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Description
So here it is! I know you've been waiting for this one...
Studying theropods really means much more than studying origins of birds. Lately I've been a lot focused on longirostrine, likely piscivore theropods. It's a very interesting matter as piscivory appeared many times (in many clades) along the theropod tree. Each time, both similarities (homoplasies) and differences among these groups could be detected. Extinct theropod clades which show possible signs of piscivory adaptations as well are Coelophysoidea, Noasaurinae and Longipterygidae, but in this sheet I decided to include only 6 species representing clades such as Spinosauridae, Unenlagiinae, Hesperornithes etc... (Pelecanimimus is actually a lonely guy under this point of view) that show this kind of ecologic adaptation somewhat extremely clearly.

I'll update this post within the next hours with a detailed description of all the differences and similarities that these groups share in relation to piscivory (regarding the skull only). Until then, I'd like to see if you can already catch those differences and similarities without explanations, so that I can see if my infographic drawing is actually useful without much speaking around.

- UPDATE after 6 hours -

Time out! Here you are my explanations, as synthetic as possible!
So, among these theropods Spinosaurus is undoubtedly the biggest one (as it is actually also one of the biggest theropods in general). Its piscivory specializations are detectable in postcranial characteristics as well. These obviously include the extremely hypertrophic and flattened neurospines which are the reason of the genus name, and in general, limbs and claws proportions extrapolated from Baryonychinae (but I'm not going into those debates about Spinosaurus posture, barycentre etc). So, the skull of Spinosaurus shows the following adaptations to piscivory:
-Elongated anterior half of the skull consisting in maxilla+premaxilla length > 2 x jugal length (even slightly more pronounced in baryonychinae);
-Procumbency of anterior-most teeth (both premaxillar and dentary, and maxillar as well) somewhat like in e.g. Masiakasaurus;
-Bulb-shaped/crocodyliform and pneumatized dentary and premaxilla tips (noticeable in all spinosaurids, including Oxalaia) accentuated by a strong discontinuity (in side view) of the jaws lines around premaxilla/maxilla suture (as in dilophosaurids);
-Retroverted narial fenestrae = external nares (really extreme in this case as the fenestra has no contact with the premaxilla and contacts on its lower edge with the maxilla indeed) with the addition of an autapomorphic subnarial foramen. This condition is also present in Irritator/Angaturama but less pronounced in Baryonychinae, where narial fenestrae still contact with the premaxilla and there are not any kinds of subnarial foramina;
-High number of premaxillar teeth (7 per ramus) and generally high global number of teeth (around 70). In Baryonychinae the number of dentary teeth further increases by at least 3 positions so that the global number of teeth is about 80.
It is clear that Spinosaurus was not a mere piscivore, but a “mega” one, just like thalassophoneans, for instance.

Let's move to “normal” piscivores (both in relation to size and ecology). Pelecanimimus is a very special case as it holds the record being the theropod with the highest number of teeth so far. While it has 7 premaxillary teeth per ramus just like spinosaurids, the number of teeth per maxilla/dentary is up to 50, making the global number of teeth up to 200 (!). As a consequence, each tooth is disproportionally small if compared to what is usual in theropods. This and many other characteristics of Pelecanimimus skull inquisitively remind me of several pterosaurs with likely homologous ecological/ethological specializations (no, I'm not referring to tapejarids) but yeah, that was the inspiration behind the “You stole my lunch” artwork. But whatever, Pelecanimimus has a very interesting chimeresque skull anatomy besides teeth issues. The jugal contacts with the antorbital fenestra and the lacrimal has no contact with the maxilla. The narial fenestrae are not retroverted so that the premaxilla is very tiny and the nasal is extremely elongated instead. Overall, Pelecanimimus skull proportions also remind me of Coelophysis bauri (what if Pelecanimimus is a late surviving coelophysid? ..or... what about spinosaurids being dilophosaurids being coelophysoids including Pelecanimimus... Ok that's probably too much speculating but you got my food for thought). The interesting fact is that coelophysids also show a incredibly high number of dentary teeth in a similar measure to Baryonyx and Suchomimus for instance. So yeah I keep insisting on coelophysoids being potentially piscivorous as well. The last thing which is enlightening about Pelecanimimus is its throat soft tissue evidence which reminds...guess what? Oh yes! Pelicans.
Now let's talk about the new entry, Halszkaraptor. So, this guy shows an insanely high number of premaxillary teeth (11) and, like spinosaurids, has a highly pneumatized premaxilla with a retroverted narial fenestra (albeit not as much as in spinosaurinae, but kinda like in baryonychinae) and a flattened, duck-like prenarial area. Both anterior premaxillary and dentary teeth show a slight condition of procumbency though not as developped as in spinosaurids or, even worse, noasaurins. The global number of teeth is between 110 and 120 (around 20 teeth per maxillary ramus and almost 30 teeth per dentary ramus). Even if the anterior half of the skull is generally elongated that contrasts with the temporal/occipital area of the skull which is well developped in height instead. That is somewhat coeherent with the several postcranial aquatic adaptations such as e.g. extremely elongated cervical vertebrae (guess whom shares the same condition? Yeah, coelophysids!) and furthermore reminds me of hesperornithes (whom also shares the long, thin, bar-like jugal with no ascending process).
But before I start talking about hesperornithes let's mention other two interesting and still separate cases inside Deinonychosauria.
Buitreraptor is the extinct theropod with the highest skull length/height ratio (up to 5) and highest antorbital/postorbital skull length ratio (up to 5 as well) although these values are potentially influenced by taphonomy bias (Peters can you hear me? I'm in California dreaming about who taphonomy interpretation used to be) so basically Buitreraptor likely had not hypertrophic nares but the elongation of the whole antorbital skull area is undeniable eveng taking taphonomy into strict consideration. An interesting feature of Buitreraptor skull is also the presence of a well developped maxillary fenestra (wide more than threefold than the antorbital fossa). Unfortunately teeth count is indeterminable but certainly very high given the rather minute size of preserved dentary teeth, I'd say probably between 100 and 180.
Note: Buitreraptor is here restored with a slanting lacrimal because of Austroraptor condition which is also one of those key factors that moved Adasaurus into Unenlagiinae in my topology (in addition to features residing in “feet”). I hypothesize (though it is still an extremely speculative conjecture so I beg you not to take this as a fact) that this particular shape of the lacrimal is a potential synapomorphy of Unenlagiinae.
Now Byronosaurus. Even in this case, teeth look unusual in their particularities, as they completely lack serrations (!) and have very short, sometimes unexisting diastemas (interdental spaces). The global number of teeth is estimable between 150 and 200, and the posterior-most portions of both maxilla and dentary are toothless so that the distribution of teeth is not homogeneous, as they gradually increase in “density” proceeding from the back to the tip of the jaws lines. Other interesting features of Byronosaurus skull include very hypertrophic external nares (not much likely influenced by taphonomy bias) and the simultaneous condition of [presence of maxillary fenestra + absence of antorbital fossa] with an interfenestral area (between antorbital fenestra and maxillary fenestra) which separates the two concavities as being on the same plan of the main maxillary body. Xixiasaurus vaguely resembles Byronosaurus in some of the aforementioned features, but still to a much less extent so Byronosaurus really remains a very peculiar case among troodontids (that doesn't mean other troodontids mustn't have been partially piscivorous).
Austroraptor is a sort of midway between Buitreraptor and Byronosaurus skull conformations. I decided not to include it in the sheet since Unenlagiinae was already represented by Buitreraptor (and also because Austroraptor has a more carnivorous-like dentition unlike the previous two).

Finally let's talk about Hesperornis. Some of the clear differences detectable in comparison to the previous theropods actually depend on well more birdy features which are synapomorphic for clades such as Euavialae, Ornithothoraces, Euornithes, Ornithurae etc. Some others are characteristic of the hesperornithes lineage which shows a rather linear adaptation to an aquatic lifestyle. Teeth are “triangular-shaped” and set in grooves. Global teeth count is around 80 with maxilla having around 15 teeth per ramus while dentary has 25 and premaxilla is edentulous (Count differences vary depending on species/genus, please don't make me talk about Hesperornis taxonomy). The whole post-maxillary area of the skull is vaguely similar to Halszkaraptor (high occipital/temporal region, thin bar-like jugal, T-shaped and “knobby” lacrimal, rounded frontal edge in side view) and the highly retroverted external nares plus the Y-shaped nasal plus extremely elongated premaxilla also vaguely resemble Spinosaurus.
These adaptations potentially indicate that while Pelecanimimus, Buitreraptor and Byronosaurus were piscivorous but still with terrestrial-based ethologies, Spinosaurus, Halszkaraptor and Hesperornis were not only piscivorous but even ethologically amphibious. Eventually, this interpretation is further supported by deeply different among each other but equally significative (in relation to water) postcranial -especially locomotory- features present in the latter three (which pop out as well evident even to the untrained eye).
You definitely deserve an applause if you read until here. Sorry for length but yeah I can guarantee this was a synthesis. Any insights questions/corrections are well welcomed! There is a huge chance I could've written some lil' imprecision because of the haste.
Hope you enjoyed both art and text!

P.S.:
Scale bars are 5 cm (2 inches).

Description
So here it is! I know you've been waiting for this one...
Studying theropods really means much more than studying origins of birds. Lately I've been a lot focused on longirostrine, likely piscivore theropods. It's a very interesting matter as piscivory appeared many times (in many clades) along the theropod tree. Each time, both similarities (homoplasies) and differences among these groups could be detected. Extinct theropod clades which show possible signs of piscivory adaptations as well are Coelophysoidea, Noasaurinae and Longipterygidae, but in this sheet I decided to include only 6 species representing clades such as Spinosauridae, Unenlagiinae, Hesperornithes etc... (Pelecanimimus is actually a lonely guy under this point of view) that show this kind of ecologic adaptation somewhat extremely clearly.

I'll update this post within the next hours with a detailed description of all the differences and similarities that these groups share in relation to piscivory (regarding the skull only). Until then, I'd like to see if you can already catch those differences and similarities without explanations, so that I can see if my infographic drawing is actually useful without much speaking around.

- UPDATE after 6 hours -

Time out! Here you are my explanations, as synthetic as possible!
So, among these theropods Spinosaurus is undoubtedly the biggest one (as it is actually also one of the biggest theropods in general). Its piscivory specializations are detectable in postcranial characteristics as well. These obviously include the extremely hypertrophic and flattened neurospines which are the reason of the genus name, and in general, limbs and claws proportions extrapolated from Baryonychinae (but I'm not going into those debates about Spinosaurus posture, barycentre etc). So, the skull of Spinosaurus shows the following adaptations to piscivory:
-Elongated anterior half of the skull consisting in maxilla+premaxilla length > 2 x jugal length (even slightly more pronounced in baryonychinae);
-Procumbency of anterior-most teeth (both premaxillar and dentary, and maxillar as well) somewhat like in e.g. Masiakasaurus;
-Bulb-shaped/crocodyliform and pneumatized dentary and premaxilla tips (noticeable in all spinosaurids, including Oxalaia) accentuated by a strong discontinuity (in side view) of the jaws lines around premaxilla/maxilla suture (as in dilophosaurids);
-Retroverted narial fenestrae = external nares (really extreme in this case as the fenestra has no contact with the premaxilla and contacts on its lower edge with the maxilla indeed) with the addition of an autapomorphic subnarial foramen. This condition is also present in Irritator/Angaturama but less pronounced in Baryonychinae, where narial fenestrae still contact with the premaxilla and there are not any kinds of subnarial foramina;
-High number of premaxillar teeth (7 per ramus) and generally high global number of teeth (around 70). In Baryonychinae the number of dentary teeth further increases by at least 3 positions so that the global number of teeth is about 80.
It is clear that Spinosaurus was not a mere piscivore, but a “mega” one, just like thalassophoneans, for instance.

Let's move to “normal” piscivores (both in relation to size and ecology). Pelecanimimus is a very special case as it holds the record being the theropod with the highest number of teeth so far. While it has 7 premaxillary teeth per ramus just like spinosaurids, the number of teeth per maxilla/dentary is up to 50, making the global number of teeth up to 200 (!). As a consequence, each tooth is disproportionally small if compared to what is usual in theropods. This and many other characteristics of Pelecanimimus skull inquisitively remind me of several pterosaurs with likely homologous ecological/ethological specializations (no, I'm not referring to tapejarids) but yeah, that was the inspiration behind the “You stole my lunch” artwork. But whatever, Pelecanimimus has a very interesting chimeresque skull anatomy besides teeth issues. The jugal contacts with the antorbital fenestra and the lacrimal has no contact with the maxilla. The narial fenestrae are not retroverted so that the premaxilla is very tiny and the nasal is extremely elongated instead. Overall, Pelecanimimus skull proportions also remind me of Coelophysis bauri (what if Pelecanimimus is a late surviving coelophysid? ..or... what about spinosaurids being dilophosaurids being coelophysoids including Pelecanimimus... Ok that's probably too much speculating but you got my food for thought). The interesting fact is that coelophysids also show a incredibly high number of dentary teeth in a similar measure to Baryonyx and Suchomimus for instance. So yeah I keep insisting on coelophysoids being potentially piscivorous as well. The last thing which is enlightening about Pelecanimimus is its throat soft tissue evidence which reminds...guess what? Oh yes! Pelicans.
Now let's talk about the new entry, Halszkaraptor. So, this guy shows an insanely high number of premaxillary teeth (11) and, like spinosaurids, has a highly pneumatized premaxilla with a retroverted narial fenestra (albeit not as much as in spinosaurinae, but kinda like in baryonychinae) and a flattened, duck-like prenarial area. Both anterior premaxillary and dentary teeth show a slight condition of procumbency though not as developped as in spinosaurids or, even worse, noasaurins. The global number of teeth is between 110 and 120 (around 20 teeth per maxillary ramus and almost 30 teeth per dentary ramus). Even if the anterior half of the skull is generally elongated that contrasts with the temporal/occipital area of the skull which is well developped in height instead. That is somewhat coeherent with the several postcranial aquatic adaptations such as e.g. extremely elongated cervical vertebrae (guess whom shares the same condition? Yeah, coelophysids!) and furthermore reminds me of hesperornithes (whom also shares the long, thin, bar-like jugal with no ascending process).
But before I start talking about hesperornithes let's mention other two interesting and still separate cases inside Deinonychosauria.
Buitreraptor is the extinct theropod with the highest skull length/height ratio (up to 5) and highest antorbital/postorbital skull length ratio (up to 5 as well) although these values are potentially influenced by taphonomy bias (Peters can you hear me? I'm in California dreaming about who taphonomy interpretation used to be) so basically Buitreraptor likely had not hypertrophic nares but the elongation of the whole antorbital skull area is undeniable eveng taking taphonomy into strict consideration. An interesting feature of Buitreraptor skull is also the presence of a well developped maxillary fenestra (wide more than threefold than the antorbital fossa). Unfortunately teeth count is indeterminable but certainly very high given the rather minute size of preserved dentary teeth, I'd say probably between 100 and 180.
Note: Buitreraptor is here restored with a slanting lacrimal because of Austroraptor condition which is also one of those key factors that moved Adasaurus into Unenlagiinae in my topology (in addition to features residing in “feet”). I hypothesize (though it is still an extremely speculative conjecture so I beg you not to take this as a fact) that this particular shape of the lacrimal is a potential synapomorphy of Unenlagiinae.
Now Byronosaurus. Even in this case, teeth look unusual in their particularities, as they completely lack serrations (!) and have very short, sometimes unexisting diastemas (interdental spaces). The global number of teeth is estimable between 150 and 200, and the posterior-most portions of both maxilla and dentary are toothless so that the distribution of teeth is not homogeneous, as they gradually increase in “density” proceeding from the back to the tip of the jaws lines. Other interesting features of Byronosaurus skull include very hypertrophic external nares (not much likely influenced by taphonomy bias) and the simultaneous condition of [presence of maxillary fenestra + absence of antorbital fossa] with an interfenestral area (between antorbital fenestra and maxillary fenestra) which separates the two concavities as being on the same plan of the main maxillary body. Xixiasaurus vaguely resembles Byronosaurus in some of the aforementioned features, but still to a much less extent so Byronosaurus really remains a very peculiar case among troodontids (that doesn't mean other troodontids mustn't have been partially piscivorous).
Austroraptor is a sort of midway between Buitreraptor and Byronosaurus skull conformations. I decided not to include it in the sheet since Unenlagiinae was already represented by Buitreraptor (and also because Austroraptor has a more carnivorous-like dentition unlike the previous two).

Finally let's talk about Hesperornis. Some of the clear differences detectable in comparison to the previous theropods actually depend on well more birdy features which are synapomorphic for clades such as Euavialae, Ornithothoraces, Euornithes, Ornithurae etc. Some others are characteristic of the hesperornithes lineage which shows a rather linear adaptation to an aquatic lifestyle. Teeth are “triangular-shaped” and set in grooves. Global teeth count is around 80 with maxilla having around 15 teeth per ramus while dentary has 25 and premaxilla is edentulous (Count differences vary depending on species/genus, please don't make me talk about Hesperornis taxonomy). The whole post-maxillary area of the skull is vaguely similar to Halszkaraptor (high occipital/temporal region, thin bar-like jugal, T-shaped and “knobby” lacrimal, rounded frontal edge in side view) and the highly retroverted external nares plus the Y-shaped nasal plus extremely elongated premaxilla also vaguely resemble Spinosaurus.
These adaptations potentially indicate that while Pelecanimimus, Buitreraptor and Byronosaurus were piscivorous but still with terrestrial-based ethologies, Spinosaurus, Halszkaraptor and Hesperornis were not only piscivorous but even ethologically amphibious. Eventually, this interpretation is further supported by deeply different among each other but equally significative (in relation to water) postcranial -especially locomotory- features present in the latter three (which pop out as well evident even to the untrained eye).
You definitely deserve an applause if you read until here. Sorry for length but yeah I can guarantee this was a synthesis. Any insights questions/corrections are well welcomed! There is a huge chance I could've written some lil' imprecision because of the haste.
Hope you enjoyed both art and text!

P.S.:
Scale bars are 5 cm (2 inches).