Primate Anatomy & Taxonomy
• Evaluate the dental formula of an unknown primate and place it within a major clade
• Interpret the difference between gradistic and cladistic methods of grouping primates
• Identify the anatomical synapomorphies that distinguish the major primate clades
• Draw a cladogram to illustrate the modern, broadly-accepted primate phylogeny
Purpose: To examine the skeletal traits that distinguish the major primate clades.
The study of non-human primates has been recognized since ancient times as relevant to understanding human anatomy. This was perhaps best recognized by a wide audience of scholars for the first time in 1735 when Carolus Linnaeus, despite his strong creationist views, included humans with other apes and monkeys in the group Anthropomorpha. By the tenth edition of his Systema Naturae in 1758 he had abandoned this term and began calling the group by the familiar name we now use: Order Primates. Linnaeus was motivated to group humans with other primates because of the many anatomical similarities that he perceived uniting them. In modern biological terms, we now refer to these structures as synapomorphies, or ‘shared derived traits.’ For instance, all primates have a broad, flat nail on their big toe, which is a structure unlike any of the narrow claws found in other mammals. We use synapomorphies like these to reconstruct patterns of shared ancestry and build cladograms to better understand the pattern of primate evolution and where humans belong in it. This method of reconstructing relationships between taxa based on shared derived characteristics is known as cladistics.
While morphology and phylogeny have shared a close relationship for centuries, the reliance on synapomorphies to group organisms is relatively recent. In the early 20th century, primatologists such as Wilfrid Le Gros Clark (famous for helping to debunk the Piltdown Man fraud) grouped primates based on their overall similarity in appearance. This method was reminiscent of Aristotle’s “Great Chain of Being,” with primitive primates at the base and humans at the apex. This way of thinking has been called gradistic, because it suggests primate evolution proceeds in a simple, uniform direction for all traits from primitive to derived. While this system has intuitive appeal, it does not correspond with the way we think evolution proceeds. Modern primatologists and anthropologists use cladistic methods based on a nested hierarchy of synapomorphies, because we believe these more accurately reflect how evolution works.
Although phylogenetic trees are built today using cladistic methods, it is clear that gradistic thinking still subconsciously underlies much of our approach to reconstructing evolution. You may have already noticed that phylogenetic trees that include humans tend to place them at one extreme end of the tree, implying some directionality or end goal to evolution, even though there is no reason they need to be placed there! Within the primate order the shift from gradistic to cladistic thinking has impacted how we perceive the relationships of many taxa, most notably the tarsier, which we will investigate more in Station 3. You will be exposed to more examples of gradistic thinking when looking at the human fossil record, where many features (such as brain size) are continuous, and identifying synapomorphies can be particularly difficult. For this lab we will investigate many of the important skeletal synapomorphies that define the largest primate clades.
Station 1: What defines a primate? (0.6 pt.)
A college happened upon a mystery skull while looking through a mammalian skeletal collection. She thinks it may be a primate and comes to you for your expert opinion. She cannot mail you the skull so she emails youaeveral photos. Examine the photos below. Use the list of primate features (found in the lab reading for this week) to help you make the distinctions.
Station 2: Dentition (2 pts.) Different primate clades can be identified by their dental formulae. Primates have 2 incisors, and 2-3 premolars (except the aye-aye, which has a very unusual dentition). Most other mammals have either more or fewer teeth. For the following questions, first determine the dental formula, then consider the cusp pattern.
1. Write the dental formula for each of the craniums or mandibles provided:
2. Answer the following questions using the above dental formulae and the written/illustrated materials provided:
a) Which specimen is not a primate? How do you know?
b) Which specimen is a New World monkey? How do you know?
c) Does specimen “E” have the bilophodont or Y-5 molar cusp pattern? Based on this cusp morphology and its dental formula, what primate group does this specimen belong to?
d) Which mandible is human? What traits did you use to make your identification?
e) Which specimens are apes? How do you know? (Hint: you are an ape)
Station 3: Strepsirrhini and Haplorhini (2 pts.)
Using the handouts, images, and websites, complete the following table illustrating some of the important morphological differences between each primate group.
Strepsirrhine: http://eskeletons.org/boneviewer/nid/12540/region/skull/bone/cranium http://humanorigins.si.edu/evidence/3d-collection/primate/loris-malaysia-usnm-84389 http://humanorigins.si.edu/evidence/3d-collection/primate/loris-malaysia-usnm-84389-0 Haplorrhine: http://eskeletons.org/boneviewer/nid/12538/region/skull/bone/cranium
with primitive primates in a grade called “Prosimia.” They are now grouped with monkeys and apes in a
Examine the tarsier skull (http://www.eskeletons.org/boneviewer/nid/12544/region/skull/bone/cranium) and study the table. Tarsiers can be placed in the prosimian grade or in the haplorhine clade. Name one primitive, prosimian feature that tarsiers retain and one derived, haplorhine feature that they possess. Add which of these features is a synapomorphy, and which is a symplesiomorphy.
Prosimian feature: Haplorhine feature:
2) Does the mystery skull at this station belong to a strepsirrhine or a haplorhine primate? List at least one trait which helped you determine this.
Station 4: Platyrrhini and Catarrhini (1.6 pts.)
The Haplorhine suborder is divided into two infraorders: Anthropoidea (Monkeys and Apes) and Tarsiiformes (tarsiers). Anthropoidea is further divided into two parvorders: Platyrrhini and Catarrhini. Platyrrhines are native to Central and South America (the ‘New World’) and Catarrhines are native to Africa, Europe, and Asia (the ‘Old World’).
Platyrrhine: http://eskeletons.org/boneviewer/nid/12546/region/skull/bone/cranium Catarrhine: http://eskeletons.org/boneviewer/nid/12547/region/skull/bone/cranium http://humanorigins.si.edu/evidence/3d-collection/primate/baboon-usnm-258502 http://humanorigins.si.edu/evidence/3d-collection/primate/baboon-usnm-258502-0
1) Based on what you’ve learned so far, identify what group the following “mystery primate” skulls belong to. To receive credit, list the character(s) you used to make your identification.
A) Is “A” a platyrrhine or catarrhine? How do you know?
B) Is “B” a platyrrhine or catarrhine? How do you know?
2) What advantages might there be to having a prehensile tail for an arboreal primate?
Station 5: Cercopithecoidea and Hominoidea (1.8 pts.)
Within Catarrhini are the two superfamilies Cercopithecoidea (Old World monkeys) and Hominoidea (apes). Use the table below to describe the features of each in relation to the other.
Cercopithecoid: http://eskeletons.org/boneviewer/nid/12547/region/skull/bone/cranium http://humanorigins.si.edu/evidence/3d-collection/primate/baboon-usnm-258502 http://humanorigins.si.edu/evidence/3d-collection/primate/baboon-usnm-258502-0 Hominoid: http://eskeletons.org/boneviewer/nid/12549/region/skull/bone/cranium
1) Which two traits in the above table would be the most useful for determining if an animal was a cercopithecoid or a hominoid in the fossil record? Hint: think about discrete (traits which are either present or absent) vs. continuous traits.
2) List one human autapomorphy – a trait that humans have to the exclusion of all the other primates. Hint:
think about what makes humans unique within the order Primates.
Exercise 2: Systematics and Primate Phylogeny (2 pts.)
Below is a hypothetical phylogeny for six different taxa (A–F). In the phylogeny, the appearance of a new character is represented as a number in a circle. For instance, Character 3 evolved sometime after the common ancestor of Taxa D, E, and F diverged from the common ancestor these taxa share with Taxon C. Character 3 would therefore be a shared, derived trait, or synapomorphy of taxa D, E, and F.
A) Which character is a synapomorphy of E and F?
B) Is Character 1 a synapomorphy or a symplesiomorphy for taxa C and D?
C) Is Character 1 useful for reconstructing the relationship between C and D? Why or why not?
D) Of the 5 characters listed, which represents an autapomorphy?
Study the primate phylogeny in your textbook and fill in the blanks below. Be mindful of spelling: some names are very similar, but have different meanings! If you are having difficulty filling out the phylogeny, you may print out the last page, neatly handwrite the answers in the blanks, and paste a picture of the phylogeny back into the document.