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\b[Living <cartilaginous fishes> are specialized descendants, not a surviving anatomical stage before the origin of bony <vertebrates>.] <fossil> discoveries have shifted several apparently “bony-fish” features toward the ancestry of all living jawed <vertebrates>. Their absence in living <Chondrichthyes> can consequently represent <character loss>, while other <shark> features remain ancestral or are new specializations. The change is a revision of <character polarity>, not a claim that every aspect of <shark> <anatomy> is reduced.
A simple comparison of living <sharks> with <Osteichthyes> can suggest a one-way sequence: an ancestor with a cartilaginous internal skeleton, followed by origin of extensive <bone> on the bony-vertebrate branch. That inference is insecure because the two living sister lineages can both have changed, and living jawless <outgroups> can also have lost structures. The <fossil record> samples extinct combinations near the relevant divergences. To determine whether a structure is a gain or loss, one must place those combinations on a rooted <phylogenetic tree> and distinguish anatomical absence from poor preservation.
The early <gnathostome> problem involves four familiar assemblages: <placoderms>, <acanthodians>, <Chondrichthyes> and <Osteichthyes>. Treating all four as separate complete <clades> can conceal transitions occurring along the stems of surviving groups. In particular, many newer analyses place <acanthodians> as successive branches on the chondrichthyan stem. Their name “spiny <sharks>” is not itself evidence for that placement; cranial, scale, <fin> and other characters must support it. If this topology is correct, the large difference between living <sharks> and early armoured jawed fishes includes a history of reduction within the cartilaginous-fish lineage.
The 2012 redescription of the <Acanthodes> braincase illustrates why anatomical interpretation and tree position must be kept separate. It revealed a mixture of conditions previously assigned to different living groups and argued for shark-like aspects of the <last common ancestor> of modern jawed <vertebrates>. Its analysis still placed <Acanthodes> on the osteichthyan stem, rather than treating every <acanthodian> as a stem chondrichthyan. Thus a resemblance initially interpreted as an osteichthyan specialization could instead be an ancestral feature retained in more than one lineage. Later hypotheses should not be projected retrospectively onto that study.<Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes by Samuel P. Davis and others>[Nature 486, pp. 247–250; braincase redescription and changes in inferred character polarity]{ref}
<Entelognathus primordialis>, described in 2013 from the <Silurian> of China, supplied a particularly important association: an armoured placoderm-like body and <skull> with large marginal <jaw> <bones> identifiable as <premaxilla>, <maxilla> and <dentary>. Such <bones> had been treated as characteristic innovations of <Osteichthyes>. Placement of this animal near the top of the jawed-vertebrate stem, together with recovery of <acanthodians> on the chondrichthyan stem, instead supported a macromeric, extensively ossified dermal ancestor. On that reconstruction, the small-scale-dominated covering and absence of those large <jaw> <bones> in living <Chondrichthyes> reflect reduction and reorganization rather than simple retention of a primitively unossified condition.<A Silurian placoderm with osteichthyan-like marginal jaw bones by Min Zhu and others>[Nature 502, pp. 188–193; Entelognathus jaw bones, macromeric dermal skeleton and acanthodian placement]{ref}
<Janusiscus schultzei>, described in 2015 from the Early <Devonian>, reinforced the point using a different anatomical region. Its combination of a bony dermal <skull> roof and informative internal cranial conditions supported a position outside crown <gnathostomes>. Braincase <anatomy> therefore did not align neatly with the traditional picture of a primitive <shark> head versus a derived bony-fish head. Together, these <fossils> move apparently osteichthyan-like states closer to the common ancestry of both surviving groups. Their significance comes from character combinations and placement, not from choosing whichever living animal looks most similar.<Osteichthyan-like cranial conditions in an Early Devonian stem gnathostome by Sam Giles and others>[Nature 520, pp. 82–85; Janusiscus braincase, skull roof and character-state reconstruction]{ref}
A separate question concerns internal skeletal tissue. \b[<dermal bone>, <perichondral bone>, <endochondral bone> and <calcified cartilage> must not be conflated.] <endochondral bone> forms during replacement of a cartilaginous template; <perichondral bone> forms around it; <calcified cartilage> remains a different tissue despite being mineralized. Therefore a bony dermal <skull> roof alone cannot establish endochondral <ossification> in the same animal.
The 2020 description of <Minjinia turgenensis>, from the Early <Devonian> of Mongolia, addressed that distinction. <micro-computed tomography> revealed extensive internal trabecular tissue in the braincase, interpreted as <endochondral bone>, rather than just an external bony shell. Its recovery close to the crown divergence permits endochondral <ossification> to be reconstructed before the separation of <Chondrichthyes> and <Osteichthyes>, followed by loss or marked suppression along the chondrichthyan lineage. This conclusion depends on identification of the tissue and the animal's position; the absence of a more complete skeleton and uncertainty among early jawed-vertebrate trees constrain how strongly it can be generalized.<Endochondral bone in an Early Devonian ‘placoderm’ from Mongolia by Martin D. Brazeau and others>[Nature Ecology & Evolution 4, pp. 1477–1484; Minjinia internal trabeculae, tomography and ancestral-state sensitivity]{ref}
The resulting inference is conditional but testable. If well-supported stem relatives and the bony-vertebrate branch possess a particular homologous tissue, while living cartilaginous fishes lack it, an ancestral presence followed by loss can fit better than several independent origins. Moving a crucial <fossil> to another branch, changing a tissue identification or allowing different transition rates can change the ancestral reconstruction. <maximum parsimony> is useful but is not proof that losses and gains have equal biological probability. Character coding should record unknown preservation as unknown, rather than use an unobserved delicate feature as evidence of absence.
\b[The major revision is from a ladder of increasing <ossification> to a branching history of gains, retention and losses.] It concerns particular tissues and anatomical elements: reduction of large <dermal bones> need not coincide exactly with loss of endochondral <ossification>. <fossils> reveal that apparently simple modern <anatomy> can be secondarily simplified, and that major living-group differences may reflect transformations on both sides of a divergence. Continued uncertainty over <placoderm> relationships and individual tissue homologies defines the comparisons still needed; it does not restore living <sharks> as unchanged proxies for the jawed-vertebrate ancestor.
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