Just for fun- some clear records of transitions:
A gradual transitional fossil sequence connects the foraminifera Globigerinoides trilobus and Orbulina universa [Pearson et al. 1997]. O. universa, the later fossil, features a spherical test surrounding a "Globigerinoides-like" shell, showing that a feature is added, not lost. The evidence is seen in all major tropical ocean basins. Several intermediate morphospecies connect the two species, as may be seen in the figure included in Lindsay [1997].
The fossil record shows transitions between species of Phacops (a trilobite; Phacops rana is the Pennsylvania state fossil.). [Eldredge 1972; 1974; Strapple 1978]
Planktonic forminifera [Malmgren et al. 1984]. This is an example of "punctuated gradualism." A 10-million-year foraminifera fossil record shows long periods of stasis and other periods of relatively rapid but still gradual morphologic change.
Fossils of the diatom Rhizosolenia are very common (they are mined as diatomaceous earth), and they show a continuous record of almost 2 million years which includes a record of a speciation event. [Miller 1999, 44-45]
Lake Turkana mollusc species [Lewin 1981].
Cenozoic marine ostracodes [Cronin 1985].
The Eocene primate genus Cantius [Gingerich 1976, 1980, 1983].
Scallops of the genus Chesapecten show gradual change in one "ear" of their hinge over about 13 million years. The ribs also change. [Ward and Blackwelder 1975; Pojeta and Springer 2001]
The horns of titanotheres (extinct Cenozoic mammals) appear in progressively larger sizes, from nothing to prominence. Other head and neck features also evolved. These features are adaptations for head-on ramming analogous to sheep behavior. [Stanley 1974]
Gryphaea (coiled oysters) become larger and broader but thinner and flatter during the Early Jurassic [Hallam 1968].
Fossil transitionals between families, orders, and classes:
Human ancestry. Australopithecus, though its leg and pelvis bones show it walked upright, had a bony ridge on the forearm, probably vestigial, indicative of knuckle walking. [Richmond and Strait 2000]
Dinosaur-bird transitions.
Haasiophis terrasanctus is a primitive marine snake with well-developed hind limbs. Although other limbless snakes might be more ancestral, this fossil shows a relationship of snakes with limbed ancestors [Tchernov et al. 2000]. Pachyrhachis is another snake with legs related to Haasiophis [Caldwell and Lee 1997].
The jaws of mososaurs are also intermediate between snakes and lizards. Like the snake's stretchable jaws, they have highly flexible lower jaws, but, unlike snakes, they don't have highly flexible upper jaws. Some other skull features of mososaurs are intermediate between snakes and primitive lizards. [Lee et al. 1999; Tchernov et al. 2000; Caldwell and Lee 1997]
Transitions between mesonychids and whales.
Transitions between fish and tetrapods.
Transitions from condylarths (a kind of land mammal) to fully aquatic modern manatees. In particular, Pezosiren portelli is clearly a sirenian, but its hind limbs and pelvis are unreduced [Domning 2001a, 2001b].
Fossil transitionals between kingdoms and phyla:
The Cambrian fossils Halkiera and Wiwaxia have features which connect them with each other and with the modern phyla of Mollusca, Brachiopoda, and Annelida. In particular, one species of halkieriid has brachiopod-like shells on the dorsal side at each end. This is seen also in an immature stage of the living brachiopod species Neocrania. It has setae identical in structure to polychaetes, a group of annelids. Wiwaxia and Halkiera have the same basic arrangement of hollow sclerites, an arrangement which is similar to the chaetae arrangement of polychaetes. The undersurface of Wiwaxia has a soft sole like a mollusc's foot, and its jaw looks like a mollusc's mouth. Aplacophorans, which are a group of primitive molluscs, have a soft body covered with spicules similar to the sclerites of Wiwaxia. [Conway Morris 1998, 185-195]
Cambrian and Precambrain fossils Anomalocaris and Opabinia are transitional between arthropods and lobopods.
An ancestral echinoderm has been found, intermediate between modern echinoderms and other deuterostomes [Shu et al. 2004]. This one is neat! |
