in the field .sx Growth form is probably the best indicator of red , calcified algae with forms corresponding to the shapes of most types of breakfast cereal .sx It may just be possible to make out , with a hand lens , the cellular structure of a solenopore , or the larger , reproductive cells of more advanced rhodophytes .sx Rhodophytes will probably have an irregular , knobbly outline attributable to truncation and branching .sx Although the stem of the green alga Chara sensu lato may become encrusted , it is the calcified walls of the cells enclosing the oogonia , the gyrogenites , ( often replaced by silica , or seen as moulds ) that are so useful as environmental indicators ( essentially fresh water) .sx Use a hand lens to identify the spiral cells .sx Marine , calcareous ( aragonite ) green algae such as Halimeda readily breaks up into segments , as does Corallina ( articulated , red alga ) ( see also section ) .sx 4.3 Animal fossils .sx 4.3.1 Soft tissue preservation .sx Soft tissue preservation is rare ( see also section ) .sx In the field consider what types of sediment may yield information on such material .sx There are five modes of preservation :sx ( 1 ) mineral coats , where tissues are outlined by a mineral , typically pyrite ; ( 2 ) permineralization , actual replacement of the tissue by phosphate ( best ) , pyrite or silica ( as with plant cells , above ) ; ( 3 ) casts or impressions of the tissues , where the adjacent sediment was stabilized prior to decay and infilling of the void :sx or , for example , by concretion formation in the Silesian Mazon Creek Formation , but probably by organic stabilization in the late Precambrian Pound Quartzite ( Fig. 4.14 ) ; ( 4 ) encasement in decay-inhibiting material ; for example , scorpions in Carboniferous coal ( peat ) , insects in amber ; and ( 5 ) encrustation of soft tissue ( e.g. hydroids , algae ) by skeletal organisms , to leave a mould of the soft tissue ( bio-immuration) .sx Evidence of bioturbation , scavenging and early reworking almost always excludes soft tissue preservation of types 1-4 ( except where concretion initiation was so early as to inhibit these factors locally) .sx Although transport per se prior to deposition does not appear to be a major factor , aerobic decay during transport can be important .sx Following deposition , it is early diagenesis that is important since anaerobic decay may be complete ( Fig. ) .sx Note that :sx .sx 1 .sx In freshwater sediments , pyritization of soft tissues is relatively unimportant due to relatively low levels of sulphate in the water .sx In marine sediments with sufficient iron , pyritization is favoured by high burial rate .sx With rapid burial more reactive ( less decayed ) organic matter is added to the sediment , leading to a rapid uptake of sulphate ions from pore water and therefore increased diffusion of sulphate ions from the overlying sea water .sx In euxinic conditions organic carbon is generally too high to favour good pyritization and pyrite is disseminated .sx figure&caption .sx 2 .sx Phosphatization of soft tissue is generally associated with low burial rate ( but without reworking ) and high organic input .sx Phosphatic materials ( bone , arthropod carapace ) often act as nuclei , but the best phosphatization seems to have taken place , or at least to have been initiated , at the sediment - water interface ( Martill , 1988 ; see also section ) .sx 3 .sx Early carbonate encasement , where calcium carbonate was precipitated as concretions , is favoured by high organic input and burial rate .sx Where terrigenous input is low ( marine or freshwater ) , rapid carbonate precipitation is favoured by high carbon dioxide production by algal and cyanobacterial mats e.g. the bedded limestones of Solnhofen ( Upper Jurassic , Germany ) and the Green River Formation ( Eocene , USA ) where fossils always seem to be compressed .sx 4.3.2 Univalves and groups with skeletons that remain more or less intact .sx Graptolites ( V - revised ) As with ammonoids ( below ) , the mode of preservation of graptolites in a sequence of mudrocks may change appreciably from bed to bed , indicating small variations in facies .sx Graptolites had a pliable organic ( collagenous ) skeleton .sx Three-dimensional preservation in early - formed concretions and certain limestones is uncommon .sx Material can be collected for later acid-treatment .sx The most common mode of preservation is as carbonized side-down ( profile ) compressions , or compressions where the organic skeleton has been replaced by white-weathering , phosphatic material ( Fig. ) .sx Diplograptids ( Fig. 4.17 ) , however , commonly landed on the sediment with one series of thecae being entombed aperture down ( scalari - form view) .sx If the form of the thecae is clear , identification is generally possible .sx Locate material with discrete rather than crowded specimens and with the proximal morphology distinct .sx Take care not to misinterpret apparent branchings for Cyrtograptus or Nemagraptus .sx Commonly , pyrite filled the thecae at an early stage .sx Only internal moulds may remain but these have the advantage that they are undeformed .sx table&caption .sx Where mudrocks contain distal turbidites ( striped mudstones ) , the graptolites therein tend to be richer and better preserved than in more oxidized sediment .sx Preservation was enhanced by rapid sedimentation .sx Conularids ( F ) are generally highly compressed in mudrocks .sx External and internal impressions occur in sandstones .sx Archaeocyathids ( E - revised ; Fig. 4.18 ) Skeleton generally with morphology distinct .sx The calcareous skeleton is now usually of calcite or has been silicified , but may have originally been Mg calcite .sx Geopetal sediment and sparry fill to chambers is common .sx Check for epitaxial algae ( Epiphyton ) as dark micritic clots on autochthonous material .sx Mouldic preservation should show evidence of regularly porous walls .sx Sponges ( E ) It is exceptional for a fossil sponge to be found relatively perfect .sx The best preserved are those with fused mineralized spicules giving a rigid skeleton .sx ( Peripheral , loosely bound spicules will still be missing .sx ) Judging by the abundance of loose spicules in many sediments ( e.g. the needle-like spicules in many Jurassic and Cretaceous rocks , or the ball-bearing-like dermal spicules ( Rhaxella ) in the Upper Jurassic ) sponges were much more widespread than is now apparent .sx Opaline spicules of demosponges and hexactinellids are readily dissolved .sx Look out for moulds ( in sand and mudrocks ) , or carbonate replacement in limestones .sx Sclerosponges have a basal calcareous skeleton with embedded siliceous spicules .sx In fossil representatives ( e.g. Chaetetes ) the skeleton may now be entirely calcitic .sx Rugose and tabulate corals ( F - revised ) In sandstones , preservation is nearly always poor .sx If the skeleton is present then it will be almost certainly replaced .sx Sediment may have infilled the calyx and infiltrated more deeply , giving variable perfection to moulds .sx Preservation in mudrocks can be good especially if they are somewhat bituminous .sx In wackestones and lime mudstones , compaction is common .sx Select material suitable for transverse and longitudinal sectioning .sx The manner in which colonial forms increase is important .sx Scleractinian corals ( F - Fig 4.19 ) Due to the originally aragonitic skeleton , mouldic or replacement modes of preservation are widespread .sx Neomorphic preservation ( calcitization ) is mostly associated with massive colonies ( because they were less permeable) .sx Colonial and solitary forms can be completely dissolved and the resulting cavity spar-filled .sx Preservation is generally poor in reefs , becoming better laterally in muddier facies .sx For full identification it is essential to have good preservation of the skeleton for transverse and longitudinal sectioning .sx Distinct banding ( probably emphasizing original seasonal growth variation ) is often evident .sx Bryozoans ( G and G - revised ) The stony bryozoans and the Palaeozoic had a calcitic skeleton and are like miniature tabulate corals , showing similar preservation modes .sx Preservation is often clear in mudstones with specimens weathering out .sx Good preservation of external morphology and , then , sectioning is necessary for close identification .sx Fenestrate bryozoans are typically exposed with the apertures ( smaller than the fenestrules ) facing down ( Fig. 4.20 ) , so it is necessary to search for material with apertures facing up .sx Slender , encrusting cyclostomes are easily damaged .sx In many cheilostomes the frontal surfaces include delicate sculpture , ovicells and avicularia , costae and spines .sx Some loss of detail may have taken place because of abrasion or dissolution .sx Also , replacement usually leads to loss of detail , though silicified bryozoans within hollow flints may be well preserved externally .sx Impressions of the external surface of Palaeozoic forms are of little significance .sx Larger foraminifera ( C ) Larger foraminifera are common at certain times and in certain facies and are generally well preserved in fresh rock .sx Fusulines ( Upper Palaeozoic ) , alveolinids ( Mesozoic onwards ) , and rotalids ( e.g. Nummulites , orbitoids of the Tertiary ) all had calcitic skeletons .sx In limestones , preservation is generally good .sx Individuals may weather out , or blocks may be collected for later slabbing .sx In sand and mud rocks , weathering may lead to dissolution .sx Break some specimens open to check that skeletal detail is present .sx Stromatoporoids ( F ) are a heterogeneous group .sx Break off a piece of rock and examine the fresh , wetted surface with a hand lens to spot the skeletal laminae and pillars ( never present in stromatolites) .sx Thin-sections are required for identification .sx Gastropods ( I ) Most modern gastropods have an entirely aragonitic skeleton .sx The operculum , when present , is horny or aragonitic .sx