When one evening in 1764 Gibbon sat in the ruins of the Capitol and first had the idea of writing the city’s history, the arch of Septimus Severus would have been foremost in his line of vision. The dynasty animates Gibbon's perhaps most dramatic chapter (VI) in The Decline and Fall of the Roman Empire. In the Colosseum exhibition the Severans' illuminated heads throng the atrium with a gamut of character types to satisfy the most demanding of script-writers, from the severe Septimus Severus, the dynasty's founder, to the depraved Eliogabalus to Alexander Severus, in Gibbons’ version almost too virtuous for his own good.

First on display, however, is Septimus's amiable Syrian-born consort, Domna Julia, the tragic super-matriarch. Here she stands in the guise of Ceres, goddess of agriculture and grain. It was grain from her husband's home region of Tripolitania, now part of Libya, that provided Rome with its seven-year surplus. Two curls of her own hair peeping out below her wig, she was a style-setter for other Roman matrons. Large eyes, long nose, high cheekbones – was this the inspiration on which Gibbon, mind-reading the marble, based his loving portrayal?

First male statue in the exhibition is Marcus Aurelius, the penultimate Antonine into whose imperial family the comparatively low-born Septimus would self-adopt.
The neatly-trimmed hair and Greek-philosopher-type beard of Marcus Aurelius are shared by Septimus – his attempt to confer authority and due continuity on what would in modern parlance be termed a military coup. Indeed in his first speech to the senate, Septimus declared Commodus, the son of Marcus Aurelius, a god, in front of the very dignitaries Commodus (of Gladiator fame) had threatened to decapitate.

The Antonines, Gibbons’ all-time favourites, had until Commodus, the last of the dynasty, used the “adoptive system”: succeeding emperors were chosen for their fitness to govern. By contrast Septimus, an ailing monarch, elevated his squabbling sons, Caracalla and Geta, to joint Caesars, sparking a sibling rivalry smacking of Cain and Abel.
One bust is of a childhood Geta. Glaring sideways, Caracalla, who had Geta murdered, boasts two busts. Geta was then erased from public memory, his likenesses pulverised or, in a family fresco, reduced to an anonymous smudge. Caracalla, Gibbon’s “enemy of mankind”, would also have an untimely end, killed in a plot involving Macrinus, head of the Pretorian Guard.

After an interim under Macrinus, who had risen through the imperial household and was proclaimed emperor on the death of Caracalla, the Severans were back by 218 AD. Faced with Macrinus’s pay-cuts, the army in the east was discontented again. Long-suffering Domna Julia, on the death of her second son Geta, had starved herself to death; Julia Mesa, her sister back in Emesa (today’s Homs in Syria), took over as dynastic matriarch. Quickly she persuaded her elder daughter, Julia Soema, respectably married to one Varius Bassanius, to feign a past affair with Caracalla. Thus the 14-year-old son, Eliogabalus, switching fathers, could directly claim the emperorship.
Arriving in Rome a year later, the youth would become a cross-dresser, husband to first a series of women, then to a charioteer, then to a vestal virgin. This between suffocating dinner-guests with rose-petals and forcing senators to attend ceremonies anything but traditionally Roman. For damage limitation, Julia Mesa persuaded him to share the throne with her other grandson, the strictly Roman 13-year-old Alexander Severus who, in a further sleight of alternative fact, was also passed off as a son of Caracalla.

Initially Eliogabalus agreed to his grandmother’s arrangement, but his wayward nature soon returned. After an outrage too far, the senate cried enough. Eliogabalus went the way of his two predecessors, his body then being thrown into the Tiber. Counselled by the wise Uplian and others, Alexander, then a callow youth, developed into the most conscientious of emperors.
From busts to buildings: models of Leptus Magna, Septimus's home town, then of his triumphal arch in the Forum, virtual imagery enabling the visitor to examine closely the scenes too high up on the real thing. Those triumphs against the Parthians, their captives lined in chains below, didn’t last. In an early example of imperial overstretch, the Persian Sassanids were waiting in the wings to win back lands from the Romans as the empire began to collapse, while  in the north of England guerrilla tactics by the local tribes drove the legions back behind Hadrian’s wall. During a rest in campaigning against the Caledonians (or Scots), Septimus died in Eboracum (York) in 2011.  
Outdoing plastic casts are the Urban Landscape section’s 3D videos: Admire Eliogabalus’s temple (in the Roman Forum) and then Septimus's Septizodium with all its statuary and fountains. There’s also a drone’s-eye view of Severus’s Arch topped by a golden seven-horsed chariot. A second video pieces back together part of the Forum Urbis, the marble map once attached in 150 slabs to the Templum Pacis, the area now largely occupied by the S. Cosma e Damiano church. Only 15 per cent – 1,186 chunks – survived depredation following Rome’s fall. The wall-holes still perforate the SS. Cosma and Damiano church façade.
From stone, via Egypt, to papyrus. The anachronistic-sounding Antonine Constitution is explained on video by Caracalla. The constitution conferred universal citizenship rights, yes. But also the responsibility to pay taxes to support the burgeoning military and the massive building and restoration projects following Commodus’s fire-prone rule. As the catalogue mentions, plagues and smallpox in previous decades had drastically eroded the tax-base. Here, at one stroke, was a means of widening it.

Lastly, trade. Another wall-map makes our own EU seem small. Metal from Britain to Dalmatia. Marbles from Greece and Lower Egypt. Wine from Ephesus or Gaul. Fish-sauce from the Black Sea. Olive-oil from Cordoba. Proof of the Severan trade boom is Monte Testaccio: the district just north of the Tiber is founded, both etymologically and archaeologically, on shards of 53 million amphorae, jug-eared “teste”, mostly from the Severan period. The stamps on the handles bear out how the dynasty established something of an imperial monopoly on the import/export business, with the names of individual entrepreneurs tending to disappear.
And luxuries? Indicating how seriously Romans took their pleasures, from New York’s Met Museum comes a tray-handle engraved with the triumph of Bacchus. With the potential to inflate cocktail prices, from Germany calyxes, cups and beakers demonstrate glassware as an art form which our mass technology has lost. More down-market is some Tunisian earthenware, but whose cupids, birds and a musical monkey combine to make drinking cinematic.

For one ticket this might seem enough. Yet, along the Forum, Romulus’s temple hosts 33 artefacts washed back into the present, following Professor Panella’s recent excavation of Eliogabalus’s Baths (between the Arches of Titus and Constantine). In a video, Time’s Vortex, archaeological layers unfold – Iron Age huts, an early imperial eating complex, Severus’s then Eliogabalus’s baths. 
Gibbon’s History of the Decline and Fall of the Roman Empire, a brilliantly grotesque if majestically worded helter-skelter of events, can read like a fiction; here in recently or not-so-recently excavated marble and travertine is its confirmation as fascinating fact.