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Herbstosaurus – What is it?

Tuesday, July 30, 2013 13:53
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Herbstosaurus pigmaeus CTES-PZ-1711 (late Jurassic) was originally described as a small dinosaur (Casamiquela 1975). All that is known of this specimen is a pelvis, limb and prepubis (ahhh, there’s a clue!). Ostrom (1978) more accurately identified Herbstosaurus as a pterosaur.

Figure 1. Herbstosaurus on the left compared to Coloborhynchus on the right. The sacrum is similar, but more swept back in Herbstosaurus. The femoral head is disssimilar.

Figure 1. Herbstosaurus on the left compared to the similarly-sized Coloborhynchus on the right to the same scale. The sacrum is similar, but the transverse processes are more swept back in Herbstosaurus. The anterior extent of the ilium and the femoral head/neck/shaft angle are disssimilar. Ornithocheirids are not known for a large prepubis. So, no match here.

But what kind?
Galton 1981 considered Herbstosaurus a member of the “Pterodactyloidea.”  Carroll 1988 narrowed that down to the Pterodactylidae. Wellnhofer 1991 suggested it was a more basal pterosaur, in view of the form of the pelvis. In Unwin 1996 considered it a basal member of the Dsungaripteroidea.

So, with so little consensus, what is Herbstosaurus?

Unfortunately,  
there is not enough here to run Herbstosaurus through phylogenetic analysis using the large pterosaur tree. So Let’s see where trait analysis leads us.

Ilial processes obliquely angled:
Most derived pterosaurs have this except basal pterosaurs up to MCSNB 8950.

Five+ swept back sacrals not coosified:
Most basal pterosaurs have five sacrals not coossified. This does not mean they are juveniles, unless the fossil record is seriously skewed. There are several clades of chiefly larger pterosaurs that coossify the sacrals. Other large forms don’t. Apparently only germanodactylids and their many descendants have swept back sacral transverse processes (Fig. 2) and basal forms, like Germanodactylus rhamphastinus and kin, do not coossify the sacrals.

Figure 2. Swept back sacrals and right angle femoral heads on Germanodactylus rhamphasitinus. Herbstosaurus was much larger overall.

Figure 2. Swept back sacrals and right angle femoral heads on Germanodactylus rhamphastinus. Herbstosaurus was much larger overall. Many bones here are difficult to gauge. The counterplate was also referenced.

Figure 3. The BMM Germanodactylus with an ilium, femur and prepubis shaped like Herbstosaurus, but much smaller.

Figure 3. The BMM Germanodactylus with an ilium, femur and prepubis shaped like Herbstosaurus, but overall the BMM specimen is much smaller.

Prepubis fan shape perhaps without perforation:
The fan shape appears in several clades The perforation is seen in most pterosaurs, but absent in an unassociated few. Germanodactylus often has a fan-shaped prepubis.

Femur with right angle head and neck, but head axis not aligned with neck axis.
This is a tough one. I wondered, is there some breakage that would, if repaired, align the femoral head axis with the femoral neck axis in Herbstosaurus? I’ve only seen drawings (Fig. 1). In most pterosaurs the axis of the head of the femur is very closely aligned with the axis of the neck of the femur. Not so in Herbstosaurus, hence the concern.

Size:
Substantial. No tiny pterosaurs.

Chronology:
Late Jurassic

Summary:
From the available evidence, Unwin 1996 was the most correct. Herbstosaurus is likely a large and primitive Germanodactylus-type pterosaur. Large taxa are already known from that clade. They are the elanodactylids (Fig. 4). Unfortunately what is known from the partial skeletons of those specimens does not overlap with the partial skeleton of Herbstosaurus. In any case, at present with these clues, this seems to be a “best match.”

Germanodactylus and kin

Figure 4. Click to enlarge. Germanodactylus and kin.

References
Casamiquela RM 1975. Herbstosaurus pigmaeus (Coeluria, Compsognathidae) n. gen. n. sp. del Jurásic medio del Neuquén (Patagonia septentrional). Uno de los más pequeños dinosaurios conocidos [Herbstosaurus pigmaeus (Coeluria, Compsognathidae) n. gen. n. sp. from the Middle Jurassic of Neuquén (northern Patagonia). One of the smallest known dinosaurs]. Actas del Primer Congreso Argentino de Paleontologia y Bioestratigrafia, Tucumán 2:87-103.
Galton PM 1981. A rhamphorhynchoid pterosaur from the Upper Jurassic of North America. Journal of Paleontology 55(5):1117-1122.
Ostrom JH 1978. “The osteology of Compsognathus longipes Wagner”, Zitteliana4: 73-118.
Unwin DM 2003. On the phylogeny and evolutionary history of pterosaurs. In E. Buffetaut, J.-M. Mazin (eds.), Evolution and Palaeobiology of Pterosaurs, Geological Society of London, Special Publications 217:139-190



Source: http://pterosaurheresies.wordpress.com/2013/07/31/herbstosaurus-what-is-it/

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