Original DocumentThe Living Environment Regents: August 2011

Note: exactly as appeared on Regents examinations with corresponding questions

When Whales Walked the EarthA newly unearthed fossil is the missing link between land and marine mammals

Standing two to three feet tall on legs adapted to wade through shallow water, the 48-million-year-old Indohyus is the missing link between modern day whales and their land-lubbing ancestors. Hans Thewissen of the Northeastern Ohio Universities Colleges of Medicine and Pharmacy recovered the skeleton in rocks from Kashmir, a disputed region between India and Pakistan, where the deer-like herbivore lived during the Eocene epoch, 56 to 34 million years ago.

The extreme thickness of its bones is a trait seen often in animals that are aquatic waders (thick, heavy bones counteract buoyancy and allow the animal to stay underwater more easily), and chemical traces in its teeth indicate that the animal ate plants in a freshwater environment. Scientists know that Indohyus belongs in the evolutionary path with whales because it has skeletal similarities to both modern whales and known primordial whale ancestors. Source: Day Greenberg, “When Whales Walked the Earth,” Popular Science, May 2008

54 Identify one structure present in Indohyus mentioned in the passage. State how that structure made Indohyus successful in its environment during the Eocene Epoch. [1]

Structure: _______________________

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55 Identify one characteristic of Indohyus that led some scientists to conclude that it was an ancestor of modern whales. [1]________________________________________________________________________________________________

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A. When Whales Walked the EarthA newly unearthed fossil is the missing link between land and marine mammals

Standing two to three feet tall on legs adapted to wade through shallow water, the 48-million-year-old Indohyus, “India’s Pig”, is believed to be the missing link between modern day whales and their original land ancestors.

Whale and offspring

Hans Thewissen of the Northeastern Ohio Universities Colleges of Medicine and Pharmacy recovered the skeleton in rocks in a region between India and Pakistan, where the deer-like herbivore lived 34-56 million years ago.

Indohyus Skeleton Kashmir (top-almost center of map)

The extreme thickness of its bones is a trait seen often in animals that are aquatic waders (thick, heavy bones allow the animal to go and stay underwater more easily), and chemical traces in its teeth indicate that the animal ate plants in a freshwater environment.

Counter Forces: Weight(mass) and Buoyancy Indohyus Underwater Indohyus Jaw and Teeth According to molecular evidence, the closest living relatives of whales are, quite surprisingly, a group of animals that include hippos. Scientists know that Indohyus belongs in the evolutionary path with whales because it has skeletal similarities to both modern whales and its early ancestors. And Indohyus lived at the same time as the earliest whales and hailed from the same place of origin – southern Asia, hippos did not. In addition, the jaws and teeth of Indohyus are very similar to those of early whales, and they have a special bone in its middle ear that is only found in whales and their ancestors, which allows them to hear underwater.

B. Closest Whale Cousin—A Fox-Size Deer?Researchers split on closest evolutionary kin to whales and dolphins

By JR Minkel  | December 19, 2007

A group of researchers says that the closest known evolutionary cousin of whales, dolphins and porpoises is not the hippopotamus, as conventional wisdom has it, but an extinct deer-like animal roughly the size of a fox or raccoon. In a new study, the team finds that a fossilized specimen of the extinct, 48-million-year-old mammal Indohyus bears several telling similarities to whales, including dense limb bones for ballast and a middle ear structure found only in the cetaceans, or sea-dwelling mammals, which is thought to help them hear underwater.

"What we think happened is that the ancestors of both Indohyus and whales were animals that looked like a tiny deer," says Hans Thewissen, professor of anatomy at Northeastern Ohio Universities Colleges of Medicine and Pharmacy, who led the study, published in Nature. The modern creature that most resembles Indohyus, however, is the African mousedeer (or chevrotain), which lives on the forest floor but scurries into the water to take cover from predators. Similarly, Thewissen says, the common ancestor of whales and Indohyus may have been a herbivore (plant-eater) that took to water to hide out, but eventually switched to a swimming, meat-eating lifestyle, which it passed down to modern cetaceans.

Other experts, however, caution that although the scenario is possible, the ancestry analysis is based on incomplete data. Researchers "really thought the book was closed on this," says Annalisa Berta, an evolutionary biologist at San Diego State University. "To suggest that this fossil somehow is closer than hippos, that's a big deal—I'm just not convinced."

Whatever its relationship with whales, Indohyus was probably not a direct predecessor of them, Thewissen says, because the specimen, unearthed 30 years ago in Kashmir, dates to roughly two million years after the earliest known cetacean fossils.

Over the past 15 years, researchers have uncovered a series of fossils intermediate between whales and land animals, but were still missing a link to landlubbing beasts, which Thewissen says Indohyus now provides.

The disagreement reflects the fact that researchers weren't around to watch successive populations of animals budding off from those that came before, resulting in a bushy pattern of new species over the millennia. Instead they have to infer the bush's branch points. They do this by categorizing extinct and living species together into the evolutionary bush that has the fewest cases of unrelated species evolving the same trait, which is in general less likely than one species passing down a unique trait to another species.

Indohyus, for example, has a half-walnut-shaped tympanic (a bony casing around the middle ear bones) with thicker walls on the sides than in front. The same thickening is found in all fossil and modern cetaceans but not in other mammals, the group notes. The fossil also has dense limb bones that would have weighed it down in water. But in contrast to the pointy molars of dolphins or killer whales, however, its equivalent chompers are squarish like those of hippos, possibly for grinding plant matter, the group suggests.

The researchers used a computer program to test the possible evolutionary bushes that could have yielded cetaceans along with artiodactyls—the mammalian order made up of two-toed, hoofed animals—which include Indohyus and hippos.

The new analysis does not yet unseat the hippo as cetaceans' kissing cousin, because it only takes into account anatomical features, not molecular ones, says Maureen O'Leary, a professor in the department of anatomical sciences at Stony Brook University on Long Island, N.Y. She says that her own categorization of artiodactyls supports the hippo as the closest relative to cetaceans, but notes that it did not include the features uncovered by the Ohio team.

C. Whales evolved from small aquatic hoofed ancestorsThis article is reposted from the old WordPress incarnation of Not Exactly Rocket Science.Travel back in time to about 50 million years ago and you might catch a glimpse of a small, unassuming animal walking on slender legs tipped with hooves, by the rivers of southern Asia. It feeds on land but when it picks up signs of danger, it readily takes to the water and wades to safety.

The animal is called  Indohyus (literally “India’s pig”) and though it may not look like it, it is the earliest known relative of today’s whales and dolphins. Known mostly through a few fossil teeth, a more complete skeleton was described for the first time last week by Hans Thewissen and colleagues from the Northeastern Ohio Universities. It shows what the missing link between whales and their deer-like ancestors might have looked like and how it probably behaved. Whales look so unlike other mammals that it’s hard to imagine the type of creature that they evolved from. Once they took to the water, their evolutionary journey is fairly clear. A series of incredible fossils have documented their transformation into the masterful swimmers of today’s oceans from early four-legged forms like  Pakicetus and Ambulocetus (also discovered by Thewissen). But what did their ancestors look like when they still lived on land?

Hooves to flippersUntil now, we had little idea and their modern relatives have provided few clues. According to molecular evidence, the closest living relatives of whales are, quite surprisingly, the artiodactyls, a group of hoofed mammals that includes deer, cows, sheep, pigs, giraffes, camels and hippos.They all have a characteristic even number of toes on each hoof and not a single one of them bears even a passing resemblance to whales and dolphins. Among the group, the hippos are evolutionarily closest and while they are at least at home in water, their family originated some 35 million years after the first whales and dolphins did.Enter Indohyus, a small animal about 70cm long that lived 47 million years ago. It was a member of a family of mammals called the raoellids, prehistoric artiodactyls that lived at the same time as the earliest whales and hailed from the same place of origin – southern Asia. By analysing a fossilised skull and limbs collected from India, Thewissen found compelling evidence that the raoellids were a sister group to the ancestors of whales.Even though Indohyus had the elegant legs of a small deer and walked around on hooves, it also had features found only in modern and fossil whales. Its jaws and teeth were similar to those of early whales, but the best evidence was the presence of a thickened knob of bone in its middle ear, called an involucrum. This structure helps modern whales to hear underwater, it’s only found in whales and their ancestors, and acts as a diagnostic feature for the group.Based on these physical similarities, Thewissen suggests that the raoellids are a sister group to the whales. Both of these groups are evolutionary cousins to all modern artiodactyls. (As a note for journalists and creationists, Indohyus is not a direct ancestor of whales, as many news sites are claiming, and nor did whales ‘evolve from deer’!)

Life in the waterIndohyus‘s skeleton also suggests that it was partially adapted for life in the water. Its leg bones were unusually thick, a feature shared by other aquatic animals including hippos, sea otters and manatees. These heavier bones stop swimming mammals from floating by default and allow them to hang in the water and dive more easily.Because Indohyus had slender legs and not paddle-shaped ones, Thewissen pictures it wading in shallow water, walking hippo-style along the river floor while its heavy bones provided ballast.Thewissen found more clues about the animal’s lifestyle from its teeth, and particularly the levels of certain isotopes in their enamel. Levels of oxygen isotopes matched those of water-going mammals, providing further support for Indohyus‘s aquatic tendencies. Its large crushing molars are typical of plant-eaters and levels of carbon isotopes in them suggested that Indohyus either came onto land to graze (like hippos) or fed on plants and invertebrates in the water (like muskrats). In terms of behaviour, they were close to the modern mousedeer, a tiny, secretive deer that feeds on land but flees into streams when danger threatens.Put together, this portrait of Indohyus‘s life also tells us about the changes that drove the evolution of whales, and it looks like it wasn’t a move to water. Whales and raoellids are evolutionary sisters and since early members of both groups were happy in the water, aquatic lifestyles must have pre-dated the origin of whales.Instead, Thewissen suggests that the key step was a switch in diet. He speculates that whales developed from an Indohyus-like ancestor that fed on plants and possibly small invertebrates on land, but fled to water to escape predators. Over time, they slowly turned into meat-eaters and evolved to swim after nimble aquatic prey.

Reference: Thewissen, J.G., Cooper, L.N., Clementz, M.T., Bajpai, S., Tiwari, B.N. (2007). Whales originated from aquatic artiodactyls in the Eocene epoch of India. Nature, 450(7173), 1190-1194. DOI: 10.1038/nature06343

Academic & Content VocabularyVOCABULARY CATEGORIES CONNECTIONS SUPPORT

Academic VocabularyWade/WadersShallowModernLand-lubbingRecoveredKashmirDisputedRegionExtremeCounteractTracesIndicateSimilarities

GeneralShallowModernRecoveredDisputedRegionExtremeCounteractTracesIndicateSimilarities

Location/Proper NameKashmir

Colloquial ExpressionLand-lubbing

Different FormsWade/Waders

Science VocabularyFeetLegsWaterAdpatedIndohyusMissing linkWhalesAncestorsPharmacyRocksDeerSkeleton/SkeletalHerbivoreEocene epochBonesTraitAquaticChemicalTeethAnimalPlantsBuoyancyFreshwaterEnvironmentScientistsEvolutionary pathPrimordial

ConfidentFeetLegsWaterWhalesRocksBonesTeethAnimalPlantsScientists

PossiblePharmacyDeerAquaticChemicalFreshwater PreciseAdaptedIndohyusMissing linkAncestorsSkeleton/SkeletalHerbivoreEocene epochTraitBuoyancyEnvironmentEvolutionary pathPrimordial

Group Tasks: When Whales Walked the Earth

1. Readers Tell Us About:A. Counter Forces B. Evolutionary Bush

B. Missing LinkC. Relatives: Earliest, Closest & Modern C. Ancestors: Direct & Like

A: Article A, B: Article B and C: Article C

2. Share Our Information:A. D: Indohyus B. K&L: Whales and the Hippopotamus

B. K&L: Indohyus and Whales C. C&CC: Whale’s Closest Cousin : ___________

D: Describe, K&L: Know and Learn, and C&CC: Claims and Counter Claims

3. Google Search: More Information on Animals/Animal GroupsA. African mousedeer (chevortain) B. Cetaceans: sea-swelling mammals

B/C. Artiodactyls: two toed, hoofed animals C. Raoellids: Prehistoric Artiodactyls

A: Article A, B: Article B and C: Article C

Group TaskAs scientists make discoveries of unknown organisms that lived in the past they are very hopeful that some of these organisms may help provide a better understanding on how different groups of species evolved. However, scientists do not always agree with other scientist’s claims when it comes to using newly discovered organisms to modify currently accepted evolutionary relationships and pathways.

Using Indohyus as an example, create a protocol (step-by-step procedure) that you believe scientists should use to establish that a newly discovered organism (Indohyus) is more closely related to an existing species (whale) than a currently established species (hippopotamus).

Your protocol should indicate the steps/areas in which you believe scientists will have the most difficulty and/or disagree the most. In addition, you should establish guidelines with specific requirements/limits for each step (rubric). And your group will need to identify and norm content vocabulary, use of evolutionary diagrams, and the format/requirements of a written argument to establish a claim.