I've been looking forward to sharing this one for multiple reasons. Despite being one of the best known ornithopods in terms of sheer abundance, Tenontosaurus hasn't been the most comprehensively figured dinosaur outside of a small handful of descriptive and osteological papers about the two species (T. tilletti and T. dossi), all of which usually only present partial or composite reconstructions, some of which are dated in several aspects.
Check here for a rigorous version of the reconstruction: sta.sh/04yg7lszpko
Sourced from Cloverly Formation rocks in Montana, April is a rather complete specimen. The overall level of completeness is notably good, but it's still significantly outranked by a small number of other Tenontosaurus skeletons, some of which are discussed below. The skull is badly crushed, and was restored with plaster prior to its display during the late 1990s. Close to every single vertebra is at least partially present, most notably missing the hemal arches and neural spines across the middle and distal portions of the tail. At least the proximal portions of the characteristic ossified tendons were once present, but since the deconstruction of the mounted skeleton in 2004, their current whereabouts is unknown - it's possible that they may have been broken beyond recognisable repair (I only started work on the skeleton in late 2017, and beyond a few old photographs I have seen no physical evidence or documentation of them). Both sets of limbs are extremely complete, mostly undistorted, and preserved in great detail. The right manus is somewhat fragmentary, but the left manus is almost entirely accounted for, with only the tiny terminal phalanges of digits II-V missing. Only OMNH 58340 (which may the most complete specimen overall), and potentially the undescribed DinoLab skeleton exhibit more complete limbs. Multiple pathologies can be observed across April's left manus.
April is about 60-70% the length of typical Tenontosaurus adults, but the overall body proportions and discrete skeletal details are closely comparable to larger, likely more mature specimens. An exact age at the time of death has not been determined. While the specimen is complete, its physical condition is quite fragile, which may be a relic of how it was originally prepared back in the 90s. This may rule out traditional histological analysis. Palaeo-artists - if you want to scale this skeletal up to use as basic reference for an adult individual, then it is probably suitable. However, always be aware of slight proportional differences and slightly different rates in allometric growth between different individuals. The tail is the biggest caveat to consider - its length is linked to ontogeny, but even across different adult (or near-adult) individuals it can be anywhere between 60 and 75% of the total length. April's tail is on the shorter end of that spectrum - but it's still flippin' long. The skulls of some of the adult specimens (e.g. OMNH 58340) can also be slightly more elongate - it's unclear if this is due to maturity or just a product of individual variation.
The handful of missing bones were reconstructed after T. tilletti AMNH 3040 (the holotype specimen), OMNH 1013, OMNH 58340 and T. dossi FWMSH 93B1.
The amount of detail preserved in the limbs makes this skeleton a key part of my research into ornithischian biomechanics. While the tibia/fibula and metatarsals of Tenontosaurus are showing signs of shortening, the hindlimbs of Tenontosaurus are relatively long, and features associated with their forceful movement like the femoral fourth trochanter (the insertion of the caudofemoralis muscle group) are still very well-developed. Combined with the fact that the tail is long enough to faciliate a broad origin for the caudofemoralis longus muscle and to function as an effective counterbalance, I would use these observations to suggest that Tenontosaurus was a capable biped. However, despite not yet forming the compact mitten-like manus of derived and robustly quadrupedal iguanodontians, mutliple lines of evidence associated with the manual bones suggest that they were well-suited for weight bearing. The metacarpals, carpals and distal heads of the radius and ulna are notably robust, and fixed semi-pronation is exhibited due to the natural articulated position. The digits are shortened, and seem to have reduced proximal flexibility. The various phalangeal pathologies seen in April's manus may also be associated with weight bearing stresses. The manual claws of digits I and II also have flattened bases. Some past reconstructions of the manus have exhibited large manual unguals across digits I-III, but this may not be correct. As figured in Tyler Hunt's 2018 master's thesis: shareok.org/handle/11244/29981… , the articulated manus of OMNH 58340 exhibits a small terminal phalanx on digit III comparable to the equivalent element on digit IV. Previous reconstructions of a three-clawed Tenontosaurus were either due to missattribution of elements from the opposite manus, or were composites. The implications of the reduced terminal phalanx of digit III for locomotion are not completely clear. In derived iguanodontians, digit III appears to be the principle weight bearer - it's usually the most robustly constructed. This role may have been taken by digit II in Tenontosaurus. All lines of evidence considered, the preferred habitual gait of Tenontosaurus is difficult to infer. It certainly seems to have been quite capable of bipedal and quadrupedal locomotion. I was able to simulate an effective quadrupedal gait during my masters research using multi-body dynamic modelling, but this model was highly schematic.
As is the case for my previous anatomical diagrams, I greatly encourage that artists use this image as reference material, as long as reconstrucions based directly on it are accompanied by some kind of attribution.
More ornithischian anatomy coming soon...
... Do I have to mention Deinonychus in this post?
Nothing about the neural spines of the tail suggests that this would be the case - the tail muscle anatomy was more likely to have been analogous to the typical sauropsid condition. Even if we were to expand the muscles of the back and tail to their plausible extremes and cover them with realisitic amounts of fascia and fat, the profile of the tail would probably still follow these contours.
The specimen this reconstruction is primarily based on is only slightly distorted for the most part. The only significantly warped areas are the poorly preserved sacral vertebrae and crushed skull. Any minor amount of distortion has been corrected accordingly during the reconstruction process, but other than the areas highlighted, the differences would not have been immediately obvious in this lateral view anyway.
Tenontosaurus really just looks... weird.
Maybe in the future, but for now, I'll only be making skeletals where the main individual being represented is one I've worked on for research purposes, either via direct in person observation or via high-res 3D scans provided to me by my collaborators and co-authors.