TAPHONOMY AND PRESERVATION (cont)
Preburial bone damage
Many bones collected in situ show pre-mortem as well as post-mortem damage. Pre- mortem damage can generally be attributed to disease or trauma, but it is difficult to be precise as to the cause. It is usually recognised as pre-mortem by the presence of new bone growth around fractures. Blows (1989) reported an Iguanodon pelvis from the Isle of Wight in which the ischium and ilium of the right side were fused. Analysis of thin sections cut through the fused region of the bones revealed the cause to be due to an impact fracture. A similar injury was reported for another Iguanodon pelvis by Charig (1979). Other pathological conditions, such as the unusual deformed neural spines encountered in an Iguanodon, remain to be explained. .
Post-mortem bone damage occurs as fractures, surface pitting or abrasion. In general, this abrasion is not due to long distant transport except for those elements usually occurring isoIated in channel sandstones and conglomerates. Surface pitting is thought to perhaps be attributable to bioerosion, with fungal and bacterial attack considered the most likely causes. Such attack may take the form of small (2-5 mm) pits, looking as though they were made by a miniature ice cream scoop. Other types of bioerosion include 0.25- 0.5 diameter, asterorhizal borings in the surface of thin bones, This latter damage is common in fish bones found in small gutter casts from the higher parts of the Vectis Formation. When infestation is slight the boring systems are readily identifiable, but when infestation is intense the bone surface simply becomes matt and slightly roughened, and is usually buff coloured compared with the dark brown to black of unaffected bones. .
Fractured bone without evidence of healing may be attributable to predation or trampling. Bite marks are occasionally found, and Naish (1999) reported tooth marks of a theropod on a fragment of theropod bone from the Wealden Group of Hastings, Sussex.
Bone encrusters
Bones from the Wealden Group are only rarely encountered as (passive) host to encrusting organisms. Bones of Iguanodon from the Vectis Formation near Atherfield Point have been found with encrusting oysters on their surface, attesting to the at least quasi-marine nature of that part of the sequence. Conversely, isolated banes within the hone bed associated with the Perna Bed are never encrusted. This is probably because the bones were mobile on the current-swept sea floor of the more open marine conditions represented here, and thus any encrusters were prevented from attaching. .
Occasionally larger bones from the Wessex Formation may be encrusted with groups of closely packed, small (circa 0.5 mm diameter), sub-spherical structures adherent to the bone surface. These structures are calcified eggs of gastropods. Examples , can be found still retaining emhryonic snails within the eggs. The eggs are linked together by a narrow neck of calcified shell, suggesting that they represent an aquatic snail. The affinities of the eggs remain to be established with certainty but could be from the common Wessex Formation gastropod Viviparus. If this is the case, then here is an unusual condition as the extant Viviparus is a live bearer.
Bone colour
Bones found in the highly organic-rich plant debris beds are often black, or very dark brown. By contrast, bones from the red clays of the Wessex Formation are usually white to cream coloured, and usually with a degree of pink staining form the surrounding clay. The white coloration of the latter bones can be attributed to the almost total oxidation of the organic component of the bones, probably due to atmospheric weathering and exposure to intense UV light while the bone was lying exposed on the floodplain under semi-arid conditions. Bones in the plant debris beds that appear black or dark brown were most likely buried extremely quickly after death (for some animals, burial may have been the cause of death). The organic fraction of such rapidly buried bones underwent a distinctive diagenetic history. This organic matter probably fuelled (at least in part) some sulphate reduction, resulting in fine scale pyrite production within the bone. This pyrite is in part responsible for the dark coloration but, also, the more refractory components of the organic matrix of the bone may be the cause.
Mineral infills
In general, bones from the Wealden group have most of their internal pore spaces (pleurocoels, intratrabecular space, osteocyte lacuanae and canaliculi system) filled with diagenetic minerals. Occasionally bones lack diagenetic infills, and are easily recognisable by their light mass, but they are usually very delicate, and can break down to a powder at even the lightest of touches. Such bones demand careful excavation and usually require on-site consolidation. Bones with complete mineral infills are often very robust and even withstand considerable rolling on the flint beach gravels during storms. The nature of the mineral infill varies, even between bones from the same skeleton. The common mineral infills found in the Wealden Group on the Isle of Wight are ferroan and non-ferroan calcites, siderite, baryte, pyrite, marcasite, sphalerite, kutnohorite and francolite (Clarke 1991, Clarke and Barker 1993, Clarke et al. 1998).
Siderite, ferroan and non-ferroan clacite.
Carbonate minerals are abundant as infills and as concretionary coatings around the outside of bones. When found as internal void fills, calcites are usually white to grey and occur with lesser amounts of pyrite. Siderite frequently occurs as an early void-filling phase and may form concretionary masses around bones. It also occurs as discrete spherules.
Non-ferroan calcite is usually an early phase, and it may completely fill void space leaving little or no spacp for later diagenetic infills. Ferroan calcite is usually a late phase void fill and it often post-dates considerable compaction. As such it can be found filling late fractures in bones and concretions.
In an Iguanodon vertebra from the Vectis Formation analysed by Clarke and Barker (1993) siderite was the first mineral deposited on the bone's internal surface. This was followed by in influx of clay material, perhaps representing a fracture event that permitted the ingress of some sediment. The clay is overlain by pyrite followed by a second phase of siderite precipitation. Some sphalerite was then precipitated, followed by ferroan calcite and finally baryte. Thus five different mineral species were deposited in one bone in six distinct episodes.
Pyrite, marcasite and sphalerite.
Pyrite is abundant both within bone void space where it forms and early diagenetic infill, and as an external coating. It also occurs as a bone replacement. In general the pyrite is finely crystalline, although larger euhedral crystals occur commonly, usually as cubic-pyritohedra rather than as cubes. In general the pyrite is only metastable, and under damp conditions, heavily pyritised bones will break down.
Because of its high density, pyrite sometimes forms accumulated masses at certain strand lines on the beaches, where pyritised bones and wood are easily collected.
Sphalerite occurs infrequently within bones as a late phase sulphide where it often forms discrete crystalline aggregates.
Kutnohorite.
This mineral has been reported occurring as a replacement for bacterial mucilage within bones from the Vectis Formation by Clarke and Barker (1993). It occurs in multiple layers, and is restricted in distribution within the bones.
Baryte.
Baryte occurs commonly in Wealden Group bones as a late phase mineral fill. It may occur as discrete blades, tabular, euhedral crystals or as fascicular (feathery) crystal growths. It usually occurs in discreet patches or aggregates, rather than as void linings.
