Monday, May 10, 2010

Furcula and Interclavicle

"The furcula ("little fork" in Latin) is a forked bone found in birds and thecodonts, formed by the fusion of the two clavicles. In birds, its function is the strengthening of the thoracic skeleton to withstand the rigors of flight."

Concerning pterosaur clavicles:
"In pterosaurs the clavicle was incorporated into the sternum."
"David Unwin, 2006. The Pterosaurs from Deep Time."
"Although it looks like a single bone, studies of an immature pterosaur show that it was made up of several elements, including the clavicles, which were thought to have been lost, but now appear to have been hiding in the breast bone all along."
He's referring to Wild, R. 1993. A juvenile specimen of Eudimorphodon ranzii Zambelli (Reptilia, Pterosauria) from the Upper Triassic (Norian) of Bergamo. Rivista del Museo civico di Scienze Naturali “E. Caffi” Bergamo 16:95-120
The [Rhamphorhynchus] pelvic and pectoral girdles fused as the animals aged, with full pectoral fusion attained by one year of age.[12]
Tyrannosaurus rex.-The only undisputed furcula of Tyrannosaurus rex belongs to an articulated postcranial skeleton (UCRC Vl) preserved in a large sandstone concretion from the Lance Formation (Maastrichtian) of eastern Wyoming (Lipkin and Sereno, 2004). The bones of the trunk are preserved in articulation with little transverse or dorsoventral distortion. The furcula is preserved in situ between the right and left coracoids with each clavicular ramus laying near the acrominal process of the scapula (Fig. 3). The distal ends of both clavicular rami are broken away at the edge of the concretion.
Among other features, birds have a structure that they share with dromaeosaurs: a fused clavicle (collarbone) called the furcula (wishbone), which serves as a brace during the flight stroke.
The [juvenile Scansoriopteryx] furcula is not present, but is instead represented by separate clavicles.

Nesbitt (2009)
11. Furcula shape in anterior view: asymmetrical
(0) or symmetrical/nearly symmetrical (1).
The furculae of most nonparavian theropods
are markedly asymmetrical (e.g., Allosaurus,
Citipati). The furculae of paravians, with the
exception of Buitreraptor, are nearly symmetrical.

It is unclear if the asymmetry of the furcula
of Buitreraptor was the result of taphonomy
or represents morphological asymmetry.
We see that the dinosaur furcula is not like the Paravian furcula.
That supports the conclusion that they are not related.

213. Clavicles: (0) present and unfused; (1)
fused into a furcula (modified from Gauthier,
1986; Sereno, 1991a; Benton, 1999; Benton
and Walker, 2002).
Clavicles are present in non-archosaurian
archosauriforms and basal crocodylian-line
archosaurs. Clavicles are not present in
crocodylomorphs (e.g., Hesperosuchus ‘‘agilis,’’
CM 29894; Protosuchus richardsoni,
AMNH FR 3024) and, therefore, they are
scored as inapplicable. Like the interclavicle, the clavicles of the pterosaur Eudimorphodon are separate ossifications in a small specimen and incorporated into the sternum (Wild, 1993). All other pterosaurs seem to lack distinct ossifications of the clavicles.
214. Interclavicle: (0) present; (1) absent
(fig. 30) (Gauthier, 1986; Sereno, 1991a; Juul,
1994; Benton, 1999).
The interclavicle is present in archosauriforms
plesiomorphically (Sereno, 1991a) and
persists through Pseudosuchia. In Pterosauria,
an interclavicle appears to be present
in young individuals of Eudimorphodon
(MCSNB 8950), but fuse to the pectoral
elements in larger individuals (Wild, 1993)
. A
distinct interclavicle is not present in all other
pterosaurs. Ornithischians and saurischians
lack an interclavicle.
Traditionally, pterosaurs have been considered to lack or have had greatly reduced clavicles and interclavicles (Gauthier, 1986; Sereno, 1991). Recently, Wild (1993) has demonstrated that the clavicles and the interclavicle are present but completely incorporated with the sternum into a single sternal element in the basal pterosaur Eudimorphodon ranzii
An interclavicle is a bone which, in most tetrapods, is located between the claviclesIn birds, the interclavicle is fused with the clavicles to form the furcula (wishbone). In chickens the furcula forms a "Y" shape and the interclavicle is the stem of the "Y".
a broad [basal pterosaur] sternum formed from the fusion of the interclavicle, paired clavicles and paired sternal plates.
Also from Nesbitt et al (2009) (page 872):
The evolutionary transformation of the furcula from separate clavicles is nicely illustrated in archosaurs and their close relatives. Euparkeriacapensis and early pseudosuchians retain both the interclavicle and clavicles. The interclavicle is lost at the dinosaur node or at an unknown node among early dinosauromorphs. From there, the origin of the furcula is well understood with the addition of the work on basal sauropodomorphs of Yates and Vasconcelos (2005) and the discovery of furculae in early coelophysoid theropods (Tykoskiet al., 2002; Rinehart et al., 2007). Some ornithischians have two small clavicles that do not contact each other (Osborn, 1924a; Brown andSchlaikjer, 1940; Sternberg, 1951; Chinnery andWeishampel, 1998). In contrast, saurischians retain clavicles with the clavicles contacting at the midline.
That meant the former sliding coracoid disc upon the interclavicle and clavicle was reduced to an immovable stem inserted into a cup, as in pterosaurs and birds.

Furcula is used for a very different purpose. Convergent evolution?
"act as a spring"

Also see the Nesbitt 2009 reference here:
interclavicle is unossified in Archaeopteryx as in juvenile stages of modern birds.
we do not know if the interclavicle has been lost or simply fused with the sternum in many extinct forms.
We know that some dinosaurs had a furcula. But we still don't know whether the avian furcula represents an ancestral interclavicle or ancestral fused clavicles, or is a neomorph. Clavicles remain enigmatic skeletal elements that illustrate the need for further research in developmental and evolutionary skeletal biology.

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