page_title: Transitional fossil
section_title: Amphistium
hierarchical_section_title: Transitional fossil / Prominent examples / Amphistium
caption_attribution_description: English: Amphistium fossil, at the Muséum d'Histoire naturelle de Genève. Français&#160;: Fossile d'Amphistium, au Muséum d'Histoire naturelle de Genève. Écriteau lié&#160;: Amphistium, fossile et reconstitution d'un poisson âgé de 45 millions d'années dont le crâne est intermédiaire entre celui des poissons symétriques et celui des pleuronectes actuels.
context_page_description: A transitional fossil is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group. This is especially important where the descendant group is sharply differentiated by gross anatomy and mode of living from the ancestral group. These fossils serve as a reminder that taxonomic divisions are human constructs that have been imposed in hindsight on a continuum of variation. Because of the incompleteness of the fossil record, there is usually no way to know exactly how close a transitional fossil is to the point of divergence. Therefore, it cannot be assumed that transitional fossils are direct ancestors of more recent groups, though they are frequently used as models for such ancestors.
  In 1859, when Charles Darwin's On the Origin of Species was first published, the fossil record was poorly known. Darwin described the perceived lack of transitional fossils as, "... the most obvious and gravest objection which can be urged against my theory," but explained it by relating it to the extreme imperfection of the geological record.
context_section_description: Pleuronectiformes (flatfish) are an order of ray-finned fish. The most obvious characteristic of the modern flatfish is their asymmetry, with both eyes on the same side of the head in the adult fish. In some families the eyes are always on the right side of the body (dextral or right-eyed flatfish) and in others they are always on the left (sinistral or left-eyed flatfish). The primitive spiny turbots include equal numbers of right- and left-eyed individuals, and are generally less asymmetrical than the other families. Other distinguishing features of the order are the presence of protrusible eyes, another adaptation to living on the seabed (benthos), and the extension of the dorsal fin onto the head.
  Amphistium is a 50-million-year-old fossil fish identified as an early relative of the flatfish, and as a transitional fossil In Amphistium, the transition from the typical symmetric head of a vertebrate is incomplete, with one eye placed near the top-center of the head. Paleontologists concluded that "the change happened gradually, in a way consistent with evolution via natural selection—not suddenly, as researchers once had little choice but to believe."
  Amphistium is among the many fossil fish species known from the Monte Bolca Lagerstätte of Lutetian Italy. Heteronectes is a related, and very similar fossil from slightly earlier strata of France.
caption: Fossil of Amphistium with one eye at the top-center of the head.
exif: {"Image ImageDescription": "", "Image Make": "NIKON", "Image Model": "E7600", "Image Orientation": "Horizontal (normal)", "Image XResolution": "300", "Image YResolution": "300", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "Microsoft Windows Photo Gallery 6.0.6001.18000", "Image DateTime": "2012:03:30 13:44:57", "Image YCbCrPositioning": "Co-sited", "Image ExifOffset": "2326", "Image Padding": "[]", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail XResolution": "96", "Thumbnail YResolution": "96", "Thumbnail ResolutionUnit": "Pixels/Inch", "Thumbnail JPEGInterchangeFormat": "6704", "Thumbnail JPEGInterchangeFormatLength": "2397", "EXIF ExposureTime": "10/601", "EXIF FNumber": "29/10", "EXIF ExposureProgram": "Program Normal", "EXIF ISOSpeedRatings": "50", "EXIF ExifVersion": "0220", "EXIF DateTimeOriginal": "0000:00:00 00:00:00", "EXIF DateTimeDigitized": "0000:00:00 00:00:00", "EXIF ComponentsConfiguration": "YCbCr", "EXIF CompressedBitsPerPixel": "2", "EXIF ExposureBiasValue": "0", "EXIF MaxApertureValue": "3", "EXIF MeteringMode": "Pattern", "EXIF LightSource": "Unknown", "EXIF Flash": "Flash fired, auto mode", "EXIF FocalLength": "17/2", "EXIF FlashPixVersion": "0100", "EXIF ExifImageWidth": "1892", "EXIF ExifImageLength": "1327", "Interoperability InteroperabilityVersion": "[48, 49, 48, 48]", "EXIF InteroperabilityOffset": "6548", "EXIF FileSource": "Digital Camera", "EXIF SceneType": "Directly Photographed", "EXIF CustomRendered": "Normal", "EXIF ExposureMode": "Auto Exposure", "EXIF WhiteBalance": "Auto", "EXIF DigitalZoomRatio": "0", "EXIF FocalLengthIn35mmFilm": "41", "EXIF SceneCaptureType": "Standard", "EXIF GainControl": "None", "EXIF Contrast": "Normal", "EXIF Saturation": "Normal", "EXIF Sharpness": "Normal", "EXIF SubjectDistanceRange": "0", "EXIF ImageUniqueID": "3E3C0D71146A46E884F2FCFB6EF61E53", "EXIF Padding": "[]", "EXIF OffsetSchema": "3868"}