8 Superorder Laurasiatheria - Cetaceans Flashcards
What led to the evolution of cetaceans?
What are the cetaceans named after?
The tethys sea closed up, it was between Gondwana and Laurasia and connected the north and the south
They are named afer the places they were found
Early whales
Where are the first fossil deposits of cetaceans?
What group have the early cetaceans been lumped together under?
What characteristic was used to group them together?
First fossils from Eocene deposits
Lumped together the “Archaeocetes” - look like a cetaceans but are not modern form
The teeth characteristic was used
Early whales – first fossils from Eocene deposits
Where did the tethys sea close up?
What is the name of the early eocene fossil?
Old view and new view, what is the new view of what it looks like?
Tethys sea close up in Pakistan region
The Pakicetus fossil
Thew new view is that it looks more dog-like
Cetaceans
Has an excellent fossil record that is almost complete
What happens to the body size over time?
Earliest form is Pakicetus
Body becomes larger and the limbs become reduced spending more time in water - Ambulocetus
By late Eocence modification of limbs adapted for swimming, could still move on land (seallike?)
Muizon (2001)
The modern forms of cetaceans are divded into sub-orders - what are they?
What are there characteristics?
Cetaceans are divded into sub-orders
- Odontoceti - the toothed whales (e.g. killer whales, dolphins & porpoises)
* Have echolocation, producing sound from monkey lips (fatty tissue) - used for orientation, locating prey - Mysticeti - the baleen whales (e.g. blue whales, humpbacks) baleen filters out phytoplankton
* Evolved song - able to communicate over 100’s of Km
Debate around monophyletic and diphyletic with regards to cetaceans
What is it now thought?
Now thought that it is monophyletic
When did the two forms of cetacans split?
When was the radiation explosion? why was this?
Split around 35 MYA
- toothed whales monophyletic (echolocation evolved once, 36– 34 MYA)
- 10 MYA radiation diversification
support for increased diversification during physical restructuring of the oceans (closure of major seaways) – see map!
(Steeman et al., 2009)
What is the cause of splitting intwo two groups?
Based on phylogenetic evidence and correlations in changes of the ocean current what seems like the main reason for split?
- Allopetric speciation e.g tethys sea closing up
- Unexploited niche -
Best guess is based on the phylogenetic evidence - suggests that it is linked to changes in water currents - thus allopetric speciation, as this seems corellated with changes in ocean currents
The main radiation conincides with closing of tethys sea
But where did the cetaceans evolve from?
The tradition view is that primitive whales and Mesonychia are sister groups. based on similarities in teeth and skulls
(Gingerich and Russell, 1981, O’Leary and Geisler, 1999)
Mesonychids * mentioned in previous lecture for this point
What are mesonychids?
are carnivorous proto-ungulates of the Palaeocene, which pre-date the split into modern ungulates
What were the similarties between mesonychids and early whales?
List the three similarities?
Similarities in teeth – both mesonychids and early cetaceans possess pre-molariform molars with simple, tall and sharply pointed protoconids and laterally compressed distal ends (narrow talonids = nt).
Auditory canal Similarities in the ear – both mesonychids and early cetaceans show elongation of external auditory meatus (Eam) and mastoid process of the petrosal (Mas)
Paraxonic foot posture Artiodactyls, mesonychids and early cetaceans also all share a paraxonic foot posture – in which the weight-bearing axis of weight bearing passes between digits 3 and 4, as opposed to through digit 3 as in Perrisodactyls.
What are the three characters that linked the cetaceans to the mesonychids?
- Molar formation
- Auditory canal
- Paraxonia
One of the problems with testing the hypothesis that cetaceans are related to the early ungulates condylarth mesonychids
Is that you cannot get genetic evidence from the condylarth
What genetic evidence was used to identify if the cetaceans and mesonychids shared a common ancestor?
What did this then reveal?
SINES was used to identified if they have a common ancestor
If you find two species that share SINE then it is likely that they shared a common ancestor
This revealed that artiodactyla is a paraphyletic group
Shimamura et al. (1997)
Position of cetaceans within Artiodactyla led to re-examination of the ankle bone – which previously considered to support monophyletic origin of artiodactyls. (paleontology evidence)
The key defining morphological feature of artiodactyls is the Double pulley Astragalus
Modern whales haven’t kept this feature - what do you have to look at?
Look at fossils of early whales & meosonychids
By
re-evaluating the paleontology evidence: i.e comparing fossils of early whale (cetacean) Pakicetus with early artiodactyl Diacodexis
and with Sus pig
You can see that the early arciodactyl and early cetacans have similar grooves
But if you look at the mesonychiads it doesnt have double groove
This DNA analyses supportes a close association between artiodactyls and the cetaceans
The emergence of the hypothesized clade “Cetartiodactyla”
(Cetacea + Artiodactyla)
How does the tree change from the morphological data to the ‘new fossil and molecular data’
Over last 10 years, more DNA analyses supported a close association between artiodactyls and cetaceans
What does the radius of the semi-circular canals found in the inner ear correlate to?
The radius of these canals correlates with locomotion and mareuverability.
Moder cetaceans have a small radius of semi-cricular canals because they are moving around in a 3D niche, this adaptation might prevent motion sickness
Spoor et al (2002)
Nature 417, 163-166
Cetaceans previously found to have reduced SCC
How can you extrapolate whether they were living in aquatic or terrestrial environments?
(Spoor et al. 2002)
By looking at fossil forms and mesuring there radius you can extrapolate whether they were living in aquatic or terrestrial environments
Molecular evolution evidence looking at developmental patterns in the Hox genes
Give an example?
What does it lead too?
There is an alanine repeat in one of the Hox genes
In mutant humans and mice you can see expansion in polyalanine tract (repeats) and leads to increased digits
The same expansion has occured in cetaceans
Some of the cetacean fins have extra digits - additional digits
Wang, Z. et al. Mol Biol Evol 2009
Molecular adaptation in the Prestin gene
Liu Y et al. (2010)
P. gene involved in amplifying high frequency sounds
Looked at substituation sin a hearing gene
number of substituations in prestin has correlation with ability of high frequency hearing
In deaf humans with loss of high frequencing hearing have loss in prestin gene
But what is the sister group of whales?
What are candidates?
What has seq evidence suggested?
Cows and Hippos are candidates
Evidence suggests that with the accumulation of seq evidence that it is the hippo
Hippos form a monophyletic group with modern cetacans
Shimamura et al. (1997)
However there are only two hippos and so not much to look at
Often the molecular data rasises a hypothesis that will make scientists go back to look at fossil record again
What did Boisserie et al do in 2005?
Boisserie et al. (2005) measured 80 (bone) ostological characters, constructed trees using parsimony analysis – placed hippos as monophyletic group with cetaceans to exclusion of pigs!
What characters do extant whales and hippos share - not found in other extant ungulates?
- Hairlessness
- Lack of sebaceous glands
- Ability to nurse offspring underwater
- Absence of paraconules on upper molars
Previously these traits were thought to be due to convergent evolution. However, these might be ancestral traits that occurred before cetaceans became fully aquatic.
But problems persist…with hippo - cetacans as sister groups
Molecular analyses of extant animals suggest hippopotamids are closest relatives of cetaceans
Yet, cetaceans arose ca 50 MYA in south Asia, whereas the family Hippopotamidae arose just 15 MYA (first hippopotamids recorded in Asia just 6 Myr old!).
So, there must be something closer even to cetaceans…
Thewissen et al. (2007) Nature
Regarding hippo-cetacan sister groups
The Eocene south Asian raoellid
Indohyus
A potential ‘missing-link’ for the cetacean sister group was published in nature in 2007, from fossils found in the Tethys sea
- The Eocene south Asian raoellid Indohyus
- looking at early Artiodactyla focusing on Indohyus
These biologists thought that this was the true sister to the cetaceans
In whales (squares), the medial tympanic wall is inflated (called the ‘involucrum’) and the lateral tympanic wall is thinned (called the ‘tympanic plate’)
In artiodactyls (open triangles), the medial and lateral tympanic walls are similar in thickness, so ratio is closer to 1
Osteosclerotic bones – thickened outer layer of bones
Indohyus had similar thickness
But also able to get some stable isotope signatures, the stable isotopes of oxygen and carbon – the plot shows (based on Oxygen axis) Indohyus is far away from the mammals – it is inferred that they were eating aquatic plants
Bivariate plot of 18O and 13C isotope values for enamel samples of early and middle Eocene mammals from India and Pakistan – result is that
Indohyus was probably eating aquatic plants.
Thewissen et al suggest these creatures took to the water as a predatoravoidance mechanism
So didn’t develop specific aquatic feeding behaviour until much later
Geisler JH & Theodor JM (2009) added genetic data to reveal grouping of hippo – state that the difference in this and the earlier study reveals importance of DNA!
Thewissen et al (2009) flight back – state that Geisler and Theodor do not say anything new (because their paper was not intended to discuss relative placement of extant groups). Also they criticise Geisler and Theodor for selective reporting!
New findings from genomics
McGowen et al. (2012) – examined ~10,000 protein-coding genes 228 under positive selection (dN/dS > 1) in dolphin
Of these, 27 associated with nervous system (incl. human intellectual disabilities, synaptic plasticity and sleep).
Yim et al. (2014) – found expansion in the whale lineage of gene families associated with stress-responsive proteins and anaerobic metabolism
Summary
- Whales & dolphins arguably most highly specialised of the mammals
- Origin appears to be in the northern Hemisphere (Laurasia).
- Molecular & morphological data suggest they are related to the artiodactyls.
- Traditional Artiodactyla is a paraphyletic group
- Proposed sister groups include entire artiodactyl order, extinct early ungulates mesonychians, an anthracotheroid clade (which included hippopotamids) and the hippos themselves.
- Contrary to the traditional view, mesonychids are not the sister group (NHM)
- Hippos better candidate
- New evidence suggests members of the extinct family Raoellidae are the true sister group!
