New World parrots Neotropical parrots
The fossil history of parrots Modern parrots New World parrots neotropical parrots are among the most distinctive birds in the world – there is little else with which they can be confused. One of the most obvious characters is the short, mobile decurved bill. This varies in its proportions and length among species but obviously is parrot-like in all. Even without the keratin sheath covering, the underlying bone retains the characteristic shape.
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Another trait is the zygodactyl toe arrangement. Most birds have the anisodactyl condition in which three toes point forwards and the fourth, where present, backward. Among living birds, only a few groups exhibit the zygodactyl arrangement: parrots, cuckoos (Cuculiformes), and most of the Piciformes (woodpeckers, puffbirds, jacamars, toucans, etc.). In these groups, the fourth (lateral) toe is fully and permanently directed posteriorly.
In several other bird groups, the feet are semizygodactyl (the fourth toe points laterally, rather than backward) or facultatively zygodactyl (the fourth toe can be moved into and out of a backward position).
This arrangement is reflected in the tarsometatarsus (the last long bone of the hindlimb, actually the foot bone). The distal end of the tarsometatarsus has protrusions, or trochlea, for the attachment of the toes. The trochlea for the fourth toe in zygodactyl birds has a laterally directed, wing-like extension and a posteriorly placed accessory trochlea or Sehnenhalter.
The latter serves as an articular surface for the reversed fourth toe. Its morphology differs among each group of zygodactyl birds. In modern parrots, there is a deep furrow between the accessory trochlea from the fourth trochlea. The presence of an accessory trochlea in a Fossil history of parrots is a good indication of that bird’s toe arrangement and is crucial for working out its relationships.
While the bill shape and toe arrangement are major features of parrots, they certainly are not the only ones that are useful for recognizing parrot relationships, but other features are much less obvious. These include such disparate characteristics as the development of projections and presence of incisions in the sternum, configuration of the articulations on the pectoral girdle,
Fig. 1 Zygodactyl toe arrangement in modern parrots.
presence of openings for nerves in the coracoid, shape of the crest on the humerus for attachment of the pectoral flight muscle, arrangement of canals and ridges on the backside (hypotarsus) of the tarsometatarsus, relative lengths of the pedal phalanges (toe bones), among others.
Modern parrots are so distinctive that it has proved difficult to determine their closest living relatives among other birds and a number of different groups have been nominated particularly pigeons. Recent molecular analyses have produced a novel and
quite unexpected result. Studies by a range of different workers have been in agreement that the nearest living relative of parrots is the passerines, or songbirds (Passeriformes).
Songbirds, which comprise about 60 percent of all living bird species, are quite different in appearance. Other than for a few species with superficially curved bills, there is no resemblance to the characteristic parrot bill. Further, all songbirds are anisodactyl, with
none having a toe condition approaching that of parrots. A clue to clarifying this seemingly anomalous connection
between two very different looking groups came with the identification of a Fossil history of parrots group of zygodactyl birds, the Zygodactylidae.
First described in 1969, these small birds were known only from isolated and fragmentary leg bones from the early and middle Miocene (34–17 million years ago [mya]) of Europe. They now are known from North America and Europe, with a temporal range of early Eocene to middle Miocene (48–17 mya). In some deposits, these are among the most abundant small birds.
Although relationships of this group were uncertain, most authors placed them with the lineage containing woodpeckers
(Piciformes). The discovery of new specimens, including nearly
Fossil history of parrots
Fig. 2 Distal end of the tarsometatarsus of a modern parrot
accessory trochlea and furrow (after Mayr 2002b).
complete, articulated skeletons from the early Oligocene of France, permitted a reassessment of this idea. The skeleton of zygodactylids was very similar to that of songbirds, with some elements being almost indistinguishable. The long, slender, and quite a parrot-like tarsometatarsus, however, exhibited a fully zygodactyl toe arrangement.
More and better-preserved specimens have led to the conclusion that this family is the sister group to songbirds. These also start to bridge the morphological gap between the parrots and songbirds, which diverged many millions of years ago. The name Psittacopasseres has been created for this lineage of parrots, zygodactylids, and songbirds.
Modern parrots belong to crown-group Psittaciformes, that is, all living parrots, their common ancestor, and all its descendants, including extinct forms. In the early section of the lineage leading up to the crown group were a number of basal and now extinct taxa.
These are known as stem-group parrots (or simply stem-parrots). They exhibit earlier stages in the development of characters that make crown-group parrots so distinctive. The assignment of some of these Fossil histories of parrots as stem parrots is still controversial because some differences from living parrots are quite marked.
Some authors restrict the use of the name Psittaciformes to the crown group. For the crown and stem groups together, they employ the term Pan-Psittaciformes. The recognition of stem parrots has been subject to considerable ongoing revisions as a result of several factors. Partial fossils, sometimes comprising only a few bones or even one fragmentary bone, can be difficult to place in the avian classification.
There are characters in parrots that are shared also by other groups in which these have been acquired independently. The history of stem-parrot classification has seen taxa originally described as parrots subsequently being shown to belong to other groups.
Conversely, other taxa first regarded as belonging to another bird group have since been regarded as Psittaciformes. Some of the Fossil histories of parrots discussed here remain controversial and their placement as parrots is still tentative.
Parrots are one of the most characteristic components of modern avifaunas in southern continents. It has been speculated that the group had its origins in the southern supercontinent of Gondwana, but this may not necessarily be so. Many recent bird families occurring only in the Southern Hemisphere now are known to have Fossil history of parrots records from the Northern Hemisphere. Although prominent in the Southern Hemisphere at the present
Fossil history of parrots
Fig. 3 Tarsometatarsi of (left) crown-group parrot and (right) zygodactylid
bird (not to scale) (after Mayr 2015a).
time, parrots are not well represented in the fossil record of these continents, particularly in the older time periods. The record is sparse for much of the Tertiary (65–1.6 mya) and offers limited clues about the development and radiation of modern parrot groups. Most fossils are known only from the Quaternary (1.5 million years to the present) and comprise modern genera and even species.
The past few decades have been notably productive in the discovery of fossil parrots. Lambrecht (1933) in Palaeornithologie listed only five taxa under Psittaciformes, two of which were driven to extinction by Europeans during the Pleistocene.
Brodkorb (1971) in his Catalogue of Fossil Birds included only two pre-Quaternary species, five extinct Quaternary species, and a number of records of modern species recorded as fossils. Both authors included a bird now regarded as a stem-parrot but then classified elsewhere.
There now are more than 12 stem-parrot species recognized, as well as more than six extinct crown-group genera and a growing number of the Fossil history of parrots species in modern genera. The oldest putative record comes from the Late Cretaceous (Maastrichtian: 74–65 mya) Lance Formation in Niobrara County, Wyoming, United States.
Stidham (1998) based this identification on a tip of the lower bill, approximately 15 mm in length; this was a toothless, completely fused symphysis of the left and right mandibles, with small holes or foramina along the middle of the lingual surface, an absence of grooves or ridges on the walls, and a specific pattern of the neurovascular canals.
The shape of the tip was judged by Stidham to be most similar to that of lorikeets among living parrot groups. In addition to being the earliest known occurrence of parrots, this specimen would be the earliest record of a modern ‘terrestrial’ bird group.
Dyke and Mayr (1999) queried the assignment of this Fossil history of parrots as a parrot, pointing out that characters used in its identification are found in other groups of avian and non-avian vertebrates. Additionally, the morphology of this fossil differed from that of early Tertiary parrots from Europe, so it was suggested that at present the recognition of parrots from the Cretaceous should be considered tentative.
In response, Stidham (1999) noted that, although some of the characters are found in other groups, the particular states observed in the fossil occur only in parrots and, indeed, the combination of characters is unique to this group of birds. This intriguing fossil has not been restudied, nor have any more specimens been recovered that have a bearing on its identification.
In several instances, birds that originally were described as parrots subsequently have been shown not to belong to this
group. Harrison (1982) described 11 incomplete bones, putatively from a single bird, recovered from the early Eocene (57–52 mya) London Clay at Walton-on-the-Naze, in Essex, Britain, and to the same species he referred an incomplete distal tarsometatarsus from the middle Eocene (52–40 mya) of Hampshire.
Notably, the referred tarsometatarsal fragment was missing the fourth trochlea. This Fossil history of parrots was regarded as representing a parrot similar in size to the living Senegal Parrot Poicephalus senegalus, and the species was named Palaeopsittacus George.
From a comparison with skeletons of modern parrots, Harrison interpreted the morphology of the fossil bird to be more generalized than in recent forms and suggested that the wings were proportionately longer than in most living parrots, with the flight being generally less vigorous.
Features of the leg, including the position of attachment for the tendons used to pull the body towards the foot while climbing, prompted Harrison to propose ‘that climbing ability and a tendency to clamber among the branches was poorly developed or absent in Palaeopsittacus.
Other features suggested that the foot was capable of a more restricted range of movements, thus lacking the dexterity prominent in modern parrots. Olson (1985) noted that these fossils lacked most of the important features that characterize modern parrots, so referral of the specimens to the Psittacidae required confirmation.
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Fossil history of parrots
Fig. 4 Putative Cretaceous parrot bill (after Stidham 1998).
Mayr and Daniels (1998) reported on new material of Palaeopsittacus from the Walton-on-the-Naze site, including
a complete tarsometatarsus, that demonstrated that this bird was actually anisodactyl or, at best, facultatively zygodactyl and so was not a member of the Psittaciformes.
This bone further showed that the distal tarsometatarsal fragment referred to by Harrison did not belong to this taxon. Based on an analysis of relative sizes of bones originally described by Harrison, Dyke, and Cooper (2000) concluded that a proximal fragment of the ulna was not associated with the other specimens. They commented that this taxon should be ‘considered to be of uncertain status at present because of problems with the questionable association of the original specimens and their absence of any characters regarded as diagnostic at the ordinal level’.
A nearly complete, articulated post-cranial skeleton of Palaeopsittacus from Messel was described by Mayr (2002a). This provided further evidence that this genus was not related to parrots, but it did show some similarities to the frogmouths (Podargidae). Another case of misallocation to parrots occurred when Waterhouse et al.
(2008) announced two new birds from the Lower Eocene Fur Formation of Denmark. One was an indeterminate genus and species of the stem-parrot family Pseudasturidae (see below). For the other bird, the authors created a new genus Mopsitta. (The name is derived from the ‘Mo Clay’ from which this bird was recovered.) The species Mopsitta
Santa was described on the basis of a humerus, which Waterhouse et al.
considered to differ from other stem parrots by their larger size – about that of a medium-sized cockatoo – and features such as the greater curvature of the shaft. While they noted some similarities of the humerus to that of the early ibis Rhynchaeites, these were dismissed as not significant. The authors considered Mopsitta to be more similar to crown-parrots than to stem parrots and, further, that it likely was a member of Psittacidae s.l. (e.g. crown-group parrots). Mayr and Bertelli (2011) questioned the identification of Mopsitta as a parrot.
They were unable to confirm a number of the characters identified by Waterhouse et al. in support of this putative relationship, but they themselves noted features that they considered precluded the fossil being psittaciform. In contrast to Waterhouse et al., these authors noted a close resemblance between the bones of Mopsitta and those of Rhychaeites from Messel. Mayr and Bertelli considered Mopsitta to be a stem-ibis, not a parrot.
Most stem parrots are known from the Eocene (56–34 mya), mainly from the Northern Hemisphere. There are only a handful of Eocene localities producing bird fossils, of which only nine have yielded fossil parrots, and three of these are particularly important, together with having produced most of the Fossil history of parrots.
London Clay, England (early Eocene 56-49 mya) has yielded bird fossils from two main localities in the southeast of the country: Waltonon- the-Naze, Essex, and the Isle of Sheppey, Kent. Fossils from these sites are found as associated but disarticulated bones in clay nodules. The Green River Formation, Wyoming, United States
Fossil history of parrots
(53.5–48.5 mya) and Gruße Messel, western Germany (48 mya) both preserve animal and plant remains on slabs. These often are complete, although compressed, and may retain feather impressions. Stem-parrot representatives are known also from sites in France, Denmark, India, Namibia, the eastern United States, and elsewhere in Germany. Birds o comparable age to Palaeopsittacus were recorded by Mourer-Chauviré (1992) from the late Eocene (40–36 mya) Phosphorites du Quercy, at Le Bouffie, France, citing them at first only as Psittacidae.
She subsequently described them in detail from the coracoid, carpometacarpus, tibiotarsus, and tarsometatarsus (Mourer-Chauviré 1992). These birds had a combination of characters of modern parrots, such as the zygodactyl foot, and other more primitive features.
She recognized two new species, Quercypsitta sure and Q. ivani, for which she created the new, extinct family Quercypsittidae, to which also was referred Palaeopsittacus George, then still considered to be a parrot. Two interpretations of the Quercypsittidae were considered. The first was that these birds were the direct ancestors of modern parrots.
The second was that they were vicariant relatives of modern parrots, sharing a common ancestor, but evolving in the Northern Hemisphere, while other relatives evolved in the Southern Hemisphere. Eventually, the quercypsittids became extinct, while the southern birds survived and became successful. Mourer-Chauviré expressed a preference for the second of these two possibilities.
Mayr and Daniels (1998) reported on parrot-like birds from the London Clay and the middle Eocene deposits at Messel, Hessen, Germany. From the Messel specimens, they named Psittacopes Lepidus, a bird that was about the same size as a
Loriculus hanging parrot.
One of the most striking features was its bill shape; compared to that of modern parrots, the bill was short and rather shallow, particularly the maxilla, with large nostrils, and an overall superfic