- Brachytrachelopan
Taxobox|
name = "Brachytrachelopan"
fossil_range =Late Jurassic
image_width = 250px
image_caption = life restoration of "Brachytrachelopan mesai"
regnum =Animal ia
phylum = Chordata
classis =Sauropsid a
superordo =Dinosaur ia
ordo =Saurischia
subordo =Sauropodomorpha
infraordo =Sauropoda
superfamilia =Diplodocoidea
familia =Dicraeosauridae
genus = "Brachytrachelopan"
binomial = "Brachytrachelopan mesai"
binomial_authority = Rauhut "et al.",2005 Overview and holotype
"Brachytrachelopan" is an unusual short-necked
sauropod dinosaur from the latestJurassic Period (Tithonian ) ofArgentina . Theholotype and only known specimen (Museo Paleontológico Egidio Feruglio MPEF-PV 1716) was collected from anerosion al exposure offluvial sandstone within the Cañadón Cálcero Formation on a hill approximately 25 km north-northeast of Cerro Cóndor,Chubut Province , in west-centralArgentina ,South America . Though very incomplete, the skeletal elements recovered were found in articulation and include eightcervical , twelvedorsal , and three sacralvertebra e, as well asproximal portions of the posterior cervicalrib s and all the doral ribs, thedistal end of the leftfemur , the proximal end of the lefttibia , and the right ilium. Much of the specimen was probably lost to erosion many years before its discovery. Thetype species was named in honor of Daniel Mesa, a local shepherd who discovered the specimen while searching for lost sheep. Thegenus name translates as "short-necked Pan", Pan being the god of the shepherds.Paleoecology and Short Neck
This
taxon 's very short neck (approximately 40% shorter than otherdicraeosaurid s and the shortest of any known sauropod) is evidence that this lineage specialized to fill an ecological niche not exploited by other members of thisinfraorder . Small for a sauropod, "Brachytrachelopan" measured less than 33 feet (10 meters) in length. Rauhut et al. (2005, 670) note that the high degree of fusion present between the preserved neural arches and their respective centra, as well as fusion between the sacral centra, sacral neural arches, and sacral neural spines is evidence that the holotype does not represent a juvenile animal. Hence, the small body size is not a relic ofontogeny .Rauhut et al. (2005, 671-672) note that the tendency towards shorter-necks seen in dicraeosaurids, and most evident in "Brachytrachelopan", runs counter to the lengthening of the neck seen most sauropod lineages (
brachiosaurid s,titanosaur s,diplodocid s, etc.) and indicates that this group of sauropods was "progressively adapting for low browsing and might have been specialized on specific food sources, as has been suggested for "Amargasaurus " and "Dicraeosaurus "." Moreover, the morphology of the cervical neural arches in "Brachytrachelopan" would have significantly restricted dorsalflexion of neck and most likely indicates that this sauropod was specialized to a diet of plants "growing at heights of between about 1 and 2 m." Rauhut et al. (2005, 672) also suggest that diet may have been a limiting factor in body size among dicraeosaurids, and that this may have placed them in the sameecological niche as "large low-browsing iguantodontianornithopod s." It is notable that such large iguanodontians are absent from the Late JurassicGondwana nsediment s that have produced all knownfossil s of dicraeosaurids, while they are abundant in similarecosystem s of the same age inNorth America , where dicraeosaurids are absent. This may indicate that large iguanodontians and dicraeosaurids (especially "Brachytrachelopan") were ecological analogs, resulting fromparallel evolution in two distantly related dinosaurian lineages.Diagnosis and Phylogeny
Rauhut et al. (2005, 670) diagnose "Brachytrachelopan" as differing from all other sauropods in the following respects: "...individual cervical vertebrae being as long as, or shorter in anteroposterior length than, high posteriorly. Further apomorphies...include a pronounced, pillar-like centropostzygapophyseal lamina in the cervical vertebrae, a pronounced anterior inclination in the mid-cervical neural spines, with the tip of the spine extending beyond the anterior end of the centrum, and anterior dorsal neural spines one to six with vertical bases and anteriorly flexed tips."
Following a
cladistic analysis of 27 sauropod taxa and 154anatomical characters, Rauhut et al. (2005, 671-672; Fig. 2) assigned "Brachytrachelopan" to theDicraeosauridae , proposing that, within thisclade , it should be considered to have asister group relationship to the Late JurassicAfrica n taxon "Dicraeosaurus," instead of to "Amargasaurus" from theLower Cretaceous of South America. Rauhut et al. (2005, 671) conclude this is indicative of a rapidevolutionary radiation and dispersal of the Dicraeosauridae following the separation of thecontinent s of the Southern andNorthern Hemisphere s during the latestMiddle Jurassic .References
* Rauhut O.W.M., Remes K., Fechner R., Cladera G., Puerta P. (2005). Discovery of a short-necked sauropod dinosaur from the Late Jurassic period of Patagonia. "Nature" 435:670-672.
External links
* http://news.nationalgeographic.com/news/2005/06/0601_050601_stubby_dino.html
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