jane pickering: goodevening, everyone. first, i'd like to thankyou all for coming out on this dreadful day. i know it's a little bit betternow than it was this afternoon, but it's quite theweather out there. so congratulations ongetting here in one piece. my name is jane pickering. i'm the executivedirector of the harvard museums of science and culture.
and i would like towelcome you to this, which is the third inour evolution matters series for the harvardmuseum of natural history. and i'm delighted towelcome our speaker, who has-- moved-- dr. zhe-xi luo,who is speaking to us today about jurassicgrandmothers from china. before i get to drluo, i would like to just mention that our nextlecture is two weeks from now. robert hazen of the carnegiegeophysical laboratory
is going to talk about the newevidence about complex evolving systems driving phenomenabeyond life sciences, such as the diversificationof minerals on earth, and how this impactsthe arguments of the anti-science proponents. and this will be on tuesday,march 12 here in this hall. i know we havesome special guests in the audience, some localteachers, who are coming and have a seminarafter the lecture.
and just to let you guysknow that the seminar room is pretty much exactly behindme, right next to the bathroom. so if you go out andjust keep going around, then you'll findyour seminar room. all the museumsclasses and exhibits can be found in this brochure,which is on the table. and so please if you don't haveone, feel free to pick one up. and if you don'tlike paper, then sign up your email addresses.
then you'll get everythingin your email box about the informationabout all our programs. and there's alsoinformation there if you would like to becomea member of the museum and help support out exhibitsand our public programs. so speaking of support,i'd like to acknowledge dr. herman and dr. joan suit--who are sitting over there-- who supported theevolution matters series. and they have been dedicatedsponsors-- thank you--
for the last several years. and thanks to theirsupport, we are able to videotapethese lectures. and they are available online. so we just put up thefirst of this year's series by dr. arkhart abzhanov who didthe first-- those of you who were here remember--which was fantastic, on dinosaur to bird evolution. that's now available,but there are many,
many others available. so i encourage youto check that out. so on to tonight'sspeaker, who's really being welcomedback to harvard rather than justwelcomed to harvard. luo is a paleontologist andprofessor of organismal biology and anatomy at theuniversity of chicago. and his research specialty ison the evolutionary biology of vertebrates.
tonight, he will focuson the originations of modern mammalianbiological adaptations in the mesozoic era, oftendominated by dinosaurs. and the story isn'tquite as simple as that-- and how jurassicfossils discovered in china shed light on the earliestevolution of placental mammals. he received hisbachelor of science from nanjing universityof china and then went on and did his phd inpaleontology at the university
of california, berkeley. he completed his postdoctoraltraining from '89 to '91 right here at themuseum of comparative zoology at harvard. and then left andspent many years as curator ofvertebrate paleontology at the carnegie museum ofnatural history in pittsburgh. and in 2004, he was named theirassociate director of research and collections.
he is considered one ofthe world's top experts in mesozoic animalsand is counted among the international team ofscientists that has discovered many significant speciesof early fossil mammals, such as the earliestknown swimming mammal and theearliest marsupial. he has received a career awardfrom the national science foundation as well asa humboldt research award for senior scientistsfrom the alexander von humboldt
foundation. the list goes on, but let'shear about jurassic grandmothers from china, origins andearly evolution of mammals. zhe-xi luo: my name is luo. and in english, itcoincides with being short. so every time i talkedabout early mammals in front of mymentor, fuzz crompton, i'm always a short guy. and i shudder.
mammals are very diverse. we have some 5,400 species. and mammals are also veryinteresting in that we have some 4,000 general fossils. so as a case study ofa major clade evolved in the evolutionaryhistory, mammals gave us a great fossilrecord to work with. mammals are alsovery interesting in the ecomorphologicaldiversity.
let me back step a little bit. this is an oldexhibition diagram by william kinggregory in the american museum of natural history. and it's a tree. it has many different. ecomorphotype maps on top of it. foremost, we haveorders familiar ground-livingterrestrial mammals.
and of course there are thingsthat can climb the tree. and once you geton the tree, it's not uncommon you canget 40 arboreals. and some of the arborealforms could be volan or it could even power flyers. you can have tonguefeeding termite eaters. and then you can havecursorial herbivores. and you can haveheavy animals that are gravid portals, suchas elephants, rhinos,
so on and so forth. and then you can havecarnivore scavengers. you can havecarnivore predators. and then finally youhave 40 aquatic mammals. and these are calledecomorphotypes. a lot of the classicnatural historians have set up ordersof mammals that largely go withthese ecomorphotypes. but of course, the modernmolecular phylogeny
have already validated fromthe genetics perspective most other orders except whatwe call the insect worth. so mammals areinteresting in ecology and in their general morphology. but all these diverseecomorphotypes share some basicfeatures in reproduction. the first group iscalled monotremes. and these are mammals. they have mammary glands.
and they nurse theiryoung, but their young were born with leathery eggs. and in contrast to theegg laying mammals, we have live birth mammals. the first group ofthese are marsupials. marsupials have very shortgestation, but rather long nursing in the pouch. and the here is afigurative metaphor of that. the most diverse groupwithin the live birth mammals
are placentals. there are some--depending on how you read the latest phylogeneticliterature-- over 5,000 species. generally speaking,the placentals have very elaborate,more invasive placenta that embed in mother's uterus. as a result, they have a longergestation and more effective nurturing in the uterus.
but all thesemammals, in addition to their basic reproductiveecomorphological patterns, they also have bones. they also have teeth. it is really throughthese osteological and dental features that wecan tie the fossil record with the modernmammalian diversity. normally, this is how we do it. so we have placentals.
we have marsupials. and these are united as atheory and characterized by live births. in addition to theirmode of reproduction, they also have teeth,jaws, and skeletal features that characterize them. this theory of mammalsis grouped together with monotremes,altogether characterized by mammary glands, and alsoderived by many bone features,
suppose we find a fossil. without this softtissue, it is really these osteological featuresthat help us to place them on the overall mammalianphylogenetic tree, or family tree. say for example, if we finda fossil called juramaia, or mother from jurassic, andits osteology and teeth place it closer to placentalthan to marsupial, but not so advanced yet tobe inside placental.
we will call them eutherians. and if we findanother fossil-- say like sinodelphys and byteeth and the bones-- we are able to associate itcloser to modern marsupials than modern placentals. we would place them closeto the marsupial line and call them metatherians. it is really theseearly fossils that gave us a timeestimate about how
a particular evolutionarylineage first appeared in the fossil record. but more important than that--and very interesting to me-- is these fossilswould have helped to provide what ourancestors would be like if these are closelyrelated to us. it is a review fromthis ancestral future, from the early fossils,that we managed to establish theevolutionary pattern,
how the modern group arrivesin the fossil record. now these fossils associatedwith individual lineage gave us the minimalage of a group. but all these extant mammalswould also carry with them the molecular sequences thatare part of their history. and through thosemolecular sequences, we can manage provide amolecular time estimate. usually there is a gapbetween the minimal fossil age and the rise, or estimateof divergence of time,
of a modern grouprepresented by molecules. but the biggest earlymammal diversity are not the group associatedwith placentals or not a group associated with marsupials. they are associated here inbetween the therian mammals and monotremes. so these are modern metatherianand eutherian groups. there are a largenumber of groups. and there are some15 different groups
in between theriansand the monotremes. and these groups,from my perspective, are the mostinteresting mammals. and many of themare from jurassic. and the modern mammalgroup in return is nested inside a groupcalled mammalial forms. and these are fossilizedmammals from very early age. and their osteological anddental features definitely associate them withthe modern groups,
but they are notquite there yet. so we call them mammalial forms. and mammalial forms,in return, are descendants from a large clusterof extinct reptilian fossils known as cynodont. normally we refer to themas mammal-like reptiles. and of course we will havethis osteological feature to construct this phylogeny. and in the more recentmonths, a much larger version
of the study is published by ateam funded by national science foundation tree oflife mammal project. but if you take thegenealogical tree and place it onthe time scale, you will see some veryinteresting sequences. and the next diagram willfollow from here to here, but vertically. we start with cynodonts thatare very common in triassic. and this is even beforethe rise of the dinosaurs.
right around thetime when dinosaurs start to appear in thefossil record-- in this time, roughly from 220 millionto 190 million years-- we start to have thesefossil groups come into the fossil record. and they definitelyhave some part of the modern diagnosticmammalian features. that's why we callthem mammalial forms. and if we move a little morerecent to the middle jurassic
from 180 millionto 165 million, we start to see the fossilsthat we can definitively place on modernclades that is either associated with modernmonotreme to some extent, or associated with moderntherians to the other. well by the time we get to latejurassic, up to 145 million, we start we have groupsthat can literally identify with eutherianssuck as juramaia. cretaceous is alsovery interesting.
but if you go frompresent date-- back step 220 millionyears-- the first 1/3 of that 220 million yearsof the mammalian history are in jurassic. and by the time weget to cretaceous, the game is almostover, so thus our theme about grandmothers fromthe jurassic today. the fossils fromthis earliest episode really can help us toframe some questions
and address how modernmammalian biology really arose in the fossil history. and this is the workthat i started with fuzz. so i'm particularlyhappy that fuzz is able to come andprobably offer me some criticism after this. the biggest questionis, what makes a mammal? mammals have hairs. mammals have mammaryglands, sweat glands.
these are allintegumentary structures associated with the skin. mammals are also characterizedby this very important feature of lactation. of course in the fossils,we cannot get the fossilized mammary glands. but because lactation hassuch a fundamental impact on how mammals grow. and we have relatedgrowth and dental features
that allow us to extrapolatewhen lactation really originated. and of course, it's ano-brainer that the brains are important for us. right? without capacity tothink, we wouldn't to be here to sit andenjoy this discussion. let's look at thehairs and integuments. and i'll start with one fossil.
and the name is not important. let's call it docodont. it's some extinct mammalfrom a long time ago. and this particular docodonthas very interesting scales associated with a bigfat tail, flattened. and in between thosetails, you can clearly recognize impressionsthat are the hairs. and in addition to this hairy,scaly tail-- and you can also tell that the tail vertebrathemselves are flattened.
and they have this so-calledtransverse processes. these are some barbssticking out to the side. and if you comparethe morphology, they are really verysimilar to modern beavers and modern otters, bothof which are aquatic. and it is with thiskind of a feature, we are able to understandthat this guy definitely flapped this tail in abeaver-like swimming motion. and has verypowerful fore limbs.
that's good for both diggingand also for swimming, such as the case of themodern platypus, which is also semi-aquatic. and then it's teeth havethese re-curved, sharp cusp, which is a typicallyinterpreted and is associated with fish feeding. so besides the fact that we'vegot a cool beaver-like mammal, the important fact is reallywe have this very ancient extinct group close tomammals, but are not
quite within themodern mammal group really fossilized with fur. basically the mammalianintegumentary structure originated well beforethe modern mammals. and our additional fossilgave us the same information. well you may say, it'snot a big surprise. mammals have fur. mammals have theseskin-related derived features. yes, it's not a big surprise,but nonetheless reassuring
that we can have fossilrecords good enough to give us an understanding ofthose common sense features. next i'm going to talk aboutlactation and the growth pattern. i will start witha fossil that's fuzz crampton's favorite. this is called pachygenelus. and this is from the earlyjurassic in southern africa. but the larger groupof this actually
has much widerdistribution in argentina and in southern africa. that record of this particulargroup related to pachygenelus really goes backto late triassic as old as 220 million years. and if you'll look at thevery interesting pattern of this pachygenelusfossil, you realize that these pc-- notpolitically correct-- they are called post-canines.
these are the teethbehind the larger canines. and if you look atthe pattern of this, you can definitely,in a single fossil, recognize at least threegenerations of teeth. so this guy hadmultiple replacements. besides the fact they replacedquite often, multiple times, they also have analternating replacement. the fact is probablynot as important if you compare it to themodern therian mammals.
and here is an australian dingo. and it belongs to placentalcanis, relevant to modern dogs. in a younger individual,we have deciduous teeth. in an older individual, we havethese permanent [inaudible] teeth, which are called molars. when these deciduousteeth are being replaced, it goes sequentially. the premolar, orrelatively simpler teeth behind the canine,when they replace they
go in a sequential wayfrom this end to this end. and then there are marsupials. and marsupials have onesingle tooth being replaced. the question we can askwith this early fossil is how we get from the ancestralcondition of our near relative to our kind of condition. the reason we care aboutthis is this pattern is related to determinantskull growth and the lactation. where as this is essentiallyour ancestral condition.
let me back step alittle bit and explain what we call determinantskull growth. i'm going to digressand use an example from the other side ofmodern vertebrate diversity, and that's the birdsand the dinosaurs. dinosaurs have veryrapid early growth. and a modern dinosaur,like ostrich and the emu, facilitated by thisintense parental care, they grow really fast early on.
but once they getto a certain stage, they reach adult body size. for the rest ofthe longevity, they have this plateauedgrowth curve. in contrast todinosaurs and birds, there are manyreptiles that would have the slow andthe sustained growth. and this littlewhat we generally call the indeterminate growth.
let me redefine a little bit. and this particulardiagram is a contrast between a rodent representativeof modern mammals and a crocodile. and generally speaking innon-mammalian, non-avian vertebrates, you havethe slow early growth, but the growthnever quite stops. whereas in birds andalso in modern mammals, facilitated by-- in amammal's case-- lactation,
you have this rapidburst of early growth. but because you grow so fast,you have to taper off somehow. so we have thisplateaued growth curve. and this gave us a veryinteresting body size difference. and if you have goodfossil preservation from some fossil sites,you can literally test. and you can distinguish if somesamples of fossils actually better indication prolongthe juvenile growth and then
a long, sustained but slowgrowth throughout the life. and if we have acase like this, we will call itindeterminate growth. it's not reallycompletely without end, but the sense is it won'tgo on for quite a while. but if we can do thestatistical analysis of a decent fossil sample, wecan distinguish a fossil group that has achieved veryhigh, rapid growth. and then their growth, oncethey reach an early age,
would be leveled off. now here is some real data. and this data are some smallrodents, small insectivores, compiled by a veryactive research lab in the 1970s in smithsonian. john eisenberg was themain leader of that. and these are thedays of individuals that you can samplefrom the field. and these are 100 personsof your adult body size.
and if you map out of the growthpattern over a time course, you get a sense of manyrodents pretty much mature, reach their adultbody size, in 30 days. it's very, very fast. and some of the insectivoreswould go on a bit longer. and they would go60 days, 70 days. but the bottom line is allmammals grow very fast. why do they grow fast? we all benefitfrom mother's milk.
and we all benefit frommother's lactation. but once we get up tothat adult body size, our body size is fixed. and therefore we havethis fixed plateau. it is a really in thisearly phase-- because we benefit from the lactation--and we can grow without teeth. and that shaped ourbasic growth pattern. so everything elsebeing equal-- because we have to plateau off-- wehave early termination
of the possibilityto replace our teeth. therefore thislactation really gave us a different timetable for growth. and if we are able to establishthe so-called limited dental replacement called diphyodontdental replacement, we are able to infer thatthis particular fossil must have had lactation. so that's my reasoning. and here is a fossil.
and it's called it brasilodon. it's from late triassic brazil. and this is a young individual. this is an older individual. that's the time course. and you can tell that thefront part of the teeth are still reptilian. whereas the backsideof the teeth-- and they are having thesequential replacement.
there is no longer thisrapid replacement of teeth. therefore, it'smore mammal-like, if not yet fully mammal-like. now if we go along in theevolutionary tree to another, more derived form,called sinoconodon. and this is one of those mothersfrom early jurassic china. this is actually a work thati did with fuzz together. and this is a veryyoung individual. this is a very old individual.
i'm not going to quiz you aboutthis very complex diagram, but it's suffice to focuson this anterior part. it's called canine. and even in theyoungest individual, we have replacement of canine. if you map it out, you'llfind the following pattern. and that is the anterior teethare replaced multiple times. and we can actuallyknow for sure, at least, the sinoconodon hasfour generations
of dental replacement. it will replace early. it will replace late. basically, it keeps onreplacing and never fully stops. and that is a reptilian pattern. now if you compare thisfossil, sinoconodon, with another fossil thatis relatively well-known called morganucodon. you will find thesize pattern is
very different betweenthe two species. essentially, morganucodon hasa very narrow growth band. and it fits the predictionof the general model. if you have a sample of afossil-- very few babies, relatively few adults-- and theyhave a very narrow size range. in contrast to that, sinoconodonhas a much wider range. and in fact for the rangeof sinoconodon we have, we estimate it can grow almostlike 44 of the body mass. but the key is thesmall sinoconodon
replace their teeth. the larger sinoconodonwould replace their teeth. and essentially, minusthe fact that sinoconodon looks like a mammalwith the jaw. it looks like a mammal withear, but its dental replacement is reptilian. so what does this tell us? it'll basically tell usthat in these near mammals, we start to have moremammal-like condition.
there is a reducedreplacement of teeth. and in sinoconodon,we already have-- very likely-- asingle generational of most of the teeth in the jaw. but the anterior teeth arestill replaced multiple times, early and also late. it is a really inmorganucodon we start to have noreplacement of molars, therefore achieves the trulymodern mammal-like diphyodont
dental replacement. that usually facilitated inextant mammals by lactation. so if you take this logicone step further, essentially between sinoconodonand morganucodon, we caught theevolutionary moment when we started to have theearliest fossil that can tell us reliablylactation has arisen. i wonder if we have dentalcare professionals here. you will wonder every time yougo into the dentist's office,
they have all these fancygadgets that make noise. you'll open yourmouth, half sedated. and they do this on your teeth. it's suffering, right? well the bottom line is we havelimited dental replacement. if we get stuck withthat permanent dentition, we have to take care of it. so essentially next timeyou tell your dentist, it's really that moment220 million years ago.
evolution took aturn for the better. so you get to suffer. and he has a job. well let's move on to the brain. we have this very nice fossil. and this is prepared by thevery best fossil preparer, bill amaral, who worked herein the mcz for many years. he just recently retired. besides the fact we can figureout a many anatomical details,
this is the one we knowabout this particular fossil. and it has a relativelylarge brain case. and we named it hadrocodium. hadro means full. codium means big head. and so this is also astudy that i did with fuzz. now if we scale the size ofthe brain against the estimated body mass, we have this eq. it's not emotional quotient.
it's encephalization quotient. it's a metric thatestimates body size after you correctedfor body mass. and so the eq is0.49, in contrast to a modern opossumthat has an eq of 0.34. so this guy is brainycompared to modern opossums. here is how we get it. here is a ct scan surface model. and in the inside, wecan see quite a bit
of anatomical structure. with quite bit of strugglefrom my co-author ted macrini, we were able to extractthis brain endocast. and you can see many featuressuch as an olfactory bulb and a relatively largecerebral hemisphere. and not just a largecerebral hemisphere, it's really the side of thecerebral hemisphere homologous to the brain corticalarea in modern mammals for sensory touch andmotor coordination that
got significantly bigger. so what does this mean? well, let's pull back anddo some comparison first. on the left is our distantrelative from early triassic called thrinaxodon. it has been endocast. first we can tell that therelatively modern mammals have very distinctiveolfactory bulbs. and the biggest differenceis modern mammals
have this largecerebral hemisphere. and modern mammals alsohave a cerebellar hemisphere that is absent in thedistant cynodonts. but when you add ontop of one another, you can tell that thisguy is not very brainy. and this guy is brainier. and so the question is how do weget from our triassic ancestor, or ancestor-like condition,to the modern mammals? well here is how we do it.
we use the evolutionarytree as a road map. and we essentially map thistransitional state one state at a time on the tree tosee how it is distributed. and its distribution patternwill give us a possibility to infer how the process ofevolution actually occurred. now with the rise ofa mammalial forms-- these are by jaw, by teeth,characterized as very mammal-like-- we start to havethis divergence of the brain hemisphere.
and once we getinto modern mammals, and we start to have verydistinctive mid-brain areas differentiated, it is areally in the modern therian. mammals that we start to havethis cerebellar hemisphere. let's focus on next thisgeneral anatomical region. in modern mammals,this general area of the brain, the neocortex, isresponsible for sensory touch. and you can actually mapthese somatosensory fields. and so you have theserepresentative animals.
and the signal should come froma tactile sense of the hair. we know that the earlymammals got hair. so that's not a worry. here is a diagram-- i'msorry this is washed out. but you've probably seenthis multiple times. it's a cut outsection of the brain in the cerebral hemisphere. and it represents thesensory touch of the surface. but if you look atthis morganucodon,
you get a sense that thebrain cast has become larger. it's not just becomeover-all large. it's really in thislimited cortical area that a modern mammal responsiblefor somatosensory fields that becomes larger. so what this means is in theearly evolution of mammals, we start to have alarge olfactory bulbs, large cerebral cortical arearesponsible for tactile sense. and this is before weget to the modern mammals
range of a brain size. again this eq isrelative brain mass after being correctedagainst the body size. and in hadrocodium, we startto have the relative brain size really within the range ofmodern mammal variation. and after that, the story islargely over about the brain evolution. so we talked about whatwe can do with the fossil from the triassicand the jurassic.
well let's forwardrelatively rapidly to talk about the splitof modern groups-- modern groups ofplacentals and marsupials. here is the fossilwe call juramaia. it's 160 million years. and i'm happy to reportthat we now already have a second fossil ofthis that we are working on. but before i go intothat, let me go back to our breakfast egg.
and here is the embryo. and the yellow partis the yolk sac. much of it is allantoic sac. and here is theamniotic fluid sac. essentially to live withinthe egg shell comfortably, we have to bring ourgroceries with us. we have to have a nice hot bath. and then not enough, wecarry our porta potty with us inside the egg shell sowe can live very comfortably.
now to take thosehomologous parts to understand the placentaof modern mammals. marsupials largely havea yolk sac placenta. so this is a general featureof marsupial placentas. and this placenta isnot terribly integrated into the endometriumof the uterus. and then placental mammalsalso got the placenta, but it's more sophisticated. it's from the amniotic part ofthe extraembryonic membrane.
and this placentastructure deeply embedded into the uterus. therefore we have much moreeffective parental care in utero. now if we look atthe broader pictures of placentaldiversity, no matter which group you arelooking at, they are all having largelythe same type of placenta. marsupials are different.
the parental care is much morelimited within the placenta. and usually marsupials alsohave much shorter pregnancies. and after the precocialfetus are born, they are sucked ontothe mother's nipple inside the pouch where they havethe rest of the development. all the marsupials tend tohave the same general placental design except thebandicoots, which are more like themodern placentals. so there is always exception.
but all of thesefeatures are also associated withteeth, with jaws. and it's by really lookingat the teeth and the jaws we are able to map someof the earlier fossils onto the overall placentaland the marsupial trees. this also matters fromanother perspective. and that is we nowunderstand mapping the genome or thecomplete genetic makeup. it is very important.
so after 2005, multipleplacental mammals have been mapped fortheir entire genome. and around 2006, at leasttwo of the marsupials have also been mappedfor the genome. and when you go down onthe evolutionary tree, the first versionof completed genome is already mappedfor platypus in 2007. but eventually in order tocompare different genomes, we have one roadmap.
and that's ourevolutionary tree. and our evolutionary tree, ina sense, is also our legacy. that's part of thereason why our interest in building thisevolutionary tree. but not just with the molecules,but also with fossils too. let me revisit an age-oldissue about the divergence time of modern mammal groups. this is the best study thathad most extensive sampling. i know you cannotread the details.
in this daisy dial, thereare 400 different families. but what's important isto bear in your mind-- and this black line isgeological time scale. this is the present time. this is the middle jurassic. this red circlerepresents the cretaceous and the paleogene boundary. that's the 65 million year eventwhen dinosaurs got wiped out. when you map all thisevolutionary lineage
according to this study,you have many groups that go before theend of the cretaceous. and here is the study by thensf tree of life mammal project. the molecular team,they have sampled the largest concatenatedmolecular data set. and many of the groups wouldgo back into the cretaceous by this molecular study. of course themorphological evidence do not always support such aclaim, excepting of one case.
and that's the split ofmarsupials and placentals. the molecular estimate ofmarsupial placental split goes roughly themiddle jurassic, roughly from 140 millionto about 180 million years. and no matter whichstudy you are looking at, it's roughly inthis time window. the discovery ofjuramaia definitely plays very closely with thismolecular time estimate. however, for the rest ofthe late cretaceous fossils,
we still have atremendous gap between the morphologicalestimate of major groups than the moleculartime estimates. and here is one of theearly fossils called eomaia. this is now from jurassic. this is from early cretaceous. and this is a sinodelphys. and this is punitively relatedto the marsupial clade-- at least, i think so-- whenyou map this fossil record
onto the time scale andfollowing evolutionary genealogical tree. about 30 years agowe were largely working with the fossilsfrom this time interval. about 10 years ago, we startto have eomaia sinodelphys. and we map the fossilrecord to this level. and in the more recenttime, we can definitely push the split betweenplacental on one side and marsupial on the other downto this time interval, which
will fall comfortably withinthe molecular estimate of these groups divergence. so it's good tohave concordance, but we do not always haveconcordance in a sense that we still have thisunresolvable issue about when the modern placental groups--not all mammals, just placental groups-- startto appear on earth. and with this kindof a fossil, we can also answer someecological functional
questions in a limited way. and here is a reconstructionof sinodelphys. let's focus on the wrist. and by looking atof the wrist, we can tell that up in the formclosely related to marsupials, there are serving bonds that arefar more robust than the form that are closelyrelated to placentals. and it's also veryinteresting the shape of the claw and of theproportion of the fingers
can also gave us an inferenceabout how this animal actually moved around. well let's backstep a little bit to follow thisline of discussion. and here is themodern opossum groups. and these are largely southamerican extant opossums. and we have [inaudible] possum. and that is arboreal. and we have mouse opossumthat is also arboreal.
but we have gray opossumthat is terrestrial. and like our backyardvirginia possum, it's a large terrestrial, butit's also capable of climbing. and among the terrestrialopossums, we can have swimmers. now it turns out all thishabitat preference or niche specialization arealso represented by the finger proportion. how so? so let's say hereis [inaudible].
it's one of the opossums. and it's definitelyknown as ground living. and here is our familiarvirginia possum, didelphis. it's largely ground living, butit's also capable of climbing. and these opossums arefull time tree living. so in the specialized formsthat stay more time on the tree because they have tograb and hold the finger proportions are different. essentially the fartherend of the fingers
tend to be a longer. and the same patterncan be also detected in modern placental mammals. if you map the earliestmarsupials and the earliest placentals from china, theirfinger proportion definitely falls onto the climber sidethan on the terrestrial side. all right so we getsome level of inference about how they moved around. now here it's a bitmore complicated.
again, we are focusing on thehand bones called metacarpals. and we're focusing on the fingerbones called proximal phalanx and the intermediate phalanx. so you map this bonealong this axis. you map this bone on this axis. and then you map thisbone on this axis. essentially this plot showsyou how, by finger proportion, you can allocate thehabitat preference of this modern primateand the marsupials
in their distribution. so that's how we understand . we can use the fingerproportion to distinguish different habitat preference. let's bring the fossilsinto this discussion. and this triconodon'smultituberculates. these are extinct fossil groups. these lived fromjurassic to cretaceous. the majority of these,so far as we know,
they're more liketerrestrial mammals. but by the time we getto a near-mammal relative called [inaudible]. they're also more likely-- orthe majority known so far-- are also terrestrial. but by the time we get to themodern placental relatives, marsupial relatives, they aredefinite closer to the tree living primates andtree living marsupials. so associated with the riseof our modern group-- that
is placental on one side,marsupial on the other-- there is definitely a shifton locomotory adaptation and the habitat preference. essentially somewhere aroundthe rise of modern mammal group, of therian, wegot into the tree. or we had a better capacityto get into the tree. now this specializationis not isolated. we used to have the stereotypebefore the cretaceous paleogene extinction, somehow wehave very limited diversity
of modern mammals. and we also have a just-sostory to go with it because these early mammalslived in the time of dinosaur. they shared the sameecosystem with the dinosaur. and suppressed by thedinosaur, they just failed to gain the greatecological possibility that it can realize untilthe dinosaurs are all gone. so that's what weunderstood then. but this is whatwe understand now.
around 2006, we start to havesemi-aquatic forms discovered in the fossilinterpreted from some of the older fossil records. and the start from2001 to 2005, we get large enough of a mammal. we know for a fact itcan eat a dinosaur, but it's generallycapable of carnivory. and that was consideredunlikely before the recent time. and in 2005, we discoveredthis form called [inaudible].
and it has a very unusualspecialization otherwise only known in aardvarksand armadillos. that is they have these peculiarteeth that's usually associated with tongue feedingof colonial insects. highly specialized, itused to be considered as only possiblewith extant mammals. yet in a totallyunrelated mesozoic group, we found the same pattern. of course we have, now, multipleforms that can climb the tree.
and once you canclimb the tree, it's only a jump away from gliding. this is what we used to know. and this is what we nowknow for our last 20 years of hard workingby paleontologists going to allcorners of the world finding the best fossil you can. and also in the case ofchina, the great ingenuity of all these peasants triedto do better for themselves,
dug out all these great fossils. so what our jurassicancestor can tell us? we can tell thatthey are not just a bunch of mesozoic road kills. they are flattened fossils. but they can definitelytell us about how the integumentary structureof mammals that characterized our whole group arisein the fossil record. and they can definitely tellus about the general growth
we can, with confidence,tie to the most fundamental mammalian adaptation. that's mammaryglands and lactation. and we can also tellthe general pattern how in the earliest phaseof mammalian evolution, we start to have a larger brain. and we can now also tell thatno later than middle jurassic, we start to have the threemain branches of extant mammal lineages.
and that's placentals,marsupials, and the monotreme. and the rise of thetherian mammals, the placentals andthe marsupials, is definitely accompaniedby some very interesting ecological diversification. it's more than just luck. it's really withgood fingers that we manage to hang onto this next episode of evolutionarydiversification in jurassic.