Matt’s last post contained a nice overview of the occurrence of epipophyses in sauropodomorphs: that is, bony insertion points for epaxial ligaments and muscles above the postzygapophyseal facets. What we’ve not mentioned so far is that these structures are not limited to sauropods. Back when we were preparing one of the earlier drafts of the paper that eventually became Why sauropods had long necks; and why giraffes have short necks (Taylor and Wedel 2013a), I explored their occurrence in related groups. But that section never got written up for the manuscript, and now seems as good a time as any to fix that.

Theropods (including birds)

Most obviously, epipophyses occur in theropods, the sister group of sauropodomorphs.

Taylor and Wedel (2013a: figure 11). Archosaur cervical vertebrae in posterior view, Showing muscle attachment points in phylogenetic context. Blue arrows indicate epaxial muscles attaching to neural spines, red arrows indicate epaxial muscles attaching to epipophyses, and green arrows indicate hypaxial muscles attaching to cervical ribs. While hypaxial musculature anchors consistently on the cervical ribs, the principle epaxial muscle migrate from the neural spine in crocodilians to the epipophyses in non-avial theropods and modern birds, with either or both sets of muscles being significant in sauropods. 1, fifth cervical vertebra of Alligator mississippiensis, MCZ 81457, traced from 3D scans by Leon Claessens, courtesy of MCZ. Epipophyses are absent. 2, eighth cervical vertebra of Giraffatitan brancai paralectotype HMN SII, traced from Janensch (1950, figures 43 and 46). 3, eleventh cervical vertebra of Camarasaurus supremus, reconstruction within AMNH 5761/X, “cervical series I”, modified from Osborn and Mook (1921, plate LXVII). 4, fifth cervical vertebra of the abelisaurid theropod Majungasaurus crenatissimus,UA 8678, traced from O’Connor (2007, figures 8 and 20). 5, seventh cervical vertebra of a turkey, Meleagris gallopavo, traced from photographs by MPT.

Taylor and Wedel (2013a: figure 11). Archosaur cervical vertebrae in posterior view, Showing muscle attachment points in phylogenetic context. Blue arrows indicate epaxial muscles attaching to neural spines, red arrows indicate epaxial muscles attaching to epipophyses, and green arrows indicate hypaxial muscles attaching to cervical ribs. While hypaxial musculature anchors consistently on the cervical ribs, the principle epaxial muscle migrate from the neural spine in crocodilians to the epipophyses in non-avial theropods and modern birds, with either or both sets of muscles being significant in sauropods. 1, fifth cervical vertebra of Alligator mississippiensis, MCZ 81457, traced from 3D scans by Leon Claessens, courtesy of MCZ. Epipophyses are absent. 2, eighth cervical vertebra of Giraffatitan brancai paralectotype HMN SII, traced from Janensch (1950, figures 43 and 46). 3, eleventh cervical vertebra of Camarasaurus supremus, reconstruction within AMNH 5761/X, “cervical series I”, modified from Osborn and Mook (1921, plate LXVII). 4, fifth cervical vertebra of the abelisaurid theropod Majungasaurus crenatissimus,UA 8678, traced from O’Connor (2007, figures 8 and 20). 5, seventh cervical vertebra of a turkey, Meleagris gallopavo, traced from photographs by MPT.

In this figure from the 2013 paper, the rightmost images show cervical vertebrae of Majungasaurus (an abelisaurid theropod) and a turkey, both in posterior view. The red arrows indicate epaxial musculature pulling on the epipophyses. They are particularly prominent in Majungasaurus, rising almost a full centrum’s height above the postzygapophyseal facets.

The epipophyses are very prominent in the anterior cervicals of Tyrannosaurus, but much less so in its posterior cervicals — presumably because its flesh-tearing moves involved pulling upwards more strongly on the anterior part of the neck. Here’s a photo of the AMNH mount, from our post T. rex‘s neck is pathetic:

amnh-tyrannosaurus-is-pathetic

You can see something similar in the neck of Allosaurus, and the trend generally seems to be widespread among theropods.

Ornithischians

Note the very prominent epipophyses protruding above the postzygs in the anterior cervicals of this Heterodontosaurus in the AMNH public gallery:

Cast of AMNH 28471, Heterodontosaurus tucki, collected from the Early Jurassic Voisana, Herschel district, South Africa. Anterior to the left.

Cast of AMNH 28471, Heterodontosaurus tucki, collected from the Early Jurassic Voisana, Herschel district, South Africa. Neck in left lateral view.

Here’s the hadrosaur Corythosaurus:

AMNH 5338, Corythosaurus casuarius, from the Campanian of the Red Deer River, Alberta, Canada. Collected by Barnum Brown and P. C. Kaisen, 1914. Cervicals 1-4 in right lateral view.

AMNH 5338, Corythosaurus casuarius, from the Campanian of the Red Deer River, Alberta, Canada. Collected by Barnum Brown and P. C. Kaisen, 1914. Cervicals 1-4 in right lateral view.

The prominent vertebra is C2: note that is has both a modest blade-like neural spine and prominent epipophyses — but that already by C3 the epipophyses are gone. Here is that C2 postzyg/epipophyses complex is close-up, clearly showing anteroposteriorly directed striations on the epipophysis, presumably representing the orientation of the attaching ligaments and muscles:

As previous image: close-up of posterior part of C2.

As previous image: close-up of posterior part of C2.

Here’s a close-up of the neck of the boring ornithopod Tenontosaurus, also in the AMNH gallery. (I’m not sure of the specimen number — if anyone can clarify, please leave a comment).

AMNH ?3554, Tenontosaurus tilletti, cervcials 2-4 in right lateral view.

AMNH ?3554, Tenontosaurus tilletti, cervicals 2-4 in right lateral view.

The interesting thing here is that it its axis (C2) seems to lack epipophyses (unlike C3), and to have a tall blade-like neural spine, as seen in mammals. We don’t really see C2 spines this big in other dinosaurs — compare with the much more modest spine in Corythosaurus, above. The texture of this part of the Tenontosaurus specimen looks suspicious, and I wonder whether that neural spine is a fabrication, created back in the day by AMNH staff who were so used to mammals that they “knew” what a C2 should look like? Anyway, the epipophysis above the postzyg of C3 is very distinct and definitely real bone.

Pterosaurs

Things get much more difficult with pterosaurs, because their cervicals are so fragile and easily crushed (like the rest of their skeleton, to be fair). While it’s easy to find nice, well-preserved ornithischian necks on display, you don’t ever really see anything similar for pterosaurs.

As a result, we have to rely on specimen photographs from collections, or more often on interpretive drawings. Even high-resolution photos, such as the one in Frey and Tischlinger (2012: fig 2) tend not to show the kind of detail we need. Usually, the only usable information comes from drawings made by people who have worked on the specimens.

Here, for example, is Rhamphorhynchus, well known as the most difficult pterosaur to spell, in figure 7 from Bonde and Christiansen’s (2003) paper on its axial pneumaticity:

BondeChristiansen2003-axial-pneumaticity-of-rhamphorhynchus-fig7It’s not the main point of the illustration, but you can make out clear epipophyses extending posteriorly past the postzygapophyseal facets in at least C3 and C5 — in C4, the relevant area is obscured by a rib. (Note that the vertebrae are upside down in this illustration, so you need to be looking towards the bottom of the picture.)

I’m pretty sure I’ve seen a better illustration of Rhamphorhynchus epipophyses, but as I get older my memory for Rhamphorhynchus epipophyses is no longer what it used to be and I can’t remember where. Can anyone help?

But also of interest is the azhdarchid pterosaur Phosphatodraco, here illustrated by Pereda Suberbiola et al. (2003):

Pereda Suberbiola et al. (2003: fig. 3). Phosphatodraco mauritanicus gen. et sp. nov, OCP DEK/GE 111, Late Cretaceous (Maastrichtian), Morocco: (a) cervical five in two fragments, ventral and left lateral views; (b) cervical six in ventrolateral view; (c) cervical seven in ventral view; (d) cervical eight in left lateral view; (e) cervical nine in posterior view; (f) cervical six in anterior view. c, centrum; co, condyle; ct, cotyle; hyp, hypapophysis; nc, neural canal; ns, neural spine; poe, postexapophysis; poz, postzygapophysis; prz, prezygapophysis; su, sulcus; tp, transverse process.

Pereda Suberbiola et al. (2003: fig. 3). Phosphatodraco mauritanicus gen. et sp. nov, OCP DEK/GE 111, Late Cretaceous (Maastrichtian), Morocco: (a) cervical five in two fragments, ventral and left lateral views; (b) cervical six in ventrolateral view; (c) cervical seven in ventral view; (d) cervical eight in left lateral view; (e) cervical nine in posterior view; (f) cervical six in anterior view. c, centrum; co, condyle; ct, cotyle; hyp, hypapophysis; nc, neural canal; ns, neural spine; poe, postexapophysis; poz, postzygapophysis; prz, prezygapophysis; su, sulcus; tp, transverse process.

The cervicals of Phosphatodraco seem to have no epipophyses. So they were not ubiquitous in pterosaurs.

What does it all mean? This post has become a bit of a monster already so I’ll save the conclusion for another time. Stay tuned for more hot epipophyseal action!

References

  • Bonde, Niels and Per Christiansen. 2003. The detailed anatomy of Rhamphorhynchus: axial pneumaticity and its implications. pp 217-232 in: E. Buffetaut and J-M Mazin (eds), Evolution and Palaeobiology of Pterosaurs. Geological Society, London, Special Publications 217. doi:10.1144/GSL.SP.2003.217.01.13
  • Frey Eberhard and Helmut Tischlinger. 2012. The Late Jurassic Pterosaur Rhamphorhynchus, a Frequent Victim of the Ganoid Fish Aspidorhynchus? PLoS ONE 7(3):e31945. doi:10.1371/journal.pone.0031945
  • Janensch, Werner. 1950. Die Wirbelsaule von Brachiosaurus brancai. Palaeontographica, Supplement 7 3:27-93.
  • O’Connor Patrick M. 2007. The postcranial axial skeleton of Majungasaurus crenatissimus (Theropoda: Abelisauridae) from the Late Cretaceous of Madagascar. pp 127-162 in: S. D. Sampson., D. W. Krause (eds), Majungasaurus crenatissimus (Theropoda: Abelisauridae) from the Late Cretaceous of Madagascar. Society of Vertebrate Paleontology Memoir 8.
  • Osborn, Henry F., and Charles C. Mook. 1921. Camarasaurus, Amphicoelias and other sauropods of Cope. Memoirs of the American Museum of Natural History, New Series 3:247-387.
  • Pereda Suberbiola, Xabier, Nathalie Bardet, Stéphane Jouve, Mohamed Iarochène, Baadi Bouya and Mbarek Amaghzaz. 2003. A new azhdarchid pterosaur from the Late Cretaceous phosphates of Morocco. pp 79-90 in: E. Buffetaut and J-M Mazin (eds), Evolution and Palaeobiology of Pterosaurs. Geological Society, London, Special Publications 217. doi:10.1144/GSL.SP.2003.217.01.08
  • Taylor, Michael P., and Mathew J. Wedel. 2013. Why sauropods had long necks; and why giraffes have short necks. PeerJ 1:e36 doi:10.7717/peerj.36

Introduction and Background

2005-09-27 CM 555 c6 480

An epipophysis in a neural arch of a juvenile Apatosaurus, CM 555. From this post.

I have three goals with this post:

  1. To document the range of variation in epipophyses in the cervical vertebrae of sauropods.
  2. To show that the “finger-like processes” overhanging the cervical postzygapophyses in the newly described Qijianglong are not novel or mysterious structures, just very well developed epipophyses.
  3. Finally, to show that similar long, overhanging epipophyses are present in other mamenchisaurids, although as far as I can tell no-one has noted them previously.

Epipophyses are muscle attachment points dorsal to the postzygapophyses, for the insertion of long, multi-segment epaxial (dorsal) neck muscles in birds and other dinosaurs. I know that they turn up occasionally in non-dinosaurian archosaurs, and possibly in other amniotes, but for the purposes of this post I’m only considering their distribution in sauropods. For some quick background info on epipophyses and the muscles that attach to them, see the second half of this post, and see Wedel and Sanders (2002) and Taylor and Wedel (2013a) for further discussion and more pictures.

OMNH emu vert 480

Before we start with the pictures, a fiddly nomenclatural point: this muscle attachment point dorsal to the postzyg has traded under at least six names to date.

  1. The ‘Owenian’ term, used by virtually all non-avian theropod workers, by Sereno et al. (1999) for Jobaria, and probably by loads of other sauropod workers (including myself, lately) is epipophysis.
  2. Beddard (1898) referred to this feature in birds as the hyperapophysis; this term seems to have fallen completely out of use.
  3. Boas (1929), again referring to birds, called it the processus dorsalis. Zweers et al. (1987: page 138 and table 1) followed this terminology, which is how I learned of it when I was an undergrad at OU.
  4. Baumel and Witmer (1993) called this feature in birds the torus dorsalis (note 125 on page 87), which some authors have informalized to dorsal torus (e.g., Harris 2004: page 1243 and fig. 1). Baumel and Witmer (1993: page 87) note that, “the use of ‘Torus’ is preferable since it avoids confusion with the spinous [dorsal] process of the neural arch”.
  5. In my own early papers (e.g., Wedel et al. 2000b) and blog posts I called this feature the dorsal tubercle, which was my own attempt at an informal term matching ‘processus dorsalis’ or ‘torus dorsalis’. That was unfortunate, since there are already several other anatomical features in vertebrates that go by the same name, including the dorsal-facing bump on the dorsal arch of the atlas in many vertebrates, and a bump on the humerus in birds and some other taxa. In more recent papers (e.g., Taylor and Wedel 2013a) I’ve switched over to ‘epipophysis’.
  6. In the last post, Mike coined the term parapostzygapophysis for this feature in Qijianglong. [Note: he now regrets this.]

As usual, if you know of more terms for this feature, or additional history on the ones listed above, please let us know in the comments.

Now, on to the survey.

Prosauropods

Leonerasaurus_cervical_vertebrae - Pol et al 2011 fig 5

I haven’t seen very many prominent epipophyses in basal sauropodomorphs. Probably the best are these in the near-sauropod Leonerasaurus, which is very sauropod-like in other ways as well. Modifed from Pol et al. (2011: fig. 5).

This combination of photograph and interpretive drawing neatly shows why it’s often difficult to spot epipophyses in photos: unless you can make out the postzygapophyseal facet, which is often located more anteriorly than you might guess, you can’t tell when the epipophysis projects further posteriorly, as in the last of these vertebrae. In this case you can make it out, but only because the interpretive drawing shows the facet much more clearly than the photo.

Basal sauropods

Tazoudasaurus cervical - Allain and Aquesbi 2008 fig 9i-j

The most basal sauropod in which I have seen clear evidence of epipophyses is Tazoudasaurus. They’re not very apparent in lateral view, but in posterior view the epipophyses are clearly visible as bumps in the spinopostzygapophyeal laminae (SPOLs). Modified from Allain and Aquesbi (2008: fig. 9).

Jobaria epipophyes

In addition to Qijianglong, some other basal eusauropods have prominent epipophyses. Probably the best known is Jobaria; Sereno et al. (1999: fig. 3) figured and labeled the epipophysis in one of the cervical vertebrae. The vertebra image in that figure is tiny (nice work, glam-magz!), so here are some sketches of Jobaria mid-cervicals (from two different individuals) that I made back in the day when I was doing the research for Gary Staab’s Jobaria neck sculpture (see Sanders et al. 2000 for our SVP abstract about that project).

Turiasaurus also has prominent, overhanging epipophyses in at least some of its cervical vertebrae. You can just make one out as a tiny spike a few pixels long in Royo-Torres et al. (2006: fig. 1K). I have seen that cervical firsthand and I can confirm that the epipophyses in Turiasaurus are virtually identical to those in Jobaria.

Other mamenchisaurids

It’s not air-tight, but there is suggestive evidence of projecting epipophyses in some other mamenchisaurids besides Qijianglong.

Mamenchisaurus epipophyses - lateral view

If you’re really hardcore, you may remember that back in 2005, Mike got to go up on a lift at the Field Museum of Natural History to get acquainted with a cast skeleton of Mamenchisaurus hochuanensis that was mounted there temporarily. During that adventure he took some photos that seem to show projecting epipophyses in at least two of the mid-cervicals. At least, if they’re not epipophyses, I don’t know what they might be.

Mamenchisaurus epipophyses - medial view

Here they are again in medial view. My only reservation is that these vertebrae were distorted to begin with, and some features of the cast are very difficult to interpret. So, probably epipophyses, but it would be nice to check the original material at some point.

Mamenchisaurus youngi epipophyses

Something similar may be present in some posterior cervical vertebrae of Mamenchisaurus youngi. Here’s Figure 17 from Ouyang and Ye (2002). The “poz” label does not not seem to be pointing to the articular facet of the postzygapophysis, which looks to be a little more anterior and ventral, below the margin of the PODL. If that’s the case, then C15 has long, overhanging epipophyses like those of Jobaria. C16 has a more conservative bump, which is to be expected – the epipophyses typically disappear through the cervico-dorsal transition.

Omeisaurus epipophysis

Finally, here’s a cervical vertebra of Omeisaurus junghsiensis from Young (1939: fig. 2). I don’t want to hang very much on just a few pixels, but my best guess at the extent of the postzygapophyseal articular facet is shown in the interpretation above. If that’s correct, then this specimen of Omeisaurus had really long epipophyses, rivaling those of Qijianglong. Unfortunately that’s impossible to check, because this specimen has been lost (pers. comm. from Dave Hone, cited in Taylor and Wedel 2013).

Diplodocoidea

Haplocanthosaurus epipophyses - Hatcher 1903

Haplocanthosaurus nicely shows that the epipophyses can be large in terms of potential muscle attachment area without projecting beyond the posterior margins of the postzygapophyses. Here is C14 of H. priscus, CM 572, in posterior and lateral views, modified from Hatcher (1903: plate 1).

diplodocid epipophyses

Epipophyses that actually overhang the postzygapophyses are not common in Diplodocidae but they do occasionally occur. Here are prominent, spike-like epipophyses in Diplodocus (upper left, from Hatcher 1901: plate 3), Barosaurus (upper right), Kaatedocus (lower left, Tschopp and Mateus 2012: fig. 10), and Leinkupal (lower right, Gallina et al. 2014: fig. 1).

NIgersaurus cervical - Sereno et al 2007 fig 3

Of course, the champion epiphysis-bearer among diplodocoids is the weird little rebbachisaurid Nigersaurus. Here’s a Nigersaurus mid-cervical, from Sereno et al. (2007: fig. 3). Note that the projecting portions of the epipophysis is roughly as long as the articular surface of the postzygapophysis.

Macronaria

Australodocus epipophysis

The epipophysis in this cervical of Australodocus just barely projects beyond the posterior margin of the postzygapophysis.

Giraffatitan c8 epipophyses

In Giraffatitan, epipophyses are absent or small in anterior cervicals but they are prominent in C6-C8. Here’s a posterolateral view of C8, showing very large epipophyses that are elevated several centimeters above the postzygapophyses. You can also see clearly in this view that the spinopostzygapophyseal lamina (SPOL) and postzygodiapophyseal lamina (PODL) converge at the epipophysis, not the postzygapophysis itself.

Sauroposeidon epipophyses

The holotype of Sauroposeidon, OMNH 53062, is similar to Giraffatitan in that the two anterior cervical vertebrae (possibly C5 and C6) have no visible epipophyses, but epipophyses are prominent in the two more posterior vertebrae (possibly C7 and C8). Click to enlarge – I traced the articular facet of the postzygapophysis in ?C8 to more clearly separate it from the epipophysis. For a high resolution photograph of that same vertebra that clearly shows the postzyg facet and the epipophysis dorsal to it, see this post.

Oddly enough, I’ve never seen prominent epipophyses in a titanosaur. In Malawisaurus, Trigonosaurus, Futalognkosaurus, Rapetosaurus, Alamosaurus, and Saltasaurus, the SPOLs (such as they are – inflated-looking titanosaur cervicals do not have the same crisply-defined laminae seen in most other sauropods) merge into the postzygapophyseal rami and there are no bumps sticking up above or out beyond the articular facets of the postzygs. I don’t know what to make of that, except to note that several of the animals just mentioned have mediolaterally wide, almost balloon-shaped cervical neural spines. In our 2013 PeerJ paper, Mike and I argued that the combination of tall neural spines and tall epipophyses in the cervical vertebrae of sauropods made them functionally intermediate between crocs (huge neural spines, no epipophyses) and birds (small or nearly nonexistent neural spines, big epipophyses). Perhaps most titanosaurs reverted to a more croc-like arrangement with most of the long epaxial neck muscles inserting on the neural spine instead of the postzygapophyseal ramus. I’ve never seen that possibility discussed anywhere, nor the apparent absence of epipophyses in most titanosaurs. As usual, if you know otherwise, please let me know in the comments!

malawisaurus-cervicals

Cervical vertebrae of Malawisaurus from Gomani (2005: fig. 9): not an epipophysis in sight. But check out the spike-like neural spines – these are so wide from side to side that from the front they look like party balloons.

And as long as we’re discussing the phylogenetic distribution of epipophyses, it is interesting that long, overhanging epipophyses are so broadly but sporadically distributed. They turn up in some non-neosauropods (Jobaria, Turiasaurus, Omeisaurus) and some diplodocoids (Nigersaurus, the occasional vertebra in Diplodocus and Leinkupal), but not in all members of either assemblage, and they seem to be absent in Macronaria (although many non-titanosaurs have shorter epipophyses that don’t overhang the postzygs). I strongly suspect that a lot of this is actually individual variation that we’re not perceiving as such because our sample sizes of almost all sauropods are tiny, usually just one individual. Epipophyses are definitely muscle attachment sites in birds and no better hypothesis has been advanced to explain their presence in other archosaurs. Muscle attachment scars are notoriously variable in terms of their relative development and expression among individuals, and it would be odd if epipophyses were somehow exempt from that inherent variability.

It also seems more than likely that ontogeny plays a role: progressive ossification of tendons attached at the epipophyses would have the effect of elongating the preserved projection. And since for some aspects of sauropod vertebral morphology, serial position recapitulates ontogeny (Wedel and Taylor 2013b), it shouldn’t be surprising that we see differences in the prominence of the epipophyses along the neck.

Back to Qijianglong

By now it should be clear that the “finger-like processes” in Qijianglong are indeed epipophyses, and although they are quite long, they aren’t fundamentally different from what we see in many other sauropods. I haven’t gone to the trouble, but one could line up all of the vertebrae figured above in terms of epipophysis size or length, and Qijianglong would sit comfortably at one end with Omeisaurus and Mamenchisaurus, just beyond Nigersaurus and Jobaria.

FIGURE 11. Anterior cervical series of Qijianglong guokr (QJGPM 1001) in left lateral views unless otherwise noted. A, axis; B, cervical vertebra 3; C, cervical vertebra 4; D, cervical vertebrae 5 and 6; E, cervical vertebra 7 and anterior half of cervical vertebra 8 (horizontally inverted; showing right side); F, posterior half of cervical vertebra 8 and cervical vertebra 9; G, cervical vertebra 10; H, cervical vertebra 11; I, close-up of the prezygapophy- sis-postzygapophysis contact between cervical vertebrae 3 and 4 in dorsolateral view, showing finger-like process lateral to postzygapophysis; J, close- up of the postzygapophysis of cervical vertebra 5 in dorsal view, showing finger-like process lateral to postzygapophysis. Arrow with number indicates a character diagnostic to this taxon (number refers to the list of characters in the Diagnosis). All scale bars equal 5 cm. Abbreviations: acdl, anterior centrodiapophyseal lamina; cdf, centrodiapophyseal fossa; plc, pleurocoel; pocdl, postcentrodiapophyseal lamina; poz, postzygapophysis; pozcdf, post- zygapophyseal centrodiapophyseal fossa; pozdl, postzygodiapophyseal lamina; ppoz, finger-like process lateral to postzygapophysis; ppozc, groove for contact with finger-like process; przdl, prezygodiapophyseal lamina; sdf, spinodiapophyseal fossa.

Cervical vertebrae of Qijianglong (Xing et al. 2015: fig. 11)

The strangest thing about the epipophyses in Qijianglong is that they seem to be bent or broken downward in two of the vertebrae (B and H in the figure above). I assume that’s just taphonomic distortion – the cervical shown in H wouldn’t even be able to articulate with the vertebra behind it if the epipophysis really drooped down like that. The epipophyses in Qijianglong seem to mostly manifest as thin spikes of bone (or maybe plates, as shown in B and I), so it’s not surprising that they would get distorted – most of the vertebrae shown above have cervical ribs that are incomplete or missing as well.

One more noodle-y thought about big epipophyses. I wrote in the last section that I’ve never seen them in titanosaurs, possibly because titanosaurs have big neural spines for their epaxial muscles to attach to. Maybe long, overhanging epipophyses are so common in mamenchisaurids because their neural spines are so small and low. Although we tend to think of them as a basal group somewhat removed from the “big show” in sauropod evolution – the neosauropods – mamenchisaurids did a lot of weird stuff. At least in terms of their neck muscles, they may have been the most birdlike of all sauropods. Food for thought.

References