Acompression fossil is afossil preserved insedimentary rock that has undergonephysical compression. While it is uncommon to findanimals preserved as good compression fossils, it is very common to findplants preserved this way. The reason for this is that physical compression of the rock often leads to distortion of the fossil.
The best fossils ofleaves are found preserved in fine layers ofsediment that have been compressed in a directionperpendicular to the plane of the deposited sediment.[1] Since leaves are basically flat, the resulting distortion is minimal.Plant stems and other three-dimensional plant structures do not preserve as well under compression. Typically, only the basic outline and surface features are preserved in compression fossils; internalanatomy is not preserved. These fossils may be studied while still partially entombed in the sedimentary rock matrix where they are preserved, or once lifted out of the matrix by a peel or transfer technique.[2]
Compression fossils are formed most commonly inenvironments where fine sediment is deposited, such as inriver deltas,lagoons, alongrivers, and inponds. The best rocks in which to find these fossils preserved areclay andshale, althoughvolcanic ash may sometimes preserve plant fossils as well.[3]
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A slab and counter slab, more often called a part and counterpart in paleoentomology[4] and paleobotany,[5] are the matching halves of a compression fossil, a fossil-bearingmatrix formed insedimentary deposits. When excavated the matrix may be split along the natural grain or cleavage of the rock. A fossil embedded in the sediment may then also split down the middle, with fossil remains sticking to both surfaces, or the counter slab may simply show a negative impression or mould of the fossil.[6] Comparing slab and counter slab has led to the exposure of a number of fossil forgeries.
Differences between the impressions on slab and counterslab led astronomerFred Hoyle and applied physicistLee Spetner in 1985 to declare that someArchaeopteryx fossils had beenforged, a claim dismissed by most palaeontologists.[7]
In its November 1999 edition,National Geographic magazine announced the discovery ofArchaeoraptor, a link between dinosaurs and birds, from a 125 million-year-old fossil that had come fromLiaoning Province of China. Chinese palaeontologistXu Xing came into possession of the counter slab through a fossil hunter. On comparing his fossil with images ofArchaeoraptor it became evident that it was a composite fake. His note toNational Geographic led to consternation and embarrassment. Lewis Simons investigated the matter on behalf ofNational Geographic. In October 2000, he reported what he termed:
a tale of misguided secrecy and misplaced confidence, of rampant egos clashing, self-aggrandizement, wishful thinking, naïve assumptions,human error, stubbornness, manipulation, backbiting, lying, corruption, and, most of all, abysmal communication.
It was eventually determined thatArchaeoraptor had been constructed from parts of anEarly Cretaceous birdYanornis martini and a small dinosaurMicroraptor zhaoianus.[8]
In order to increase their profit, fossil hunters and dealers occasionally sell slab and counter slab separately. A reptile fossil also found in Liaoning was described and namedSinohydrosaurus in 1999 by theBeijing Natural History Museum. In the same year theInstitute of Vertebrate Paleontology and Paleoanthropology in Beijing described and namedHyphalosaurus lingyuanensis, unaware they were working with the counter slab of the same specimen.Hyphalosaurus is now the accepted name.[9]