Osteology Everywhere: Vertebral Incidens

Try as I might, I can never escape osteology. Never. Just the other day, I was walking through my school’s expansive, boneless atrium, when these haphazardly scattered letters stopped me in my tracks:

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DЯSTUDENSN

Amidst this alphabet soup, there it was, calling out to me. Whispering. Longing….

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Ah, the dens. What is the “dens” you ask? It is a special little projection on a special little bone, the second cervical vertebra (C2). Why is it special? Well, most vertebrae look pretty similar to one another, with a body in the front being held in awkward embrace by a bony neural arch in the back.

 

But not the first two vertebrae, C1 and C2. No, these rebels are spinal celebrities. C1, whose rock name is “Atlas” (presumably in honor of its favorite episode of Wishbone) cradles the skull’s occipital condyles on its concave shoulders. Lacking a true body or centrum, Atlas viewed from the top resembles the gaping maw of a manta ray:

Top: Manta ray. Bottom: Atlas viewed from top, anterior is on the bottom (from Scheuer and Black, 200). A and F refer to the age at which the bony portions appear and fuse, respectively.

Top: Manta ray. Bottom: Atlas viewed from top, anterior is on the bottom (from Scheuer and Black, 2000). A and F refer to the age at which the bony portions appear and fuse, respectively.

Atlas is a jerk and so it sits right on top of C2, whose rock name is Axis (after the second album by the Jimi Hendrix Experience). More gawky and angsty than Atlas, Axis differs from the rest of the vertebrae in having an extension, the dens, which reaches skyward to boop the inside of Atlas’ maw:

Top: Axis viewed from the front. Bottom: Axis getting pwnd by Atlas. Modified from White et al. 2012.

Top: Axis viewed from the front. Bottom: Axis getting pwnd by Atlas. Modified from White et al. (2012).

The most distinctive feature of Axis, aside from its smoldering adolescent rage, is the dens (or odontoid process). If you find a bone fragment that is verily vertebral and has a perpendicular projection, you can bet good tenge you’ve got an Axis. Even a densless fragment can be distinguished from all other vertebrae by its superior articular facets, which are rather flat and face mostly superiorly.

What I thought would be a casual jaunt after class last week turned out to be a horrific reminder of the most amazing vertebrae. This must be  how Scott Williams always feels.

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Osteology Everywhere: Vertebeer Fest

This past weekend was witness to the Summer Beer Festival, the annual showcase of Michigan’s brewing splendor. Dozens of breweries brought out batches of beer, from classics we know and love, to inspired innovations meriting a MacArthur Fellowship. There was an embeerrassment of boozes. Dark Horse Brewing Company, from Marshall, MI, put on quite the show:

Dark  Horse Brewing Co. pumping out the brews and blasting t-shirts into the crowd.

Dark Horse Brewing Co. pumping out the brews and blasting t-shirts into the crowd.

Besides towering over the bacchanal hordes, the Dark Horse beer fort also offered IPAs infused with pretty much anything that might pair well with hops. They even steeped habañero peppers in one, and it was maximally boss.

Beer still my heart.

Beer still my heart.

Having sampled only a small part of rich the smorgasbord on tap, a rest by the river was in order. The Festival was on the banks of the mighty Huron River, an excellent place to sit and sip Arcadia‘s scotch ale, taking in the evening under cloud-peppered, cerulean skies. Such a calm and relaxing setting would surely offer respite for a brain besieged by bones. Right?

Every year for the Festival they replace the river water with beer.

Every year for the Festival they replace the river water with beer.

Wrong! Peering through beer goggles over the shimmer of the river, seeking signs of Bigfoots lurking on the opposite shore, I locked eyes with a large, wooden vertebral body.

No ordinary tree stump

An eyeless frown marks the ventral surface of this centrum.

The human spine is composed of anywhere from 31-34 vertebrae (not counting the coccyx or tail bone). The body or “centrum” is the large, blocky portion of the bone, which is separated from other such bodies by intervertebral discs; it is literally a pile of bodies, stacked one on top of the other. And the intervertebral discs are remnants of the notochord, the embryonic structure that unites you and me and all other humans with all other animals known as chordates. Anyway, kiss my grits if this old tree stump across the mighty Huron River here doesn’t look like a lower thoracic or upper lumbar vertebral body, the metaphoric shark fin of a giant trunkless human waiting to pounce from the placid waters.

a) Our mystery vertebra. b) a lumbar vertebra from White et al. (2012). c) views of the right and front side of the Australopithecus africanus fossil StW H41, from Sanders (1998, Fig. 1).

a) Our mystery river vertebra. b) a lumbar vertebra from White et al. (2012). c) views of the right and front side of the Australopithecus africanus fossil StW H8/H41, modified from Fig. 1 of Sanders (1998).

Thinking on it, our mystery river vertebra doesn’t just look like any old human centrum, it is a ringer for the second lumbar vertebra of StW H8/H41, a series of the 11th thoracic to 4th lumbar vertebrae of Australopithecus africanus from Sterkfontein (see the red arrow in c, above). Sanders (1998) notes that this short segment of an early hominin spine shows clear adaptation to walking upright like we humans do today, although the size of the vertebral bodies is both absolutely and relatively small compared to ours, just as is seen in other Australopithecus fossils.

And what better way to celebrate this monumental discovery than returning to the Beer Festival – hooray beer!

Osteology everywhere: Pollicem verte(b)r(a)e [Latin puns are hard]

I just got back from the meetings of the European Society for the Study of Human Evolution in Florence. As you can guess, bones and genes and anatomy and apes and biomechanics and energetics and everything were on everyone’s minds. Even in the midst of an unseasonal surprise typhoon of lunch time ice:

Ambush of hail.

Aw hail no.

Along the way, I passed a gift shop window and this book cover immediately caught my eye:helert

No, it’s not an ancient Roman gladiator’s helmet. It’s clearly a lumbar vertebra, probably of some quadruped. We’re looking down onto the top (or front of it) from the cranial view. The body or centrum is the rounded part toward the bottom of the picture, the short transverse processes jutting off to the sides. The spinous process, pointing toward the top, is even thick and blunt distally as is characteristic of lumbar verts. Here’s a comparison:

Middle lumbar vertebrae, from the cranial view (modified from Figs. 3-4 of Moyà-Solà et al., 2004). 0=modern baboon, A=Proconsul nyanzae (KNM-MW 13142-J)(B) P. catalaunicus (IPS-21350.59). (C) Cast of Morotopithecus bishopi (UPM 67.28) from Moroto (Uganda). (D) D. laietanus (IPS-18000) from Can Llobateres (Spain). (E) Pongo pygmaeus

Middle lumbar vertebrae of various Miocene apes (A-D) in cranial view (modified from Figs. 3-4 of Moyà-Solà et al., 2004). 0=modern baboon, A=Proconsul nyanzae (KNM-MW 13142-J), B=Pierolapithecus catalaunicus (IPS-21350.59), C=Morotopithecus bishopi (UPM 67.28), D=Hispanopithecus laietanus (IPS-18000), and E= modern orangutan.

Modern apes use an upright posture more frequently than living monkeys, who are quadrupedal. An anatomical correlate of these postures is the position of the transverse processes. Compare the baboon (0 in the figure above) with the orangutan (E). In the monkey the transverse processes come off the sides of the centrum (below the horizontal line), while in the orangutan the processes come off the pedicle further back. In your lumbars the transverse processes arise a little bit more toward the back than in the orangutan.

This is a pretty characteristic pattern, meaning that we can reconstruct the habitual posture of an animal based on a single bone – even just part of a single bone as in the case of Hispanopithecus (D, above). Proconsul nyanzae (A), dating to around 19 million years ago and therefore one of the earliest apes, has a monkey-like lumbar vert; the rest of its skeleton is monkey-like and so we think many of the earliest apes moved around like modern monkeys. In contrast, Morotopithecus bishopi (C), at 20.6 million years ago, is also one of the earliest apes but has a more modern-ape-like lumbar. And so with Pierolapithecus and Hispanopithecus.

The vertebra gracing the cover of our gift shop book is clearly more monkey-like, presumably from a simian who long ago walked on all fours across the blood-soaked floors of a cacophonous Colosseum.

Back to the backbone of Homo erectus

Of course the title is referring to all of the back bones. An alternate title may be “The backbone’s connected to the – what bone?” but that’s also kinda lame. I’ll do better next time.
Martin Hausler and colleagues (in press) report on newly identified vertebral fragments of the WT 15000 Homo erectus skeleton, perhaps the most complete of an early hominid (this one ~1.5 million years ago). This skeleton, and other early hominids (i.e. Australopithecus africanus), were described as having six lumbar (lower back) vertebrae; the modal number in humans is 5, and 3-4 in the great apes. The issue of vertebral formula (the number of cervical, thoracic, lumbar, and sacral verts) in hominids is interesting because it is unclear what the ancestral condition is: was ancestral pattern to have more lumbars (like australopiths) from which humans and apes lost verts, or is ape pattern is ancestral, and lumbars were gained then lost over the course of human evolution?


The fragments found by Hausler and team establish that the WT 15000 individual – and presumably all H. erectus – possessed only 5 lumbar vertebrae. In the past, the only evidence of the 6th-to-last pre-sacral vertebra was the vertebral body. It was unclear whether this vertebra would have had articular facets for ribs (like a thoracic vertebra) or not (like a lumbar vertebra). The pedicle fragments identified by Hausler and colleagues (figure to the right) have a rib facet, and so indicate that the 6th-to-last vertebra of this skeleton was thoracic. Thus, WT 15000 – and again presumably all Homo erectus – had a modern-human-like vertebral formula.
The evo-devo of the spinal column is interesting because it seems to me that it may not be so outlandish to try to identify and test hypotheses about how spinal column development (segmentation) changed over the course of hominid and ape evolution. In trying to determine how development of vertebral segments evolved it is important to know how ancient the human pattern is, and so the identification of 5 lumbars in WT 15000 at 1.5 million years ago is an important finding. I need to think on this a bit, I’ll hafta get back to you . . .
ResearchBlogging.org
* figures are from Hausler et al. in press


Reference
Martin Haeusler, Regula Schiess, Thomas Boeni (2011). New vertebral and rib material point to modern bauplan of the Nariokotome Homo erectus skeleton Journal of Human Evolution : 10.1016/j.jhevol.2011.07.004