Bioanthro lab activity: Estimating Miocene ape body mass

We’ve arrived at the Planet of the Apes, also known as the Miocene, in my “Bones, Stones and Genomes” course. The living apes are but a small remnant of what was a pretty successful radiation starting around 20 million years ago. There were so many apes that it can be a bit confusing for students, but it’s important for setting up the biological and ecological contexts of hominin origins.

Possible evolutionary relationships of myriad Miocene apes and subsequent hominins. From Harrison (2010)

Possible evolutionary relationships of myriad Miocene apes and subsequent hominins. From Harrison (2010)

This week also marks my students’ first lab assignment, analyzing CT scans of bones. Here, we looked at how we estimate body size in extinct animals, using the KUPRI database and the free CT analysis software InVesalius. Because some of the KUPRI primates have body masses recorded, students can examine the relationship between animals’ weight and skeletal dimensions. The purpose of the assignment is to help familiarize students with skeletal anatomy, CT data and principles of linear regression.

One of the KUPRI specimens, an old female gorilla, with known weight.

One of the KUPRI specimens, an old female gorilla, with known weight.

I selected a few specimens for students to examine. After students download the massive files, they can load them into InVesalius for analysis. This program allows students to easily identify bone versus other tissues, and to create a 3D surface rendering of a highlighted region (tissue) of interest.

A grivet, Chlorocebus aethiops, with bone highlighted in 2D sections and as a 3D model.

A grivet, Chlorocebus aethiops, with bone highlighted in 2D sections and as a 3D model. This little guy weighs only 4 kg!

It’s pretty easy to take simple linear measurements (and angles), assuming students can get oriented within the skeleton and identify the features they need to measure. It can be a little tricky to measure a femur head if it’s still in the acetabulum (below). Luckily, InVesalius lets you take measurements on both 2D slices or the 3D volume.

Let's measure that femur head diameter.

Let’s measure that femur head diameter.

So students do this for a few specimens and enter the data into Excel, which can then easily plot the data and provide a regression equation. They then use this equation to estimate masses of the specimens – if there’s a good relationship between mass and skeletal measures, then the estimates should be close to the observed values. Students use their equation to predict body mass of some Miocene apes based on femur head diameter and femur midshaft diameter, noting how confident they feel in their estimates given how well their regression performed on the training dataset. They also compare their mass estimates to those using another equation generated by Christopher Ruff (2003).

It might be a little intense for students totally unfamiliar with apes, bones and CT scans, but it should be a good way for them to learn lots of concepts we’ll revisit over the semester.

Here’s the lab assignment, in case you want to use it in your own class: Lab 1-Miocene masses

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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.

What do you call a Middle Miocene hominoid that doesn’t belong to you?

Nacholapithecus kerioi.

Oh man, that was a bad joke. N. kerioi is an ape known from the site of Nachola in Kenya, dating to 17-14 million years ago. There is a fairly complete skeleton, KNM-BG 35250, and completeness is always exciting for the paleontologist. It seems to have had a fairly large and robust forelimb compared to its hindlimb, possibly indicating locomotor behavior unlike anything modern primate. Based on evidence from the skull and teeth it was likely a hard-object feeder, a characteristic in many Miocene hominoids starting with the 17 million year-old Afropithecus. Here’s a picture of the skeleton (Nakatsukasa and Kunimatsu 2009, Fig. 1):
This specimen is the holotype for N. kerioi. A nice contrast to my last post griping about specimens that have been selected as holotypes.

Reference
Nakatsukasa M and Kunimatsu Y. 2009. Nacholapithecus and its importance for understanding hominoid evolution. Evolutionary Anthropology 18: 103-119.