Transitional Fossils

This page is dedicated to an analysis of the variety of transitional fossils currently known.

Prominent Examples of Transitional Fossils in the Animal Kingdom



Archaeopteryx

The original Archaeopteryx fossil

Archaeopteryx is one of the most famous transitional fossils and gives evidence for the evolution of birds from theropod dinosaurs.

Archaeopteryx is a genus of theropod dinosaur closely related to the birds. Since the late 19th century, it has been accepted by palaeontologists, and celebrated in later reference works, as being the oldest known bird.

It lived in what is now southern Germany in the Late Jurassic period around 150 million years ago, when Europe was an archipelago in a shallow warm tropical sea, much closer to the equator than it is now.

Anatomical illustration comparing the "fan-tail"
of the modern bird with the "frond-tail"
 of Archaeopteryx

 Similar in shape to a European Magpie, with the largest individuals possibly attaining the size of a raven, Archaeopteryx could grow to about 0.5 metres (1.6 ft) in length. Despite its small size, broad wings, and inferred ability to fly or glide, Archaeopteryx has more in common with other small Mesozoic dinosaurs than it does with modern birds. In particular, it shares the following features with the deinonychosaurs (dromaeosaurs and troodontids): jaws with sharp teeth, three fingers with claws, a long bony tail, hyperextensible second toes ("killing claw"), feathers (which suggest homeothermy), and various skeletal features. These features make Archaeopteryx a clear candidate for a transitional fossil between dinosaurs and birds, making it important in the study both of dinosaurs and of the origin of birds.

The first complete specimen was announced in 1861, and ten more Archaeopteryx fossils have been found since then. Most of the eleven known fossils include impressions of feathers—among the oldest direct evidence of such structures. Moreover, because these feathers take the advanced form of flight feathers, Archaeopteryx fossils are evidence that feathers began to evolve before the Late Jurassic.

Below is a timeline of Archaeopteryx discoveries and their locations:



Here is a compilation of the variety of Archaeopteryx fossils that have been discovered across the globe as mentioned in the above table:

Munich Specimen

London Specimen

The feather specimen

Maxburg Specimen

Haarlem Specimen

Daiting Specimen

Solnhofen Specimen

Thermopolis Specimen

Eichstatt Specimen

"chicken wing" Specimen

Pakicetids

The ancestors of whales

The cetaceans (whales, dolphins and porpoises) are marine mammal descendants of land mammals. The pakicetids are an extinct family of hoofed mammals that are the earliest whales, whose closest sister group is Indohyus from family Raoellidae. They lived in the early Eocene, around 53 million years ago.

 Their fossils were first discovered in North Pakistan in 1979, at a river not far from the shores of former Tethys Sea.  Pakicetids could hear under water, using enhanced bone conduction, rather than depending on tympanic membrane like most land mammals. This arrangement does not give directional hearing under water.

No pakicetids have been found in marine deposits, and they were apparently terrestrial or freshwater animals. Their long limbs and small hands and feet also indicate they were poor swimmers. Their bones are heavy and compact and were probably used as ballast; they clearly indicate pakicetids were not fast runners notwithstanding their otherwise cursorial morphology. Most likely, pakicetids lived in or near bodies of freshwater and their diet can have included both land animals and aquatic organisms.

Here are some examples of Pakicetid fossils.

Pakicetus attoki

Pakicetid skull found in Pakistan

Ambulocetus natans, which lived about 49 million years ago, was discovered in Pakistan in 1994. It was probably amphibious, and resembled the crocodile in appearance.

 In the Eocene, ambulocetids inhabited the bays and estuaries of the Tethys Ocean in northern Pakistan.  The fossils of ambulocetids are always found in near-shore shallow marine deposits associated with abundant marine plant fossils and littoral molluscs. Although they are found only in marine deposits, their oxygen isotope values indicate that they consumed water with a range of degrees of salinity, some specimens showing no evidence of sea water consumption and others none of fresh water consumption at the time when their teeth were fossilized.

 It is clear that ambulocetids tolerated a wide range of salt concentrations.  Their diet probably included land animals that approached water for drinking, or freshwater aquatic organisms that lived in the river. Hence, ambulocetids represent the transition phase of cetacean ancestors between freshwater and marine habitat.

Fish

Tiktaalik is a genus of extinct sarcopterygian (lobe-finned fish) from the late Devonian period, with many features akin to those of tetrapods (four-legged animals).  It is one of several lines of ancient sarcopterygians to develop adaptations to the oxygen-poor shallow water habitats of its time—adaptations that led to the evolution of tetrapods.  Well-preserved fossils were found in 2004 on Ellesmere Island in Nunavut, Canada. 

Discovery Site

Tiktaalik lived approximately 375 million years ago. Paleontologists suggest that it is representative of the transition between non-tetrapod vertebrates such as Panderichthys, known from fossils 380 million years old, and early tetrapods such as Acanthostega and Ichthyostega, known from fossils about 365 million years old.

Its mixture of primitive fish and derived tetrapod characteristics led one of its discoverers, Neil Shubin, to characterize Tiktaalik as a "fishapod". Unlike many previous, more fish-like transitional fossils, the "fins" of Tiktaalik have basic wrist bones and simple rays reminiscent of fingers. They may have been weight bearing. Like all modern tetrapods, it had rib bones, a mobile neck with a separate pectoral girdle, and lungs, though it had the gills, scales, and fins of a fish. 

Tetrapod footprints found in Poland and reported in Nature in January 2010 were "securely dated" at 10 million years older than the oldest known elpistostegids  (of which Tiktaalik is an example), implying that animals like Tiktaalik, possessing features that evolved around 400 million years ago, were "late-surviving relics rather than direct transitional forms, and they highlight just how little we know of the earliest history of land vertebrates".

Devonian Era Fish Species

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.

Modern-Day American Sole Flatfish

Amphistium is a 50-million-year-old fossil fish which has been 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."

Plants

Runcaria

A middle Devonian precursor to seed plants has been identified from Belgium, predating the earliest seed plants by about 20 million years. Runcaria, small and radially symmetrical, is an integumented megasporangium surrounded by a cupule. 

Modern-day Dahlia
(descendant of Runcaria)

The megasporangium bears an unopened distal extension protruding above the multilobed integument. It is suspected that the extension was involved in anemophilous pollination. Runcaria sheds new light on the sequence of character acquisition leading to the seed, having all the qualities of seed plants except for a solid seed coat and a system to guide the pollen to the seed.