Dinilysia

Dinilysia is an earlier example of a snake, from the Santonian Stage (86.3 Ma ± 0.2 to 83.6 Ma ± 0.5) and/or Coniacian Stage (89.8 Ma ±0.3 to 86.3 Ma ± 0.2) of the Upper/Late Cretaceous Period. Though portrayed with use of the extant Red-tailed Boa in the series, and shown living in North America 65.5 Ma (the Maastrichtian Stage of the Late Cretaceous Period) this is an anachronism in truth, out of place and out of time.

Walking With Dinosaurs

Dinilysia featured in the final episode of Walking With Dinosaurs - Death of a Dynasty. The T.rex babies were investigating it, as it was strange looking to them and they investigated it - by nipping at at it, which the snake did not appreciate.

The significance of its inclusion within Walking With Dinosaurs, was to briefly show that snakes have evolved and are now here and this is around the time the first ones evolve.

However, there is also a scientific theory expressed on screen by including the snake and its way of seeing - thermal imaging senses that reveal the Dinosaurs to be clearly endothermic, standing out against the backdrop. This is part of a huge argument over what type of blood at least some or maybe all Dinosaurs had. It is thought by many, that active carnivores such as the Tyrannosaurus rex would have had to be endothermic (almost certain)

Specials

Dinilysia was not in any specials.

Trivia

• Dinilysia was "live-acted" for the series by a red-tailed boa. Only one other creature was "live-acted" for the series, that being Steropodon, which was "live-acted" by a South American Coati. However, other animals were live-acted for Walking with Monsters and Walking with Beasts.
• Dinilysia actually lived 20 million years before T.rex, and in South America,not North America. Like with a few other matters in the Walking With...series, there are convenient amalgamations of different animals from somewhat different times, for convenience, sometimes due to the availability of fossil evidence at the time in 1999 with regards to Walking With Dinosaurs itself, though not necessarily. In other words, some inclusions were honest mistakes, others might have been more of a deliberate combination of animals with limited fossil evidence (in either a 1999 context or present day one) 
• There is such a thing as dramatic license, and "winging" certain things for the sake of artistic license; although strictly speaking, a number of species in Walking With Dinosaurs are indeed somewhat anachronistic to each other e.g. Plateosaurus had not yet evolved by the time the first episode of Walking With Dinosaurs - that is, the episode, New Blood - shows it very prominently, living in the same time and place as 220 million year old Coelophysis. An entire herd of Plateosaurus appear at the end of the episode in dramatic fashion; however, because of the limited number of episodes the series had, it was a fair choice to appropriately convey the symbolism of the world becoming one which the Dinosaurs would rule, much greater in size compared to older land animals that had come before.                                                                                                                               
• It got the message across, in spite of depicting Plateosaurus roaming the land roughly 6 million years earlier than it did according to what fossil records we have of Plateosaurus. For similar reasons, Dinilysia was likely picked to simply get the message across about the different visual spectra in the animal kingdom, and to take a look at a familiar reptile in a natural history documentary style. The ins and outs of which actual snake fossil inspired the one shown in the episode, "lived-acted" by a red-tailed boa, were not the primary concerns.                                                                                                                                                                                                       
• The snake is representative of all Cretaceous snakes in a sense, reminding the audience that such recognisable and ancient looking animals, were still relatively recently evolved in the grand scheme of things. Therefore, the inclusion of Dinilysia in Walking With Dinosaurs, is more of an esoteric complexity which most will not question (only those scientists in the know or pedantic enthusiasts/inversely critics, whom question their presence) The criticism of using the wrong species or a corner cutting measure to avoid having to animate the snake in a more original way, is understandable.         
• However, it might be a bit unfair to criticise the inclusion of such species via an obviously inaccurate "live-action" stand in for a better animated alternative, considering that animating a convincing snake interacting with the baby Tyrannosaurus would have been a bit excessive for a short scene, on the limited "bargain basement" silver-screen budgets they were working on back in 1999.                                                                                   
• It was a convenient and logical way to avoid an excessive and prohibitively expensive (as well as visually very tricky and potentially unconvincing) animation task. It got the message across and the brief but memorable scene, with the tense music and colourful visuals, explained to the audience that snakes had evolved in the Cretaceous Period. That was what they were going for, and it clearly works.
• The last episode is set 40.5 million years after the previous episode, interestingly enough. In that huge gap, there had passed multiple stages of the Cretaceous Period, including the one in which true/modern snakes probably first evolved from stem-snakes with legs (likely in the Cenomanian Stage or Albian Stage) 
• Exactly how far snake evolution had been pushed back in 1999, is difficult to be precise about in retrospect, though was presumably at least a bit more inaccurate back then in 1999. One would have to know precisely how old people thought that Ophidia and Serpentes were, in 1999.                                                                                                                   
• Snake specialist Palaeontologists, had by 1999, worked on snakes and snake-relatives (such as Pachyrhachis problematicus, a Cenomanian Stage snake with hindlimbs still in evidence, complete with tiny hip, knee and ankle joints) or snakes considered to be very basal (primitive)
• Now we know about very early stem snakes such as Eophis from the Bathonian Stage of the Middle Jurassic Period.
• It would be fair to assume that even in 1999, Palaeontology understood snake evolution to have gone back to at least the Cenomanian Stage, over 93.9 million years ago as we understand it now (though biostraphigraphy and subsequent chronostratigraphy would have been notably less developed in 1999) Though 1999 isn't all that long ago in the grand scheme of things, a lot can happen in science 21 years, so it is worth keeping in mind. 
• A fair assumption would be to say that the makers of Walking With Dinosaurs (not particularly getting their scientific advice from mostly the very limited number of specific snake specialists among Palaeontologists in general, though actively working with over 400 Palaeontologists in the process of making the series it should be said) had a general idea that snakes went back over 100 or so million years. How precisely this was understood in 1999 is difficult to be sure of. Speculation may already have existed of snakes evolving over 100 Ma, though. 

• Seeing as the penultimate episode of the main series of Walking With Dinosaurs, Spirits of the Ice Forest the fifth episode of six, was set 106 Ma/Mega-annum/Millions of years ago (and not featuring any snakes) it is fair to point out that the final episode was therefore the last opportunity that this specific series had in actually showing a snake.                                                  
• Neither modern snakes or stem snakes were ever shown prior. In the case of true/modern snakes, that is because they simply had not evolved yet in the times of most of the other episodes (though it is reasonable to point out that Snakes and Ophidia to which they generally belong, go back to different times and that the very earliest stem snakes may have been over 170 million years old, which ironically puts them back to the time in between the first and second episodes of the series, in the Middle Jurassic Period)                                                 
• Today, it is considered that recognisably "modern" Snakes evolved around 100 million years ago (according to 2020 chronostratigraphy as it stands, that would put the evolution of the first Snakes, to provably, be in the Cenomanian Stage of the Upper/Late Cretaceous Period)                                                                                   
• The Cenomanian Stage was from 100.5 to 93.9 Ma, according to the most recent 2020 Chronostratigraphic Chart, from the International Commission on Stratigraphy. 
• Dinilysia patagonica, had really lived in roughly the Coniacian Stage and/or Santonian Stage of the Upper/Late Cretaceous (in the very middle of the Upper/Late Cretaceous Period, that is) in South America. That is long before when the snake was shown to be portrayed in WWD.
• Dinilysia patagonica is one of the best-known terrestrial snakes from the Cretaceous Period, with abundant (and rare in being abundant) fossil finds in Argentina, particularly from the Anacleto Formation of Neuquen Province.
• The Anacleto Formation in Neuquen Province, Argentina, includes sandstone sediments in which Dinilysia is rather commonly found.
• Dinilysia patagonica has 24 mid-posterior trunk vertebrae.
• Unfortunately, in spite of rarer exceptions, the fossil record of snakes and their like in general, is relatively poor, due to the relatively fragile nature of snake bones, and especially the delicate and complex skulls, which when lost, take with them so much useful morphological information about the animal as it appeared in life (cranial morphology in snakes is very helpful in identification of which snake it is, extinct or extant; as cranial morphology is so often helpful in understanding an organism, with postcranial morphology sometimes being more generic, at least, in general)
• In other words, you can learn a lot about an animal by understanding their skulls and they can be identified more easily that way. The lack of such fossils, makes for both bigger gaps in the fossil record, as well as potential misunderstandings about the ones we have, in the full context of their evolution.
• The exact relationships of different fossil species and families within the overarching Order Squamata, are still debated in an ongoing situation where many disagreements emerge over one or other species or family. Therefore, it is reasonable to consider the possibilities and, lacking the level of fossil evidence we would prefer, speculate with logical guesses at the evolutionary mysteries relating to Snakes.                                           
• There will be more fossils found, more gaps in the fossil record filled - although preservational/taphonomic biases may long make that learning process delayed and drawn out. It does not help that snake fossils are harder to find than some other fossil animals. There is a reason why snake specialist Palaeontologists are comparatively rare, even among an already rare subject and profession.   
• Perhaps a somewhat better match for the snake portrayed, may have actually been another Cretaceous snake named Coniophis which lived at the same time and same place as Tyrannosaurus. However, the current fossils of Coniophis are nowhere near the size of Dinilysia.
• There is a huge debate in palaeontology, often relatively heated among those whom are involved or so this seems, about the evolutionary origin of snakes and their kin; whether they evolved on the land or in the oceans.  
• There are many relatives of snakes that had evolved as semi-aquatic reptiles which then became fully-adapted to marine life. Of course, the semi-aquatic reptiles had evolved from terrestrial ancestors that ironically, further back owed their own existence, to having crawled out of the sea.                                       This is secondary marine adaptation, where animals go back to the oceans for whichever evolutionarily driven reasons (often likely coming down to resource competition and palaeoecological pressure) With the Dinosaurs dominating the land, turning to the sea to do better, would not have been easy with the kinds of competition from large sharks and other marine reptiles. But whether snakes evolved in the oceans first and returned to the land again, or evolved first on land and then some of them became marine adapted snakes/sea snakes, is still debated. 
• Dinilysia was definitely a terrestrial snake, specialising in hunting small mammals perhaps.