Unenlagia comahuensis: The Bird's Missing Link?


by Laurence D Smart B.Sc.Agr., Dip.Ed., Grad.Dip.Ed

Email: laurence@unmaskingevolution.com

Webpage: www.unmaskingevolution.com

[Free to print and distribute. Copy must be in full.]


The Background

Holotype: Specimen MCF PVPH 78 (see diagram below)

Discovery Date: 1996 (?)

Discovery Site: Sierra del Portezuelo, Patagonia, Argentina.

Figure 1 - The Patagonia Region

Estimated Size: Nearly 2m long.

Bones Excavated: The exact number of bones is unknown, but the collection totals about 13.

Bone Arrangement: The specimen was found with its bones partially attached, and in their proper anatomical position.


Figure 2 - The authors' reconstruction of the Unenlagia fossil.

(After figure 1, Nature, Vol. 387, 22 May 1997, p:391)


Rock Source: Upper Cretaceous

Associated Fossils: These rocks have also produced remains of the theropod "basal bird" Patagonykus piertai, as well as some other non-avian (non-bird) theropod dinosaurs.

Observations Made:

(1) Unenlagia had already "acquired novel, almost avian, forelimb movements". [p:391]

(2) Unenlagia combines a mixture of dinosaur and bird characters. "Unenlagia is a true mosaic ..." [p:350]

Method of Analysis:

(1) Eighty of Unenlagia's bone features were scored and put into a table. The data "was processed using the implicit enumeration option in Hennig 86" [p:392] to produce many cladograms.

(2) The best cladogram chosen was the "most parsimonious tree" which consisted of "116 steps, a consistency index of 0.68, and a retention index of 0.71." [p:392]


Figure 3 - The authors' chosen cladogram showing the possible relationship between

Unenlagia and other fossils.

(After figure 3, Nature, Vol. 387, 22 May 1997, p:391)


Results of the Analysis: "Unenlagia slots in cladistically between dromaeosauid [dinosaurs] and Archaeopteryx." [p:349]

Conclusion Drawn: Unenlagia represents "an intermediate" [p:391] between dinosaurs and birds.



What the General Public needs to Know from the Scientific Report



(1) For the past 20 years there has been continuing "dissent" [p:349] among scientists that the birds evolved from the theropod dinosaurs. (see also Biol. J. Linn. Soc. Vol. 8, 1976, p:91-182)

(2) The coelurosaurs (eg Deinonychus and Velociraptor) have not been proved to be the ancestors of the birds. Instead, they are "usually regarded as the closest relatives" of the birds. [p:349]

(3) It is still a theory that the dinosaurs evolved into birds, and this fossil does not prove the transformation. "The new evidence, however, reinforces the hypothesis that a bipedal, cursorial theropod was ancestral to birds." [p:392]



(1) The evolution of flight is only a theory. The authors state that Unenlagia's forelimb design and 'function' "do not suggest that there was a gliding phase before the acquisition of the avian flight". The features are "more consistent with the idea" that there was a flapping stage. [p:392]

(2) Again, they reiterated, "[A]n arboreal climbing model may be as valid as a terrestrial predation model in explaining the transition to birds." [p:350]



(1) Unenlagia was inferred to be flightless - there is no proof that it was. "The large size of Unenlagia, together with its proportionally short forelimb, ... argue that this animal was flightless ..." [p:391]

(2) The authors guessed that Unenlagia "probably" [p:392] used its arms to maintain its balance and controlled its body while it ran and leapt to catch its prey.

(3) There is no evidence that the arrangement of the forelimb bones allowed Unenlagia to fly. "Although it is tempting to speculate that feathers, and perhaps even incipient flight, honed these bird-like attributes of Unenlagia, there is as yet no evidence for this." [p:350]

(4) No feathers were collected with the fossil, nor were there any feather impressions in the rock. "Whether Unenlagia was feathered or not is unknown." [p:392]



(1) The basis for the intermediate status of Unenlagia is mainly based on the forearm attachment to the scapula (shoulder blade). This attachment was interpreted by the scientists to indicate that the forelimb could be folded and stretched like a bird. It is an interpretation of a few bones, not an observed fact.

(2) The orientation of the scapula in the analysis was "based on the assumption" [p:391] that it faced the same direction as the theropod's and the bird's. This is interesting when the characteristic that gives the fossil bird-like qualities is the movement of the forelimb, which was actually determined by the way the scapula faces. This angle determines the extent to which the humerus moves, providing flying movements.

(3) The bird-like forearm movements are only an interpretation of the bones - they are not facts. "The structure of the forelimb suggests that the avian mode of forelimb folding, and the extensive forelimb elevation necessary for powered, flapping flight, was already in cursorial, non-flying theropod dinosaurs." [p:390]

(4) The Nature review article stated that the authors' interpretation of the angle of the scapula "will raise the most controversy" (p:350). This is because the scapula can be placed in a position like a bird, or it can be placed in a position like a reptile. "This interpretation will be controversial because the orientation of the shoulder socket is dependent on the orientation of the scapula, which is not fixed to the rib cage, but rather 'floats' in muscle" [p:350].



(1) The reviewer stated that Unenlagia "may help to fill in the details of the functional transition between reptiles and birds" [p:349]. This is not the same as the actual intermediate between reptiles and birds.

(2) Unenlagia' status as an ancestor between the dinosaurs and Archaeopteryx is inferred by a cladogram. However, a cladogram really only shows similarity in the possession of characteristics, not genealogy.

(3) Note that the cladogram is magically converted into a phylogeny (genealogy) to 'prove' the evolution of birds. Unenlagia "... is a critical testament of the phylogenetic transition ..." of reptiles to birds. [p:350]



(1) This fossil is well out of chronological sequence in the geological column but it is still accepted as an intermediate. "Despite the relatively late appearance of this dinosaur in the fossil record (Upper Cretaceous), several features of Unenlagia are more bird-like than in any other non-avian theropod so far discovered." [p:390]

(2) Note that the youngest fossil is the one that 'evolved' into birds, even though birds had already evolved. It is "much more recent than the oldest birds". [New Scientist, 1 Feb, 1997 p:28]




90 myo

Unenlagia comahuensis


90 myo

Patagonykus piertai

"basal" bird *

115 myo

Eoalulavis hoyasi

archaic 'opposite' bird

120-130 myo

Iberomesornis romerali

archaic 'opposite' bird

121-142 myo

Confuciiusornis sanctus

archaic 'opposite' bird

121-142 myo


archaic 'true' bird

121-142 myo

Sinosauropteryx prima

"feathered" dinosaur **

130 myo

Archaeopteryx lithographica

primitive 'true' bird

190 myo

Paleopteryx thomsoni

'true' bird

205 myo

Protoavis texensis

'true' bird

*- a dinosaur, not a bird (see Journal of Vertebrate Paleontology 1Vol.17. No.1, 1997 p:137)

** - not real feathers. (see John Ostrom in New Scientist, 12 April, 1997 p:13)



What can be Concluded?


(1) Unenlagia may fit anatomically between dinosaurs and birds, depending on the scapula angle.

(2) Unenlagia is over 100 million years out of sequence and can't be intermediate based on the geologic column.

(3) All other conclusions are speculation, based on circumstantial evidence




Source - Nature, Vol. 387, 22 May 1997, p:349-350 & 390-392