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Southern fossils have changed the geography, timing and anatomical character of mammaliaform evolution, while exposing how strongly a northern fossil record had shaped earlier expectations. Their importance is not that every southern lineage arose in the south. A dated occurrence establishes presence and a minimum age; an evolutionary origin requires an argument from phylogenetics and biogeography. Nor do modern continental outlines reproduce the connections of Gondwana.

A particularly consequential example is Ambondro mahabo, represented by a jaw fragment with lower teeth from Middle Jurassic Madagascar, approximately $167\pm 2$ Ma. The teeth possess a basined heel associated with the crushing component of tribosphenic molars, which combine shearing and crushing in a single occlusal system. Its age extended the then-known record of this dental construction substantially backwards and challenged a simple northern origin inferred from better-sampled northern localities. The material nevertheless consists of a very restricted part of the animal, and the matching upper dentition was not directly observed. Assigning the entire skeleton, physiology or reproductive mode from these teeth would go beyond the evidence. ^[ref]

The discovery also changed the character of the phylogenetic question. One hypothesis groups several southern forms in Australosphenida, associated with the ancestry of monotremes, and proposes that a tribosphenic-like construction arose independently of northern Boreosphenida, including therians. In that interpretation a highly functional tooth arrangement is convergent evolution, not a unique certificate of therian ancestry. A competing analysis supports a single origin of tribospheny and rearranges the relationships of the relevant southern taxa. ^{[ref] [ref]} The important lesson is not to declare one historical topology established by the striking tooth alone. The choice depends on which cusps and basins are judged instances of biological homology, character coding, inclusion of other anatomical regions, and the taxon sample. Nor does a Middle Jurassic tribosphenic tooth by itself date the split between living marsupials and placental mammals.

South American monotreme material further undermined the equation of present distribution with evolutionary birthplace. Early Paleocene Monotrematum, approximately 60–65 Ma, has teeth with affinities to the platypus lineage. Living monotremes are restricted to Australasia, but that restriction cannot be projected unchanged into the past. ^[ref] South America, Antarctica and Australia offered changing opportunities for connection, fragmentation and movement. A South American occurrence is compatible with a geographically extensive southern history; it does not uniquely establish the direction of dispersal or prove that Monotrematum was the direct ancestor of the living platypus. The distinction between a crown group and related members of its stem group is essential when attaching such fossils to divergence-time estimates.

Latest Cretaceous Madagascar has contributed more than isolated tooth surprises. The cranium of Vintana, around 70 Ma, supplied extensive new characters for the formerly tooth-dominated Gondwanatheria. Its robust feeding apparatus and unusual cranial combination permit tests of relationships and ecology that were unavailable from a few teeth. Yet a mosaic of familiar and unusual features is precisely the situation in which a single apparently diagnostic character can mislead. The placement of gondwanatherians therefore needs broad anatomical and taxonomic comparison, including other extinct groups, rather than assignment to a living order by superficial resemblance. ^[ref]

The approximately 66 Ma skeleton of Adalatherium expanded that evidence into the postcranial skeleton. It documents a relatively large, several-kilogram non-therian mammal with an unusual combination of cranial, dental, vertebral and limb features. This contradicts the caricature that all Mesozoic mammaliaforms were anatomically interchangeable tiny insect-eaters. Madagascar's long isolation provides a plausible context for its evolutionary distinctiveness, but the claim that isolation caused a particular feature remains an explanatory hypothesis. One needs comparative island and continental samples to distinguish an island effect from inherited lineage-specific anatomy. ^[ref]

Australia's early Eocene Djarthia, approximately 55 Ma, addresses a different issue: the relationships and movement of marsupials. Its ankle material supports a position close to the base of Australidelphia, a group containing the living Australian orders and the South American Microbiotheria. This is stronger evidence than assuming that every small Australian jaw belongs inside a modern Australian family. ^[ref] It is compatible with a history involving South America and Antarctica, but a branching order is not a map of a single migration. An extinct distribution can be extensive, intermediate populations can disappear, and the oldest sampled fossil need not occur near the place where a lineage originated. Also, the older and more inclusive Metatheria must not be confused with the living marsupial crown group.

Together these discoveries change how large-scale history is tested. First, expanding southern sampling can overturn a supposed absence that arose from collecting intensity, exposure of suitable rocks or preservation. taphonomy and the concentration of many early records on durable teeth must be modelled before comparing continental diversity. Second, new skeletons can change the inferred phylogenetic tree, and therefore whether a tooth adaptation counts as one origin or several. Third, fossil calibration constrains the minimum age of a lineage, with explicit uncertainty in both geological age and phylogenetic assignment. Fourth, continental drift changes which dispersal routes were physically plausible. Shared distributions across former Gondwana may reflect vicariance, later dispersal, or a mixture; the geological separation must be compared with a defensible lineage age rather than used as its automatic explanation.

From the Jurassic through the Palaeogene, these independent discoveries have cumulatively changed both the anatomical evidence and its interpretation. They reveal a more diverse southern mammaliaform history and force explicit tests of convergence, missing data and geographic sampling, rather than replacing an assumed northern centre with an equally assumed southern centre.

Living great apes are indispensable comparative evidence, but none is a preserved version of the Pan–Homo last common ancestor. The ancestor was a population with its own combination of traits, and every living descendant has subsequently evolved. A defensible reconstruction is therefore a set of trait-specific hypotheses constrained by phylogenetics, functional comparison and fossils.

The comparison starts with relationships. Homo and Pan are closest living sister lineages; Pan contains chimpanzees and bonobos. Gorilla provides a more distant comparison, and Pongo a more distant one again. Humans are themselves great apes, rather than a group standing outside them. A character shared by Pan and Homo may have been inherited from their last common ancestor, but it can also have changed in either lineage or arisen in parallel. A shared feature of Pan and Gorilla does not automatically reconstruct the ancestor either: it may be ancestral, convergent, or retained in some branches and lost in others.

Living great apes are especially valuable because their soft tissues, growth, behaviour and mechanical performance can be studied. Muscle paths, joint motion and hand loading can make competing functional interpretations of a fossil wrist testable. Observing climbing and terrestrial movement establishes which combinations are mechanically possible; it also exposes the inadequacy of labelling a whole animal simply “arboreal” or “terrestrial”. Appropriate controls compare individuals of different ages and sexes, adjust for body size, record substrate and loading, and distinguish performance in captivity from behaviour in the wild. A surface that resembles another surface is weak functional evidence unless its position in the complete joint and the range of loads are considered.

The same accessibility creates temptations. chimpanzee locomotion, male coalitions or territorial behaviour cannot be transferred wholesale to the common ancestor. bonobos demonstrate variation within Pan, not a second unchanged ancestral template. Ecology, group composition, developmental plasticity and histories of selection differ among living populations. Current behaviour can generate hypotheses about feasible ancestral social systems, but proving that a particular ancestral behaviour existed requires additional evidence; most social behaviours do not fossilize. Treating the contrast as a choice between a uniformly aggressive chimpanzee ancestor and a uniformly peaceful bonobo ancestor replaces biology with caricature.

knuckle-walking illustrates the genuine scientific alternatives. One reconstruction takes the shared use of knuckle support in Pan and Gorilla as evidence for an ancestral condition, with later loss or modification in the hominin lineage. wrist features in early hominins have been interpreted as retained support for that hypothesis. ^[ref] A competing study emphasized differences in the development and functional organization of chimpanzee and Gorilla wrists, consistent with independent origins of their knuckle-walking systems. ^[ref] Similar external posture can therefore conceal distinct load-bearing arrangements. Neither simply counting two living knuckle-walking genera nor quoting the conclusion of one comparative study settles the ancestral condition. The inference depends on biological homology, functional interpretation and inclusion of relevant extinct forms.

Ardipithecus ramidus, approximately 4.4 Ma, is particularly important because its skeleton combines a grasping great toe and arboreal capabilities with anatomy interpreted as supporting terrestrial bipedalism. The authors argued that its hands and feet did not represent an unchanged modern chimpanzee locomotor package. ^[ref] This weakens a reconstruction that derives early hominins by merely standing a modern chimpanzee upright. However, Ardipithecus lived after the inferred Pan–Homo divergence; its features may include specializations of its own. Its proposed hominin position and functional reconstruction must also be evaluated against incomplete preservation and distortion, especially in the pelvis. It is evidence about a nearby branch, not direct observation of the common ancestor.

Older Miocene apes extend the comparison beyond surviving lineages. Pierolapithecus, approximately 12.5–13 Ma, combines an upright-trunk construction (orthogrady) with hand anatomy lacking the complete suspensory specialization of some living apes. ^[ref] The example shows that trunk posture, climbing and suspensory locomotion can evolve as a mosaic. Its exact position is debated, and it is not thereby identified as the Pan–Homo ancestor. Rather, extinct combinations expand the set of plausible ancestral forms and can reveal that a feature widespread among living apes was assembled in stages.

Genetic evidence establishes relationships and helps constrain timing, but a molecular divergence estimate is not a fossil anatomical reconstruction. Estimates are sensitive to mutation rate, calibration and generation interval. Pedigree observations of wild chimpanzees and Gorilla demonstrate why substituting one universal generation length can shift the inferred dates of great-ape divergences. ^[ref] A gene tree can also coalesce before a population split, and incomplete lineage sorting means that not every genomic region follows the species branching order. There is consequently no reason to force every candidate fossil into an overly precise split date selected from one molecular estimate.

The best procedure is to specify competing ancestral states and their predictions: for example, whether a particular wrist character should accompany habitual knuckle support, or whether an inferred climbing apparatus requires suspensory finger proportions. Score those characters across Pan, Homo, Gorilla, Pongo and well-preserved fossil taxa; examine sensitivity to uncertain character coding and alternative fossil placements. Related features must not be counted as a large number of independent confirmations when they are mechanically or developmentally coupled. Extant-only ancestral state reconstruction is especially vulnerable to the extinction of lineages that possessed combinations absent today.

Living great apes supply the comparative mechanics, development and diversity needed to interpret fossils; fossils prevent the living sample from dictating a false ancestral stereotype. A cautious composite reconstruction, with separate uncertainty for posture, hand use, diet and behaviour, is more defensible than choosing one living species as the ancestor's complete anatomical and social model.

taphonomic bias is systematic distortion of biological information between a living community and the fossil record. Death, carcass decay, scavenging, transport, burial, chemical alteration, exposure and collection all filter what is observed. A deposit therefore samples organisms, body parts and environments unequally. This is more than incompleteness: preferential preservation of a particular tissue or habitat can produce a misleading pattern of abundance, anatomy or evolutionary timing.

An autochthonous fossil assemblage contains remains preserved where the organisms lived; an allochthonous fossil assemblage contains material transported from elsewhere. These describe provenance, rather than two levels of fidelity. In an allochthonous fossil assemblage, currents can separate elements by size, shape and density, concentrate resistant teeth or scales, remove delicate bodies and mix organisms from different habitats. Erosion and redeposition can also introduce older fossils into younger sediment. Such reworking differs from simple transport of recently dead animals. time averaging mixes deaths from different times even without long-distance transport. A concentration of robust remains consequently need not represent either a locally abundant living population or a single ecological event.

An autochthonous fossil assemblage avoids some transport distortion, but still selects organisms and characters. A bottom-dwelling community rapidly buried in place can be unusually well represented while swimmers above it, unmineralized tissues and organisms living outside that depositional setting are missing. Conversely, an intact carcass can travel before burial, whereas locally decayed remains can become disarticulated without transport. Articulation, abrasion, preferred orientation, sedimentary context and age agreement must therefore be evaluated together. Local preservation does not imply an unbiased biological census.

The contrast between cyclostomes and early members of the gnathostome lineage is particularly revealing. Living cyclostomes, hagfish and lampreys, have no extensive mineralized external skeleton comparable to the armour of many early jawless relatives of gnathostomes. Their bodies are dominated by tissues whose preservation usually requires exceptional conditions. The abundance of isolated mineralized scales, plates or teeth can thus make a hard-bodied lineage conspicuous where a soft-bodied contemporary is virtually invisible. Importantly, the armoured jawless assemblage traditionally called ostracoderms is not simply the ancestral cyclostome record: many of its members belong on the gnathostome stem. Absence of jaws does not by itself establish cyclostome affinity.

There is also a character-level filter. Robert S. Sansom and colleagues followed experimental carcass decay of living chordates, and then examined hagfish and lampreys in detail. Anatomical characters did not disappear at random: loss of diagnostic features could make a derived animal appear more primitive. This produces stemward slippage if investigators mistake an unpreserved feature for a feature absent in life. The result is relevant both to isolated soft-bodied fossils and to the apparently long primitive intervals preceding well-preserved groups.^[ref][ref]

A useful experimental design records fresh anatomy first, samples replicated carcasses through time under stated temperature and chemical conditions, and scores the same characters in each specimen. Comparisons between animals and between preservational conditions test whether the loss order is reproducible. Such experiments identify plausible biases; they do not recreate every burial chemistry or prove that every uncertain fossil has undergone the same sequence. In a phylogenetic character matrix, genuinely unobservable anatomy should normally be coded as unknown rather than automatically absent. Comparison across differently preserved specimens can sometimes recover information that one specimen lacks.

Exceptional discoveries show what the ordinary record omits. The Late Devonian lamprey Priscomyzon from South Africa preserves a recognizable oral apparatus and branchial region without a massive mineralized skeleton. It shows that early lamprey anatomy can be recovered when the appropriate soft-tissue window is available; the gap before such a discovery is not evidence that the lineage had just originated.^[ref] The Cretaceous hagfish Tethymyxine preserves evidence including slime-producing structures, allowing characters unavailable in most older deposits to be brought into an analysis using phylogenetics. It supplies a minimum occurrence and anatomical information, not the date when hagfish originated.^[ref] Conversely, Early Silurian Qianodus is known through mineralized tooth whorls: such durable small remains can establish an early occurrence of a toothed gnathostome even where complete bodies are missing.^[ref]

The defensible comparison concerns preservation probabilities as well as biological histories. First fossil occurrences are minimum ages, and unequal counts need not measure unequal diversity. Independent anatomical characters, comparable depositional settings, searches for small hard parts and exceptional soft-tissue deposits together give a better test of early vertebrate history than a literal reading of the most conspicuous remains.

The transition places vertebrate origins within a prolonged assembly of animal body plans, ecological interactions and preservational opportunities. It does not identify one boundary at which a complete modern vertebrate suddenly appeared. Three questions must be separated: when the relevant lineages diverged, when diagnostic features evolved, and when those features became recoverable as fossils.

The Ediacaran precedes the Cambrian, whose beginning is approximately 539 million years ago. Ediacaran communities already contained macroscopic organisms and evidence of animal movement and feeding. Kimberella, for example, has been interpreted from its body organization as a mollusc-like bilaterian; its relevance is that substantial animal differentiation predates the boundary, rather than that it is a vertebrate ancestor.^[ref] trace fossils record activities that may otherwise leave no body remains. Increasingly elaborate movement and sediment disturbance reveal ecological capacities, but an isolated burrow does not identify a notochord, a head or a particular branch of the animal phylogenetic tree.

Across the transition, animal skeletons, predation, burrowing and water-column communities became more prominent. Their appearance changes both biology and observation. Resistant skeletons improve preservation, while bioturbation can destroy the sedimentary surfaces that had preserved some earlier soft-bodied communities. Different preservational windows reveal different subsets of life. An apparent replacement in the fossil record can therefore combine real ecological turnover with taphonomic bias. One should compare anatomy and environments across several deposits rather than treating all Ediacaran absences as biological absences.

The Cambrian exceptional deposits provide positive evidence. Haikouichthys and Myllokunmingia from Chengjiang reveal small fish-like animals with combinations of axial, muscular and branchial structures interpreted as early vertebrate organization.^[ref] Metaspriggina from the Burgess Shale adds evidence from a distinct assemblage, including prominent eyes, segmented musculature, organized into myomeres, and serial branchial supports.^[ref] These are more informative than an undifferentiated claim that “fish existed”: they allow particular characters to be mapped onto alternative phylogenetic trees.

A plausible sequence involves a swimming chordate organization with a notochord and segmental musculature, followed by elaboration of anterior sensory and feeding systems along the vertebrate lineage. Nevertheless, fossil compressions can obscure boundaries or superimpose unrelated structures. Confidence should come from repeated specimens, bilateral agreement, positional relationships and combinations of characters, rather than resemblance of one dark patch to an organ. A structure absent from a compressed specimen may be unpreserved. Nor does a plausible early vertebrate have to be a member of the living vertebrate crown group, still less a direct ancestor of a living species. A stem group can contain extinct side branches showing different combinations of ancestral and derived states.

Living organisms constrain the reconstruction without forming a ladder. Molecular analyses place tunicates, rather than cephalochordates, as the closest living relatives of vertebrates.^[ref] A modern lancelet is consequently a comparative relative with its own history, not an unchanged stage through which vertebrates passed. The same warning applies to living hagfish and lampreys. Their anatomy can reveal inherited features and secondary specializations; simplicity alone cannot polarize a character.

The New Head hypothesis connects cranial sensory and feeding innovations with neural crest and ectodermal placodes. This is a developmental explanation for assembling a more elaborate head. Experiments on living chordates can test where cells originate, which structures they form and which regulatory interactions are needed. The fossil presence of eyes or branchial supports cannot directly demonstrate the embryonic migration of neural crest. Conversely, a conserved regulatory gene alone does not establish that a complete modern head existed in an Ediacaran ancestor. Developmental, molecular and anatomical evidence address complementary parts of the argument.^[ref]

Ecological context helps explain why these capacities might diversify. More active swimming, prey detection and food processing can create opportunities and selective pressures for improved sensory and locomotor coordination. molecular oxygen availability constrains the metabolic activities that can be sustained. Erik A. Sperling and colleagues compared modern low-oxygen animal communities and related the scarcity of carnivory at low oxygen levels to hypotheses about Cambrian ecological expansion.^[ref] This motivates a mechanism; environmental correlation alone neither dates the vertebrate ancestor nor establishes oxygenation as a sufficient single cause.

The useful conclusion is a constrained historical sequence, with explicit gaps. Ediacaran evidence establishes antecedent animal complexity; Cambrian exceptional preservation documents recognizable early vertebrate combinations; comparative phylogenetics and evolutionary developmental biology test their placement and assembly. molecular clocks can estimate earlier divergences, but their dependence on rates and fossil calibration prevents them from turning every absence into a precise origin date.

Phylogenetics infers evolutionary relationships from evidence such as anatomy and DNA sequences. A phylogenetic tree summarizes a proposed pattern of ancestry; phylogenetic characters supply the observations, and an inference procedure states how alternative trees are compared.

For sequence data, a nucleotide substitution model assigns probabilities to changes along branches. The phylogenetic pruning algorithm efficiently sums over unknown ancestral states to compute a likelihood function. This connects a historical hypothesis to an explicit calculation rather than treating the most visually similar species as automatically closest relatives.^[ref]

Rhabdoviridae is a family of Negative-sense RNA viruses that includes vesicular stomatitis virus. Many particles have a characteristic bullet-like or rod-like form with a viral envelope, but neither that shape nor an envelope is universal across the family. The official report also distinguishes the common single-molecule genome from members with two or three segments.^[ref]

The family illustrates why a useful example must not become an absolute definition. Recognizing a bullet-shaped particle supplies structural evidence; classification also uses genome organization and phylogenetics. Different members infect animals or plants, so one host or transmission pattern cannot stand for all of them.^[ref]

A classic mechanistic experiment used vesicular stomatitis virus. In 1970, researchers detected particle-associated RNA-dependent RNA polymerase activity that generated RNA complementary to the viral genome.^[ref] This supported an explanation of how an incoming negative-sense genome can initiate transcription, rather than assuming that its genomic RNA is already an ordinary protein-coding message.

Ribosomal RNA oligonucleotide cataloguing compares organisms through collections of short sequences obtained from ribosomal RNA. Instead of determining one complete modern sequence, investigators generated fragments, identified their sequences and compared which longer fragments were shared. The resulting catalog supplied molecular evidence about relationships beyond external appearance.

The 1977 Carl R. Woese and George E. Fox paper describes digestion of small-subunit ribosomal RNA with ribonuclease T1, followed by separation of the products using two-dimensional electrophoresis. Sequencing the resulting oligonucleotides produced a catalog characteristic of the examined material. Comparing catalogs helped establish the distinct position of the methanogenic organisms discussed in the paper. This was an RNA-fragment experiment, not a modern amplified-marker survey.^[ref]

The RNA oligonucleotide association coefficient used nucleotide counts in cataloged sequences of six or more residues:Here $N_A$ and $N_B$ count nucleotides represented by the eligible fragments in the respective catalogs, while $N_{AB}$ counts those represented in their shared portion. It is therefore not simply the fraction of fragment labels found in common, nor the percentage identity of a complete aligned RNA.^[ref]

For an original accounting example, let $N_A=40$, $N_B=60$ and $N_{AB}=25$. Then $S_{AB}=50/100=0.50$. A catalog with fewer recorded nucleotide residues can share the same amount of material while giving a different normalized comparison. Agreement in short fragments alone also need not imply identical full sequences.

The scientific advance was a reproducible molecular comparison usable across morphologically different microbes. Its interpretation still depended on the marker, sampling and method. A catalog similarity supplies evidence for analysis in phylogenetics, not a complete reconstruction of every gene, a direct date of divergence or a guarantee that two organisms have the same physiology.

The RNA oligonucleotide association coefficient measures the overlap of two catalogs produced through ribosomal RNA oligonucleotide cataloguing, weighting the retained shared fragments by their nucleotide content. In the 1977 comparison, catalogued sequences had at least six residues, andwhere $N_A,N_B$ count nucleotide residues represented in the two eligible catalogs and $N_{AB}$ counts those represented in their shared portion. It is a catalog-overlap statistic, not the percentage identity of a complete sequence alignment.^[ref]

An original example uses distinct fragment labels with specified lengths. CatalogA contains $a_6,b_8,c_{10}$; catalogB contains $a_6,b_8,d_6$. The subscripts denote nucleotide counts, and differently named fragments are different sequences. ThusCounting only matched labels would instead give two shared labels out of three per catalog, producing $2/3$. The difference exposes the effect of weighting: a shared eight-residue fragment contributes more than a shared six-residue fragment.

For a nonzero denominator, $0\le N_{AB}\le \min (N_A,N_B)$ gives $0\le S_{AB}\le 1$. A useful equivalent form isThe numerator in the subtracted fraction represents the catalog material outside the overlap under this counting model. If both catalogs are empty, the original formula has denominator zero; assigning a value would require an extra convention.

The coefficient is also sensitive to catalog coverage. Add the same new six-residue fragment to both example catalogs: the totals become30 and26, with20 shared, so the coefficient rises to $40/56=5/7$. Add it only toA instead, and the coefficient is $28/50=0.56$. These constructed changes alter the statistic without specifying any new evolutionary history.

The original ribosomal RNA oligonucleotide cataloguing experiments used comparisons of this kind as evidence for phylogenetics. Turning a similarity into a particular phylogenetic tree or a divergence date needs additional assumptions and evidence. The statistic itself records the specified molecular overlap.^[ref]

A species concept states what should make organisms members of the same species; an operational criterion states what observations will test that claim. Reproductive isolation, distinguishable morphology and ancestry inferred through phylogenetics answer different questions. In a lineage-based interpretation, they are evidence for separation of evolving biological populations, and need not appear together. ^[ref]

Squamata is the reptile lineage containing lizards, snakes and amphisbaenians. It belongs within lepidosaurs, whose other living branch is represented by the tuatara. Snakes are nested among lineages commonly called lizards, so excluding snakes does not leave a complete evolutionary branch.

The relationships require evidence beyond superficial similarity: elongated, limbless bodies can arise in different lineages. A 2015 analysis combined hundreds of anatomical characters with dozens of genes from living and fossil groups to compare competing arrangements.^[ref] This makes Squamata a useful example of how phylogenetics tests anatomical interpretations against molecular evidence while retaining fossils that have no recoverable sequence data.

Taxonomy names and classifies organisms. Classification can organize known diversity and make comparisons useful, but ranks and names are not themselves evidence of ancestry. Phylogenetics infers that ancestry; a clade includes an ancestor and all its descendants. A shared feature such as lacking a cell nucleus need not identify an exclusive clade. The organism branch here organizes cell types for reading and is not a proposed phylogenetic tree. See OpenStax Biology 2e, 20.1: classification and evolutionary history.

A paleontologist who investigated hagfish fossils and the relationship between morphological and molecular evidence in early vertebrate phylogenetics.^[ref]

Tod W. Reeder studies evolutionary relationships within Squamata. In a 2015 study, he and collaborators combined 691 morphological characters with sequences from 46 genes to examine relationships among living and fossil taxa. The analysis explicitly tested conflicts between anatomical and molecular evidence.^[ref]

A fossil often preserves anatomical characters but no usable DNA. Combining evidence lets such a taxon be included in phylogenetics alongside living relatives with molecular data. The contribution illustrates why additional characters can change a fossil's inferred position: a family tree is an inference from a specified dataset and model.

A type species is the species serving as the nomenclatural type of a genus under a naming code that uses this form of typification. It anchors application of the genus name. It is not necessarily an average, an ancestor or the most familiar member of the genus.

In prokaryotic nomenclature, the Code explicitly identifies the type of a genus or subgenus with a type species and specifies how that species is fixed. If a genus originally included only one species when validly published, that species is its type even without an explicit designation. These are naming rules, not measurements of evolutionary distance.^[ref]

An original hypothetical example separates the reference from a summary statistic. Suppose a genus contains species T, U and V, with T the designated type species. A measured trait has values1,10 and11, whose arithmetic mean is $22/3\approx 7.33$. U lies nearer that mean, but this does not make U the type. The designation and the statistical comparison answer different questions.

Now suppose new evidence from phylogenetics divides the hypothetical group into T+U and V. The existing reference is associated with the group containing T; how the remaining group is named requires the relevant naming rules and publication history. Merely discovering that V has an unusual trait does not transfer the reference to it.

The anchor has another level beneath it. In the ordinary prokaryotic case a species itself has a type strain, whereas the genus has a type species. A stored culture accession, a species reference and a genus reference therefore do not all denote the same rank or the same physical object.^[ref]

A type species stabilizes the connection between a name and its reference while taxonomy remains open to revised evidence. The detailed code governing the organisms concerned must be stated; one nomenclatural example should not silently impose every prokaryotic rule on other branches of biology.