25 Days of Skeletal Facts: Day 7 – Thoracic Vertebrae

Happy Monday everyone, today we will be continuing with our wintery celebration of skeletal anatomy with the thoracic vertebrae. Humans have twelve thoracic vertebrae that dominant your mid spine, sitting between the cervical vertebrae of your neck and the lumbar vertebrae of your lower back. You may notice that these vertebrae largely resemble the lumbar vertebrae from yesterday. The vertebrae of your spine overall have the same attributes, a body, traverse processes, spinous processes, etc, etc. However, each region has its own characteristics, and the thoracic region features facets (small joint surfaces) for the ribs, see the image below. Each thoracic vertebrae serves as an attachment for right and left ribs – 12 sets of ribs for 12 thoracic vertebrae.

Across the animal kingdom, there is quite a lot of variability in the thoracic spine. Remember yesterday when I said we have 1 to 2 more lumbar vertebrae than our closest ape relatives? Well we also have one less thoracic vertebrae, and this is a change that came along with, you guessed it, our bipedalism. Paleoanthropologists have established that our thoracic spine has consisted of an even dozen of vertebrae for at least 3.3 million years (Ward et al. 2017). A specimen of Australopithecus afarensis, an earlier member of our hominin family tree made famous by the fossil ‘Lucy’ discovered in the early 1970s, dated to about 3.3 million years ago from Ethiopia, with great preservation of its spinal column, revealed 12 thoracic vertebrae, all with evidence of rib attachments (see image below).

Spine of an Australopithecus afarensis specimen. Source Figure 1 Ward et al. 2017

Across mammals, about 12-15 thoracic vertebrae is common – although the sloth boosts an incredible 25 thoracic vertebrae, though with only 21 pairs of ribs. And that’s all for our mid spine, join me tomorrow as we explore the last of the spinal column with the cervical vertebrae of your neck.

REFERENCES & MORE TO EXPLORE

Thoracic vertebra. KenHub Anatomy. Ferng & Mytilinaios. Last reviewed October 2020

Evolutionary backing found in analysis of mammalian vertebrae. ScienceDirect. May 2019

3.3-million-year-old fossil reveals origins of the human spine. ScienceDirect. May 2017

Thoracic vertebral count and thoracolumbar transition in Australopithecus afarensis. Ward et al. PNAS. 2017

8 Facts You (Really) Didn’t Know About Sloths. Symeou, Amelia. The Sloth Conservation Foundation. 2016

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