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The tree of life: exploring evolutionary pathways through

fossil evidence

Daniel C. FisherMuseum of Paleontology, Departments of Geological

Sciences and Ecology and Evolutionary Biology

The Treeof Life

Neontology(thousandsof years)

Paleontology(billions

of years)

The Treeof Life

of years)

The Treeof Life

of years)

The Treeof Life

Press & Siever 1982

“And if you were to say that such shells were created [in the rock] … –such an opinion is impossible …, because the years of their growth can be counted on the shells, and both smaller and larger shells may be seen, which could not have grown without food, and could not have fed without motion ….” Leonardo da Vinci, ca. 1500 (italics added)

Fossil

Modern

Shark’s teeth and “tongue-stones” arealike … “as one egg resembles another.”

Steno 1667

Modern

Fossil

Press & Siever 1982

Unconformities:“Here are three distinctsuccessive periods of existence,and each of these is, in ourmeasurement of time, a thing ofindefinite duration ….”

Hutton 1788

C. Badgley

Steno’s Law of Superposition:units low in a sequence must have been deposited first; unitshigher in the sequence musthave been added later; thesequence of superposition hastemporal implications.

Steno 1669

Three days postmortem … … three weeks postmortem …

… one year postmortem … … and two years postmortem.

Coe (1980)

Digital Elevation Model of U.S.Future fossils are being preserved today in lakes, intermontane basins,river valleys, coastal plains, and offshore areas of sediment deposition.

Bedrock Geology of U.S.Fossils are found today in areas where sedimentary rocks are exposedor encountered in excavation or drilling, even in the deep sea.

Southern end of Zinda Pir Anticline, PakistanGingerich & Zalmout

D. Miller

T. Baumiller

S. Peters

J. Whitlock

tufts.edu/geology

P. Gingerich

P. Gingerich C. BadgleyT. Moore

T. Baumiller J. Whitlock

Hunting for fossil whalesin Pakistan … under armedguard …

… or flying over remoteSiberian tundra … in old Russian helicopters loadedwith mammoth tusks …

… paleontology entails risks.

How does paleontology help us explore evolutionary pathways … and perceive Life’s history?

1. Temporal structure:organisms from differenttime periods are not allthe same; similarity is oftengreatest between formsthat occur close togetherin time.

“Stratigraphic successionof floras, faunas” …corroborated and extendedby correlation between regionswith differing exposure …with the most recent biotasmost like organisms alive today…

A unique contribution of thegeological and paleontologicalrecord that set the stage forDarwin’s ideas …

2. Spatial structure:organisms from differentplaces are not all thesame; similarity is oftengreatest between formsthat occur near one anothergeographically.

Geography

Biogeographic patterns are thejoint province of neontologyand paleontology …and represent fundamentalobservations that support theidea that similar organismsshare some common historywithin a finite geographic region.

3. Trait distributions amongorganisms: patterns of traitdistribution can be graphicallyrepresented by branchingdiagrams on which traits canbe mapped “parsimoniously”.

D1 change

Unrooted “network” Rooted “cladogram” Rooted “tree”(cladogram w/ time)

AA BB CC

1 change 1 change

Patterns of trait distributionare also the joint province ofneontology and paleontology …though temporal data used forrecognizing ancestors anddescendants come from fossils.

[Selected] Vertebrates – 10 taxaMacClade Example File; Maddison & Maddison 1992

• Ray-finned fish• Lungfish• Salamanders• Frogs• Turtles• Lizards• Snakes• Crocodiles• Birds• Mammals

How are these organismsrelated?

What are the patterns ofrelative similarity and dissimilarity among them?

What are the patterns oftrait distribution?

If they were “connected”in some way, would it help to explain the distribution of traits?

[Selected] Vertebrates – 10 taxaStates Min. Steps

• Char. 1-(amnion): (0, 1) 2 1• Char. 2-(appendages): (0-2) 3 2• Char. 3-(body covering): (0-4) 5 4• Char. 4-(thermoregulation): (0, 1) 2 1• Char. 5-(internal nares): (0, 1) 2 1• Char. 6-(atrial septum): (0, 1) 2 1• Char. 7-(temporal fenestra): (0-2) 3 2• Char. 8-(hemipenes): (0, 1) 2 1• Char. 9-(suspensorium): (0, 1) 2 1• Char. 10-(gizzard): (0, 1) 2 1• Char. 11-(antorbital fenestra): (0, 1) 2 1• Char. 12-(lateral sphenoid): (0, 1) 2 1• Char. 13-(pedicellate teeth): (0, 1) 2 1

Total 31 18

[Selected] Vertebrates – 10 taxa

This is the most parsimoniousunrooted network found by PAUP.

If rooted,root here.

There is only one instance of characterconflict on this network: the occurrenceof advanced thermoregulation in birdsand mammals, but in no other groups.

[Selected] Vertebrates – 10 taxa

Character 1 mappedOnto cladogram.

It explains the distributionof 13 characters (with 31character states) across the10 taxa with only 19 steps, implyingonly ONE instance of homoplasy.Retention index = 0.95

Vertebrates – 10 taxa

Only 1 “best” hypothesisNot many “next-best” hypotheses

Many 1 “poor” hypotheses

g1 measures “skewness” of treelength distribution

“Poor” hypotheses require similar traits to have arisen many timesindependently, leaving their similarity unexplained.

[Selected] Watches – 10 taxaStates Min. Steps

• Char. 1-(numerals): (0, 1) 2 1• Char. 2-(seconds display): (0, 1) 2 1• Char. 3-(analog/digital): (0, 1) 2 1• Char. 4-(date): (0, 1) 2 1• Char. 5-(strap): (0-2) 3 2• Char. 6-(front material): (0-2) 3 2• Char. 7-(power): (0, 1) 2 1• Char. 8-(face color): (0, 1) 2 1• Char. 9-(waterproof): (0, 1) 2 1• Char. 10-(number of buttons): (0-4) 5 4• Char. 11-(crystal material): (0, 1) 2 1• Char. 12-(back opening): (0, 1) 2 1• Char. 13-(lunar display): (0, 1) 2 1

Total 31 18

[Selected] Watches – 10 taxa

CDF Timex NEF Fossil

RCD Casio

DCF Casio

CDF BrookstoneSGB Timex DCF Relic

CDF Gruen

CDF Bucherer

CEF Timex

“Strict consensus” of 15 most-parsimonious diagrams …Conflicts among patterns of trait distribution prevent simple

mappings of state changes on network.No fewer than ELEVEN homoplasies. R.I. = 0.68

Archetypal products of design … and implicitly, little or no“shared history” … do not show the same kind of patternsof trait distributions we see in organisms.

What do the patterns of trait distribution in organisms mean?

Watches – 10 taxa

g1 shows less “skewness” of treelength distribution

Many “best” hypotheses

Even more “next-best” hypotheses

A “flatter” distribution of “poor” hypotheses

Robinson 1992

Fortson 2004

Language “Geography”and

Language History

Language similarities maponto geography in simpleways, show patterns of traitdistribution indicative of ashared history of descentwith modification, and arefurther elucidated by a “fossilrecord” of ancient texts and inscriptions.

Bloch et al. (2001)

Another example:15 taxa, 33 characters, occurrences in 9 intervals

Bloch et al. (2001)

Cladistic result: one cladogramStratocladistic result: one cladogram (the same one)

with eight genealogies (three equivocal ancestors)

Trait distributions are not just evidence that allows us toreconstruct a history of descent with modification. Theyare evidence that organisms share a history of descent.

Bloch et al. (2001)

With the temporal control provided by fossils, hypothesesof relationship can be more severely tested.

Are transitions documented in the fossil record?Globorotalia cibaoensis to Globoratalia crassiformis

Arnold 1983Measurements

Form Change

Beginning

Middle

Here’s another,from the sameissue of Paleobiology.

Globorotaliaplesiotumida toGloborotalia tumida

Malmgren et al. 1983

Form ChangeTi

Reptile-Mammal Transition

Permian synapsid – starting point

Modern opossum – ending point Opossum ear ossicles– enlarged

Triasssic cynodont – transitional form

Human ear ossicles

Hopson & Allin 1992

Reptile-Mammal Transition

Jurasssic mammal – Morganucodon

Placed within a historicaland environmental context,the transition to mammalscan be better studied.Hopson 2001

Hopson 1994

K/T Extinction

P/Tr Extinction

Start of PaleozoicStart of Phanerozoic

Sepkoski (1981)

The Big Picture … well, some of it.

What does fossil evidence bring to the study of evolution?

• Historically, fossils forced us to address the antiquity of the earth and the reality of a past history of life.

• Fossils forced us to recognize the reality of extinction.• Fossils provide evidence of different conditions in the past.• Fossils provide much of the temporal framework that

contributes to reconstructions of earth history.• Fossils provide a temporal framework that facilitates

reconstruction of evolutionary trees (esp. w/ ancestors).• Fossils enlarge our perception of biodiversity and past

biogeographic patterns, changing our view of life’s history.• Fossils place past evolutionary transitions in a temporal,

geographic, and environmental context, making it possible to study causes of past events and transformations … and better understand the dynamic system that is Life.

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