What I came to know was that 12617/8 Mangala Lakshadweep express is the descendant of this Jayanti Janata express. But it wasn't a direct descendant though. Lot of changes happened in due course, before today's Mangala Lakshadweep came into picture.
1973/4: 131/2 Delhi Nizamuddin – Mangalore/ Cochin (Jayanti) Janata Express (splitting/joining at Shoranur) and 17/18 daily Madras – Delhi Janata Express were started.
131/2 was initially bi weekly, then tri weekly and later 5 days a week. 17/8 was daily since inception and was extended to JAT somewhere around the introduction of TN....
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1988: Big 4 of south became daily. During same time, Janatas were discontinued and were converted to standard express.
When Kerala became daily, 131/2 service was discontinued and was merged with Kerala and for very short time, Kerala ran as Kerala-Mangala express with splitting/joining at Shoranur(Kerala part towards TVC and Mangala part towards Mangaluru).
At the same time 17/18 MAS – JAT Janata was converted, split and extended into three trains in 1988, the Andaman (MAS – JAT), Himasagar (CAPE – JAT) and Navyug (MAQ – JAT) Expresses of today. Now till SVDK.
Don't know when and why the slip portion was discontinued.
1998: With opening of Konkan line, the lost connectivity of Mangaluru was restored with 12617/8 Mangala Lakshadweep express running till ERS.
Not just these 2 trains, but other JJ category trains introduced during 1973/4 period are now running as some or the other train.